Show Posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.


Messages - sookwinder

Pages: [1] 2
1
BTW, not withstanding what Fender/Rhodes called it, it isn't actually a bass boost ... it is a bass cut.
Just the same principle as the bass control on an amp... a high pass filter.
One extreme allows all the signal through, the other extreme cut the bass signal.

2
I'm re-opening this topic with a clarification question:

Were Audio Taper potentiometers used in all cases?

Certainly on the controls I designed/built the pots are all audio....  basically it is just a copy/revamp/bastardisation of stuff that Leo did on his amps.

3
The Fender Rhodes Electric Piano / Re: Rhodes Stage MKI Hiss
« on: February 24, 2019, 05:25:23 AM »
I too have been "fighting" hiss/noise that comes and goes on my Mk1 stage piano.
It occurs when the tonal controls are bypassed or set at 10.

It has taken about a year of on-again and off-again experiments to determine that the 73 "guitar pickups" that is the rhodes are picking up something emanating/radiating locally... whether that is a function of the normal street lights / power transmission lines that are nearby or modern switch mode power supplies that are everywhere I have not been able to determine. [I have spent many a lunchtime discussing that latest data with my manager who is an electrical engineer]

I know it isn't self generated by the Rhodes (obviously), it isn't from any of the amplifiers or preamps that I have put in or out of the signal line, recording system, house wiring, solar panels, other electrical equipment within the home that is causing the issue.

While I have no real idea what is causing the issue, I believe I have a fix, that being lining the under side of the cover with copper foil and securing it at the two ends/sides to the earth/chassis of the piano.  Trying to create a faraday cage was impractical , so I went for the shielding approach.  My experiments have been quite successful and am now awaiting the roles of 0.3mm copper foil.

I will document this in a thread once I have completed the "upgrade".

4
Yes, if you read my original post, I had the hand drawn circuit in my Rhodes. 
I agree .. next to useless.

And no I do not play with the preamp sitting on the Rhodes top....  that was an artistic set up for the photo  ;D
I understand the concept of electrical interference.

5
Jenzz, G'day there...
The Rhodes was completely restored by CEPCo 10 months ago...  no issues from that perspective.
The noise I speak of is the standard noise you get from single coil pickups. The Rhodes, last time I looked, had quite a few single coil PUPs.

I am still trying to come to grips with the concept that a Rhodes needs (or needed) a bass boost.  Every amp I have ever used, or player I know who plays a Rhodes, have always had to reduce the bass via the amp controls.

FYI, with the circuit that was in my Rhodes, as the Bass Boost was rolled off  (ie the level of bass relative to the treble reduced) the overall volume of the treble decreased as well.  I measured this with a signal generator. That circuit, 10k/10k/0.1uF, is a dud circuit.  Even the 50k/10k/0.047 version circuit would have been better.  I even built up a copy of the 10k/10k/0.1uF circuit with brand new components and the results were the same.

Irrespective of the above ...  the Rhodes I have was manufactured with a crappy designed volume/tone control which was useful at basically one combination of the volume/tone controls.  I have looked at the issue from the way I would if I was designing an amp/guitar combo and come up with a circuit that is useful at the entire range of positions.

the Rhodes sounds awesome ... even better than it did.
And people can use the circuit if they wish or ignore it ...  that's what forums like this are for.

David

6
I have 10+ amps of various sizes, designs (6 of them designed/manufactured by me) all very quiet.  All exhibit the "noise" level that I mentioned I experienced when plugging in the Rhodes.  The same noise never occurs with any guitars or other EPs.
The noise IMO is because of the miss match of components (in/out impedance mismatch) is a function of the circuit ... an EE would immediately comment of  the miss match of impedances and warn about loss of high frequencies which would result in noise being more prominent.

Further in my assessment of the circuit I did not even plug into an amp as such ... I used the valve preamp I have designed which is a the front end of an amp design, but doesn't have the power section for speakers as I then go DI.  I.e. it boosts the signal to line levels adding the "valve" sound and levels of compression associated with valve amps.  So I have negated any issues associated with good or bad speakers.

I do not know if you have played a stage Rhodes with the particular circuit I had in my stage (the one with the 2 x 10K pots and 0.1uF cap) but in reality when it was used, and the "bass boost" utilized, the overall volume reduced even further.  So you had a situation where, if you wanted bass boost you turned it up only to find that the overall volume has been reduced. Yes a tonal change has occurred but so has the volume dropped, almost counter acting the desired affect. IMO a crap design.

Volume/tone control circuits are a strange beast. 
There are many that are used or have been designed for amps over the years, but historically the one that is used is the FMV (fender Marshal Vox) style.  Not  because it is the best  (it certainly isn't) but because it has been the most commonly used, because it appears to be the simplest design that musicians (not amp designers) can understand and use successfully and because the component numbers are low - good for mass production.  Another reason is that FMV circuit is a good (general) circuit for ALL types of music.   
Likewise I would put forward that the circuit that was in my Stage Rhodes (10k/10k/0.1uF) was based upon cost reduction for production rather than any particular notion of "let's design it to allow a boost here or there in the bass because of speakers that may be not as efficient at particular areas".

David

7
I have had my 1973 Mk1 Stage Rhodes for just shy of a year now. Love it.
What I don't love is the crappy volume/tone controls. I know in Leo's day, everything was done to reduce production costs, for example design attributes in valves amps that we would just take for granted these days (including safety features that lower the risk of destroying the valves) were just not even considered back in the day.

Most likely that mind set was continued once Leo retired and obviously the Rhodes production was not immune.
But seriously the volume and tone controls circuit fitted to my Mk1 stage is amateurish at best.
The tea lady (or coffee lady) would have had a better chance of designing volume and tone controls for the Stage Rhodes.

There are many issues that arise.
The circuit is noisy.
Most of the signal is lost to ground because of ridiculous component (size) choices.
The bass boost circuit really doesn't do what its name suggests or at least does it partially but at the same time has a detrimental affect on the volume.
The circuit disobeys fundamental design rules as far as output/input impedance when connecting the Rhodes to a standard guitar amp, which causes reduction in frequency response  (ie. make it sound like there is a wet blanket over the speaker).

In short it actually causes the player to rely on the control on the amplifier to fix up the short comings of the signal coming out of the Stage Rhodes.

So given we have a long weekend here in Oz, I decided to investigate.

Initially I just set up a cable connected directly off the harp RCA through to the Phono jack on the name rail, bypassing all of the circuit.  Yes this did seem to improve the frequency response, but it also seemed to add a continuous static, not (white) noise but a static noise. Grabbed another cable and the results were the same. Encouraging, but not a solution.

Next idea was to change the value of the volume pot in the circuit.

One of the members here (Sean) had shown in another thread the following circuit for the Mk1 Stage:



The circuit in my 1973 Mk1 is different again.
I checked each of the pots and they both are 1973 production. 
Thanks to Max from CEPCo for the hand modelling services.


So I built up a wiring loom exactly the same as what appears to have been shipped with the EP back in 1973, except I changed the volume pot to 1M.  Given that many guitars utilise 1M volume pots, this should not have caused any issues.  (Typically Strats with single coil pickups have 250k pots, Les Pauls etc with humbuckers have 500k and sometimes 1M volume pots. Jaguars and Jazzmasters use 1M pots)

While I still felt the frequency response of the output was under par, there was a distinctly noticeable volume increase.

After some deliberation I decided to approach this issue as if I was designing a section of an amplifier.

The design would need to take into account that it needed to be compatible with amps that have high input impedance. (ie the classic valve amps for the 60s and 70s that we all love).  The correct choice of components, in particular the potentiometer for volume control, should allow the vast majority of signal to get to the amp and not be lost to ground.
 
The Bass boost function is a complete waste of time.  I have never known of anyone, be it in a studio or live, stop and say "my Rhodes stage EP does not have enough bass, damn! I need more bass boost".  Every player I know is always reducing the bass controls on the attached amplifier in the vain hope of dialling in some brightness and chime.

So I thought about how one would go about designing a bass roll-off control. It is better to try and get the signal coming out of the Rhodes as close as possible to what you want, rather than relying on the amp to control it.

I went for the classic High Pass Filter route.

For those who were asleep in classes, this is a high pass filter.  As the name suggests it lets through the high frequencies and blocks the lower frequencies.  There is a frequency called the cut off frequency, this is where the signal had dropped 3dB.  It doesn't mean everything below that frequency suddenly vanishes, but rather at that point that is the start of the roll off of all lower frequencies.



I wanted a design that allowed the cut off frequency to be adjustable.
If you wanted everything, that should be available.
If you wanted a little attenuation on the bass notes, likewise this should be able to be dialled up.
And lastly if you wanted the bass way down this should also be achievable.

Half a day of experiments and some calculations led me to the following design:

 , where the relationship between R1 : R2 is about 1:4.5 or 1:5. 
I had chosen the Volume pot to be 1M and the Bass roll off pot to be 250K, which meant that (using the standardised fixed resistor values) R1 could be either 47K or 56K.
The relationship between the combined R1+R2  (remembering that R2 is variable) and the capacitor then gives the range of the possible cut off frequencies.


I also added an extra function, using a CTS 1M pot that has an inbuilt switch. The switch is a SPST, and is independent of the functionality of the rest of the pot.  I.e. it does not turn the pot on or off, but rather the switch is activated by pulling the shaft/knob out and deactivated by pushing it in and allows 'something else" to be activated by this SPST switch.  I used it to have an additional capacitor added or removed from the circuit.  The circuit has two possible ranges of cut off frequencies (that overlap slightly).



The table below shows the cut off frequency relative to the percentage of Bass Roll-off control being selected. (and for each of the two possible - useable - values of R1 (47K or 56K)


Here is the layout drawing and some installation photos.
Note 1: there is just enough room between the harp and the name rail, at the far left, to be able to carefully locate the larger CTC 1M pot (with switch) without the need to remove the name rail
Note 2: you will need to cut off (with a hack saw) about 5mm of the shaft for the CTS 1M pot. Take about 10 minutes from go to whoa.  For what ever reason these CTS 1M pots with the switch feature have extended shafts.






So how does it sound... well great .. just what I hoped for.
The signal level is much louder, there is less (white) noise, no static noise and the bass cut off function is very easy to operate.
Fully counter clockwise the is no bass roll off (the starting frequency at those position is 26Hz, below the piano, below your ears).
Then just rotate the knob clockwise to gradually reduce the bass notes relative to the higher notes.
What I particularly liked is that I can set it up so I can clearly hear in the signal the physical striking of the tines on the upper register keys. This has often been overwhelmed by the lower notes.

The circuit is in the Rhodes and is staying put.
Enough said.

Time for dinner

David







8
The Fender Rhodes Electric Piano / Re: New Vintage Vibe Electric Pianos
« on: January 31, 2018, 04:57:37 AM »
Hi guys... another post to this older thread.

This delightful Vintage Vibe 64 EP in Lilac sparkle arrived on my doorstep here in Australia a couple of days ago.
My first impressions of the EP leave me speechless … I am over the moon. 
Earlier posts have said how great these VV EPs are , but let me just reiterate how awesome the key action and the sound is on the VV64.

I already have a 1973 Rhodes, Wurli 206A/200A, Pianet N & T and this VV64 has a voice of its own that fits in perfectly and fill a gap at the same time.
What is great is that the sound with the active electronics can achieve a multitude of styles and feels   …   totally different to my Mk1 Rhodes or my Wurli 206A/200A

The fit and finish of the instrument is first class, the quality of the parts used, the design, the fitment is very impressive.
Little things have blown me away… like the location of the on/off switch … yes you need an on/off switch, but it doesn’t need to be staring you in the face when you play. 
Again discussing the action of the keys … they make playing just so easy … to use an auto industry metaphor … while I love my 1973 Rhodes , comparing the action of the Rhodes to the VV64 is like comparing the steering of a 70s Chevrolet to a 2017 Chevy SS  -  chalk and cheese.
The Lilac sparkle top is perfect .. both in colour and in the finish surface.
I also like the fact that the active electronics allow a guitar to be plugged into the return and utilise the preamp FX.

In short …  this VV64 is a superb instrument.
Plus communication with Chris and the VV guys was easy.

David




9
Loved the sound since I first heard it in the mid 70s (and realized it was something different from an acoustic piano).
Finally bought myself one earlier this year.


10
- turns out the volume pedal is necessary for proper output!

Not exactly correct.

you can fit a shorted plug in lieu of the pedal and the Pianet N will operate with "proper output"

11
The Fender Rhodes Electric Piano / Re: Rhodes sustain rod nut
« on: November 08, 2017, 09:23:55 AM »
Metrification of the UK totally failed.
In Australia in the early 70s we changed over within 2 years.

I recall seeing a news article in the 90s where a British government "white paper" has come to the conclusion that metrification of the UK had failed because items were still being sold in "imperial" sizes, but just called how ever many metric units there were.
I.e. a pint of milk was still available , regardless of how many mili-litres it said on the outside of the bottle
 
When I was in the UK on business earlier this year, the speed limits were still in mph.

12
You like the NAKED version ....  try this for NAKED   ;D
(Only ever recorded in mono.)
David

http://www.offsetguitars.com/personal/sookwinder/various/The%20Beatles%20-%20Let%20It%20Be%20%5bBy%20the%20numbers%5d.mp3

13
aaaahhh yes ..  the two different but same versions of LET IT BE.
The original version as recorded in January 31 69, George OD in April 69, and released as a single march 70. Produced by George Martin

Then we have the LET IT BE album version, which is also from the original tapes, but edited to be longer and with ODs that were done without the Beatles  .. orchestra, backing "heavenly" sounding singers, echo in Ringos Hihat that goes out of sync after the first verse, some additional percussion by Ringo  ... and maybe as you say a Pianet N.  Spector also chooses to use a somewhat rougher George solo, than what George Martin used.

I never listen to the LP version of the song, only the single version  (or the "NAKED" CD version)

So yeah ... Fred, maybe you are correct that on the Spector version there is a Pianet N ... but I suspect at the very best it is a Billy Preston OD 8 or 10 months later, or at worst some unknown musician ..maybe even Spector himself.

David


14
more images of BP with the Beatles and the Rhodes




15
I'd put my money on the "N" being played in the "Let It Be" (F Em Dm C etc.) descending line.

The final takes of LET IT BE (the song) were recorded on 31/1/69, in the Apple basement studio.
All of the photos and footage (both officially released in the movie, as well as film outtakes) I have seen of the studio set up (22nd - 31st January 1969, excluding the 30th Jan which was the rooftop concert) there is no sign of the Pianet N.
Further, in the 60 odd hours of audio I have of the 22nd - 31st January recording sessions/rehearsals/jams, no Pianet N was evident to my ears.

If I was to speculate, Mal Evans & Neil Aspinal (the roadies/gofers) rocked up to EMI Studios on the 29th January and asked to borrow the Pianet N for the next day's rooftop concert, as a back up to the Rhodes. The Beatles never owned a Pianet N, the one used in the 65-67 recordings belonged to the EMI studio (Abbey Road).
Mal Evans & Neil Aspinal also had to purchase many pairs of women's stockings on the 29th January, for use as "wind socks" for the microphones used in the rooftop gig the following day.

David

16
My mistake....  it was late when I typed it up ... Pianet N on the roof (I will edit my original post)

17
Let me say from the start, I am a "Beatle freak", I have hundreds of hours of Let it Be / get back sessions from January 69.

I do not think the Pianet N was used on the song LET IT BE.
Macca played  the BLUTHNER piano
Billy Preston played Hammond and Rhodes

On other tracks (DON'T LET ME DOWN, ONE AFTER 909, GET BACK, DIG A PONY, I GOT A FEELING) Preston played a Rhodes.

The RHODES had been given / bought from Fender-Rhodes in 68 and when Macca heard it, it wasn't the sound he wanted... it appears he was actually wanting a wurli (sound).  But then it was used during the GET BACK sessions in 69.

During the ROOF TOP CONCERT 30/2/69, the pianet N can be seen up on the roof, but it was there just in case problems occurred with the Rhodes.  It was never turned on during the roof top concert. Billy Preston played the Rhodes during the entire rooftop session.

David







18
I have a Pianet T, N, a Wurli and  Rhodes....   in a very high overview perspective ... I would say the N is like a Wurli, the T is like a Rhodes.

but in reality they all are different

Zombies used an N.
Beatles used an N on most of the HELP album (especially THE NIGHT BEFORE and YOU LIKE ME TOO MUCH).  Also used on I AM THE WALRUS.

19
Other Keyboards & Software Synths / Re: Tube preamp for Clavinet D6?
« on: October 14, 2017, 02:04:26 AM »
A 12AX7 preamp valve is too hot...  you'll be playing nicely, then hit a note slightly harder and you get instantaneous  Hendrix sounds.
Try a 12AY7 instead.

20
anything is possible...

I've designed/built valve amps before so I know what an earth loop or major issue in the grounding sounds like  ... usually based upon 50Hz (or 60Hz depending where you are). What the grounding of the hammers did was more up in the high freq "noise"  and hiss area.

I am not an EE (my area is physics) but it is like when you have plugged a strat into a valve amp and for what ever reason (occasionally) you hear a high freq hiss/buzz  ...  but once you pick up the strat and place your finders on the strings it goes away. My gut feeling is the preamp design in the Pianet N works perfectly but it wasn't "boiler plated designed"  and there are times when it is plugged into wiring systems of modern houses and noise occurs.   

I know when I plug my Hammond in to one half of my house's wall socket system there is an awful buzz.  Plug it into the other half of the house's wiring circuit and it is quiet as a church mouse.

To me, and I felt this when I did electronics at uni,  ....  electronics is not a science ... it is "black art" and sometimes you just do things to solve a symptom rather than attempting to find/route cause the under lying problem.

David

21
That process of grounding the keys was designed to eliminate static discharges caused when using the silicone sticky pads sold by Clavinet.com.  The pads sold by Ken Rich are like the originals, and do not create a static discharge.  My Pianet N (with Ken Rich pads) plays very quietly without any added grounding of the keys or shielding.  So, I think your problem is elsewhere.

Alan

I also have Ken Rich pads and the grounding of the hammers also reduced the overall noise level.... in a similar way that sometimes when you touch an amp the noise level drops... either way a roll of the specific tape only cost about $15

22
Have you "earthed" all the hammers using the 3M conductive tape product that has the sticky glue also conductive (so when you stick it and the electrical continuity continues).
This dropped the noise considerably.

Also you need to put the lid back on before you check for noise, as the lid is part of the electrical shielding system

23
Other Keyboards & Software Synths / Re: Pianets - are they usually so noisy?
« on: September 17, 2017, 06:14:54 AM »
I have a Pianet N and have replaced components  (caps and CC res) on the preamp circuit. 
Also have used the copper tape to ground the hammer shafts. 
All this worked well to quieten the output considerably.

But... compared to modern mechanical EPs or even my restored Mk1 Rhodes, the Pianet N is a noisy beast

My Pianet T is super quiet ... but then it should be  ;D

24
YET AGAIN CONTINUED

So I had my OD circuit working … maybe the light at the end of the dark tunnel I had been in for some months, wasn’t a train coming in the other direction…

Next I went back to the tremolo circuit it and amazingly it worked.  Everything was coming together.  This trem circuit uses a red LED as the method of biasing the triode. (rather than the usual 3.3k res parallel with a 25uF capacitor).  Every time the trem circuit oscillates the LED will flash off and on, so you get a visual reference as well.  A number of amp manufacturers use this feature so you can see if the trem is on or not on the front of the amp control panel.

While I was experimenting with this circuit I did discover something about this trem circuit and the use of the red LED to bias the valve.  I noted that when the trem speed was raised, the pulses seem to blur. I do not mean that they were so fast I could not perceive them, but rather there seem to be a loss of the high point of each pulse (that was noted aurally not visually on a scope). Yet when I replaced the red LED with 3.3k//25uF set up, this blurring at high trem speeds seemed to no longer occur. In the end my ears preferred the set up with the cathode using the 3.3k//25uF rather than the red LED.

When the design and build is almost finished it is almost an anticlimax because you know that there is just a few more things to do (take photos of the completed circuit, clean up any lead dressing that is messy, finish of the artwork for the schematic and layout drawings, mount it in the cabinet).

But I did have a couple of ideas that I wanted to pursue.

The first being the on/off foot controls for the tremolo circuit.  Up until now I had been just lifting the cathode which immediately would stop the tremolo.  Or start it again, once the cathode was back in the circuit.  One of the problems with valve tremolo circuits is that, especially at low speeds, it takes a couple of seconds for the tremolo to kick in. Lifting/dropping the cathode was an instantaneous switching option, but it is dangerous for the valve and eventually it poisons the valve. (how long that takes, I have no idea).  But after talking with Merlin, he suggested an add on to my circuit that allowed me to safely keep using the lifting/lowering of the cathode as my tremolo switching position.

The second idea I wanted to look into was also to do with the tremolo circuit. 
This would be to add a switch that allowed me to lower the range of speeds that the tremolo functioned at.

I grabbed a toggle switch from my stash of trusty NOS “eastern block cold war” Russian military spec switches (available cheaply from a number of ebay stores) …  these switches are “real men” switches… when they switch, they feel and sound like a switch should… definite, solid, dependable…. not like much of the crap one can buy these days.  Also grabbed a 47nF capacitor to be added in parallel with the other caps of the tremolo circuit.   

If I want to, I can now have a tremolo speed of less than 1Hz.

The last thing to do was change the valves from my “working B team” valves (anything I had lying around), to the “A team”.
1962 GE 12AY7
1958 RCA 12AX7
1955 RCA 12AU7

By this stage I had replaced the final valve (a 12DW7) with a 12AU7, as I only required one of the 12AU7 triodes for the final cathode follower.

The circuit now was back to three valves:
Input
12AY7 triode 1  Gain stage 1 – valve 1
FMV Tone stack
12AY7 triode 2  Gain stage 2 – valve 1
12AX7 triode 1  Gain stage 3 with variable OD – valve 2
12AX7 triode 2  Tremolo circuit – valve 2
12AU7 triode 1 Empty/unused triode valve 3
12AU7 triode 2  Cathode Follower (for line out) – valve 3

While all of the above had been occurs over the past 6 months I had ordered from MOJOTONE a cabinet for this preamp unit that matched the colouring of my Rhodes.  It arrived some months ago, but I was still awaiting the faceplates that I had ordered from my supplier in Thailand.   Jay had helped me with the artwork for the faceplates and I assumed all would be ok.  Never assume anything I keep telling myself but always forgetting.

When I went to fit the faceplates to the chassis, it dawned on me that they had been manufactured incorrectly.  There should not be an outer white edge/band.  The outer edge should be black. The faceplates did not fit. 


I contacted my supplier in Thailand and managed to convince him that he needed to have the plates manufactured again.  The next day I was flying to Germany for a 10 day business trip and hopefully by the time I got back the correctly manufactured faceplates would be waiting for me. And they were.

So before we get to the final photos some points of note:
- Even though it was a nightmare during some of the development stages, in the end the design I came up with achieves exactly what I wanted. A preamp unit  that allows me to plug my Rhodes directly into a modern desk or amp or DAW.
- The unit sounds like a valve guitar amp, again what I wanted. It sounds like I have plugged it into a SF Twin Reverb or a Princeton reverb.
I was not attempting to mimic the sound of the solid state preamps used in the suitcase versions of the Rhodes EP.  There are organisations out there who have done that with lesser or greater levels of success and if you want that sound then they are available.
- I can plug in an electric guitar and achieve a “valve” amp sound without blood spurting from my ears. From chimey clean to Hendrix like OD.
- I can also plug in my Wurlitzer EP or modern day Roland EP and “warm up” their sounds as well.
- My Pianet T also sounds awesome when run through this unit.
- While the “Bright” control adds sparkle to a guitar plugged in, it doesn’t seem to add much to the Rhodes sound, which seems to gel with the notion that the frequency range of the Rhodes is quite a bit lower than for the harmonics in an electric guitar.

Thanks to James and Jay (from www.offsetguitars.com) for the invaluable help with this project.

David








25
CONTINUED FROM PREVIOUS POST

But this is also where I f**ked up big time…. and basically wasted almost 2 months of building, redesigning, starting over yet again … this is the dark period when building a new design…  no one else can help you, they can give advice, they can suggest solutions, but in the end it comes down to checking and rechecking everything from the schematic, to the layout drawing (did I create my layout drawing exactly how the schematic had the circuit?), to the actual amp and even as I discovered , even the way the valves I was using were manufactured.

I have told myself over the years, after barely passing university electronics, never assume anything when dealing with electronics and circuits. But I never seem to remember my own advice.

The 12DW7 has two different triodes within the glass envelope, unlike a 12AX7 or 12AT7 or 12AU7 9 pin valves, were both triodes are the same.  If one wires up backwards any of these type valves it doesn’t really matter, each triode is the same. But with a 12DW7, each triode is doing a completely different function in the circuit so you must wire the 12DW7 correctly.

From the spec sheet for a 12DW7 I know that the first triode is the 12AX7 type triode and the second triode is the 12AU7 type triode. Pretty simple, not hard to get the wiring correct I hear you all chuckle. 

The standard 9 pin miniature type valve uses pins 1,2,3 for one triode and pins 6,7,8 for the other triode.  Now any sane person, any logical person, anyone when asked would say Pins 1,2,3 would be the “first” (#1) triode and pins 6,7,8 would be for the second (#2) triode,  And that is how I wired up the 12DW7 valve into my circuit.

But I was wrong that is not how the two triodes are specified . Pins 1,2,3 are for the #2 triode and pins 6,7,8 are for the #1 triode. I would love to know which idiot back in the 1940s or 1950s thought it would be a good idea to designate it this way... totally illogical.




When I finally decided to plug the amp in and turn it one, while I was able to get a signal through, which sort of sounded OK, there was no tremolo at all and the OD circuit would have been perfect for Ozzi Osborne, but not for a Rhodes player. Plus even without the OD circuit engaged, if I turned the volume past about 6 it was just “crunch city”

mmmmm… damn!

So I had 3 issues:
- Signal is OK, up till 6, but then not usable past that point
- Specific OD circuit appears to have very little variation no matter where I set the controls
- who stole the tremolo?

At least the pilot lamp was turning on and the PT/rectifier/heater circuits were performing as they should and the measured DC voltages all roughly lined up where I thought they should be.

Mind you I did have another stuff up courtesy of the turret board used for the filtering circuit in the dog-house.  If you look at the photo above where I high lighted a section of the circuit in yellow, notice that unused hole… it is joined under the black surface coating with the turret directly above it. Four months later I can now see that quite clearly, back then I did not notice it in the photo or even when looking at the turret board itself.

When I mounted the power supply filter circuit into the dog house I decided to use that spare hole as a mounting point, rather than drilling a new hole.  Seems logical. Well it is until one turns on the power, … I had created a short circuit to ground and suddenly the PT is getting  very very hot. I eventually tracked down this stuff up and it started to confirm my hatred for turret boards.  Give me eyelet boards (just as Leo used) any day of the week.

For five or six weeks I was in almost in continual email conversation with my friend James about possible reason for the design not working as I thought it should. My lunchtimes at work were almost solely dedicated to discussing the same issues with my manager who is an electronics engineer.

Even though I use my third bedroom as my workshop, my lounge room had this on the coffee table, scope, sig-gen, alligator clip leads, probes and electronic components for many weeks:


By this stage the restored Rhodes that I had commissioned back in December 16 arrived (albeit 2 months after the promised arrival date).  The pressure was on to get this circuit correct and finish the project.

After yet another evening that ended up with …


... I came to the realisation that I needed to start again, dump the turret board (you can barely fit 2 or 3 wires into the holes, you are limited to the layout on a grid   aaaahhhhhhhhhhhh) and design the circuit again using the old faithful eyelet board process.





I had decided to use the variable cathode biasing type tremolo circuit that is used on the Fender Vibro Champ … I had used this circuit before on eight previous builds (of various designs) … it is a simple circuit, working on the cathode of the second or third gain stage and produces a lovely sounding tremolo.

At this stage I still had not realised that I had the 12DW7 wired up incorrectly, but the 12AX7 triode was still operating sort of OK as a Cathode Follower, rather than the correct 12AU7 triode in the 12DW7.

For the VC tremolo I needed to use a complete 12AX7, so an additional hole in the chassis needed to be drilled.  I had also surmised that no matter how I set up the first valve (a 12AX7), I could not get enough clean headroom even without engaging the OD part of the circuit.  It was just too hot having 3 gain stages. The unit lost any semblance of clean output once the volume went past 4.

So I decided to change the first valve from a 12AX7 to a 12AY7.  This is the same type of preamp valve used in the Tweed Bassmen and has about half the gain of a 12AX7. It also has a smoother gradient of OD as the signal increases than the 12AX7…  all good points.

The layout was

Input
12AY7 triode 1  Gain stage 1 – valve 1
FMV Tone stack
12AY7 triode 2  Gain stage 2 – valve 1
12AX7 triode 1  Gain stage 3 – valve 2
12AX7 triode 2  OD  stage2 – valve 2
12AX7 triodes 1 & 2  Tremolo – valve 3
12DW7 triode 1 Empty unused triode valve 4
12DW7 triode 2  Cathode Follower (for line out) – valve 4


What’s the old saying … pride come before a fall …  two more problems arose … and it is times like these that just stopping and doing nothing for a couple of days is good advice.  When dealing with electricity (house voltages) never rush things, never just do an extra how before midnight of work …  always stop before you get too tired physically and mentally.

The first issue was something so stupid I am embarrassed to show it … but if it helps others then that is good.

If you look at the photo below of the underside of the populated eyelet board showing the connecting wires that are underneath, I still have not trimmed the legs of the components.  Problem is that I forgot to trim them before I wired the board into the chassis. And notice how some of the extraneous wires/legs are touching other components. Oh s**t !

So I wired everything into the chassis, power it up and nothing seems to be as it should be (I wonder why?) 

I grabbed the DVM and started to measure voltages throughout the circuit and at one stage while measuring the voltage on one of the pins of the 12AX7 used for the tremolo part of the circuit I slipped and shorted something out.

Pooooooooffff .. bang  …magic smoke escaping from somewhere …

To cut a longish story short, I stuffed up the power transformer.  Never done that before.  It only puts out about 2/3 of the DC voltage it should now.  Normally someone in this situation would have to go and buy another PT, but because I purchase my PTs from the US and a significant portion of the cost is shipping them to Australia, I always purchase a second (spare) PT.  Never had to use the spare PT before…


Another day/evening lost

The eyelet board was removed (yet again), wires trimmed and along with the spare PT everything was re-wired back into the chassis.  Another replacement PT was  ordered from the USA.

Powered up the unit again.

The change to the 12AY7 worked a treat … now OD cause by running the volume control at higher levels only kicked in at about 7 to 7.5. But the tremolo circuit, the VC style tremolo circuit that I had just changed to would not work. Swapped valves in the trem circuit, measured voltages, swore a lot, but even though I had successfully used the VC style tremolo many times before and it is a circuit I know inside out (I even understand how it actually works), I could not get the damn thing to oscillate. No matter how many times I measured voltages, compared the schematic to the layout to the board itself .. nothing, zip

So I decided to watch an episode of the latest series of THE BLACK LIST and came back to the project a week or so later.  I had a 6 day business trip to the Manchester/Liverpool UK, the same week that the bombing occurred.  Things like that make you realise what is important in life and not to get too worried about a preamp that doesn’t want to work.

Upon returning to Oz, a week or so later, I still did not have any idea why the tremolo circuit would not work, but my time away from the project was useful.  It still amazes me how one’s brain can be working on a problem even though you have set it aside for the moment.

I had finally realised that I had been wiring up the 12DW7 incorrectly.
Amazing what information one can learn from the 1960s valve spec sheets when one actually ready everything rather than just skimming through it.

This explained why the first version of the tremolo circuit did not oscillate.  The 12AU7 section of the 12DW7 did not give out enough voltage swing to even come close to oscillation.  If I had have used the 12AX7 half of the 12DW7 it would have worked.  So for the moment I was going to leave the tremolo circuit alone… the VC style trem circuit had beaten me and eventually I would go back to the original circuit I have wired in, all those months ago.  You live and learn.

I also had some revelations concerning the DC coupled OD circuit.
After spending some time looking at the scans from the scope of the OD created by the DC coupled gain stage and the cathode follower pair, it was apparent that if I was into northern European style death metal then this type of OD would be perfect. 

But no matter how I adjusted the set up of the grid stopper resistor voltage located between the two triodes that made up the OD circuit I could not achieve anything near a smooth or creamy OD sound. Effectively it went from clean to flat top OD and nothing in between. All that it would produce was the sound of a Kenworth truck crashing into a steel girded bridge.


I needed to final another circuit that would allow me to adjust the levels of OD.

Just like ground hog day, out came the circuit board yet again…


And a modified version was put back in.  This time with the original trem circuit design, that only required one half of a 12AX7 and a new variable OD circuit.


This new variable OD circuit uses the method of varying the cathode resistor on the cathode of the third gain stage to achieve different levels of and different types of OD.

In general the size of the cathode resistor for a single 12AX7 triode is 1.5k.  If this cathode resistor is can be varied from the normal 1.5k down to say 680 ohms, the value will run “warm” and then to “hot’ and produce grid current clipping.  This is what that “creamy” OD is. Take it too far and it gets ugly, but control it at the right level relative to the input signal and you get a dynamic creamy three dimensional OD that every guitarist loves and it will make the Rhodes sound awesome… even adds a bit of a Wurlitzer style grit to the sound.

On the other hand if you run the triode in a cold state one can achieve cut off clipping style OD.  In this situation rather than the cathode resistor being 1.5k, let’s make it 5k, or anything in between. This produces cool or cold biasing of the valve and the OD is more two dimensional, less dynamic, but a style of OD  that I prefer with the Rhodes when you are just wanting the sound to be thicker (slightly more creamier or “hairy”).

There is also another dimension to this OD set up, that being the level of signal input into the valve, obviously controlled by the volume control at the start of the circuit.  Have the volume really high, then the 3rd gain stage OD circuit described above just adds to the existing OD. Have the volume in the middle, then the 3rd gain stage OD circuit can add a delicate level of thickness to the sound or a lot more or even none, depending upon where you set the OD control.  (that being a 5k linear pot).

This is the schematic for the circuit.  Very simple and, again, the idea was from gleaned from various sections of Merlin "The Valve Wizard" Blencowe’s book “Designing Tube Preamps for Guitar and Bass, 2nd edition”



26
OK… as some of you may know, a couple of months back I bought a restored Fender Rhodes Mk1 electric piano.  It was the “Stage” version, meaning effectively it is a giant passive electric guitar with 73 single coil pickups, unlike the “suitcase” version that has its own preamp, power amp and speaker cabinet.

 
We all know that like an electric guitar the stage version of the Fender Rhodes needs to be plugged into a guitar amplifier that has a preamp that can raise the signal level, in a similar way they do for electric guitars. Modern keyboard amplifiers assume the signal from the keyboard is already at LINE OUT level and hence are basically no good for use with the “Stage” version, other than with the addition of a preamp or booster pedal unit.

This thread is the adventure that took about 7 - 8 months to complete, designing/building a preamp unit for the Fender Rhodes Stage EP.

Those who have read any of my build thread over at www.offsetguitars.com know that I write like I talk... a lot.... so grab yourself a cuppa tea, a coffee, a beer, maybe even a couple of sandwiches … whatever  … and delve into preamp design work and all the highs and lows that went with it.  And for those who are not trainspotters for amp designs, just cut to the chase and page down to see the photos.

Last December when I first ordered the EP, I knew that I needed some sort of preamp unit for the EP as I was not going to mic it up when I used it for recording. I had already built an amp that I have been using when I wanted to record my Pianet T, but it also had issues that many others had found in as much as overdrive, too much overdrive (distortion) can occur…. In short the attributes that a valve amp is well known for when used with a guitar, often are not those desired when using an EP. The designs that utilise 12AX7 valves in many preamps for electric guitar amps are just too hot.

So initially I discovered that a company in German,  Reußenzehn, has a “preamp box” available for use with a Stage EP.  The box had tremolo and reverb functions and also depending upon volume controls, different levels of overdrive could be achieved. The blurb on the website said it was designed to simulate the Suitcase model preamp.  It can do mono or stereo tremolo output.

As my intention was to use this EP when recording in my home studio, mic'ing up a full size valve amp is somewhat prohibitive, so this Reußenzehn unit seemed like the perfect solution. 

The unit utilises a 12AX7 and a 12AT7. I fitted NOS 1960s valves and I was happy with the sound. But what immediately threw me off and concerned me was that there is a "clicking" sound functioning when the tremolo section of the box is engaged. The click is quite prominent, especially with held notes/chords.  The clicking is coherent with the cycling of the tremolo.

Here is a trace of the resultant tremolo using the Reußenzehn unit, when compared to a tremolo that uses cathode bias varying design.


Large Fender amps that have tremolo circuits that use an a light dependant resistor (LDR) in the opto-isolator (AKA as  the 'roach' ) have a "click" when the tremolo is used.  But the “click” was either very quiet or fixed by an “infield fix” back in the day.

When I contacted the manufacturer of the Reußenzehn unit I received the following reply. (these are the salient points from the reply with all the hype removed):
Quote
So the click of the Vibrato / tremolo is part of the sound. Like in most of all original Tube Tremolo the tube make an ignition and alternating two high voltage gas lamps, the clipping of the electrons does that floating alternating sound. You can hear that in each original Fender Twin Reverb Amp when the vibrato is on.

 ... And the click is very lower while playing, because the system works while playing with lower Z.

So in short it seems that the Ressenzehn's "Suitcase Preamp" is a preamp that attempts to reproduces the sound of a Fender Twin Reverb tremolo and not that of an actual suitcase preamp.  In fact it attempts to reproduces the sound on the SF Twin including the annoying click.

Now let me say that in a live situation the Ressenzehn's  "Suitcase Preamp" would work perfectly OK ...It would sound great ...  no one would even notice the clicking, not the audience, not the players.   But I do not gig ... all I do is record in  my home studio and I notice the clicking immediately.  So the Ressenzehn's  "Suitcase Preamp" now sits on my shelf collecting dust never to be used by me again… anyone want to make me a an offer for it ?

So by late January 2017 I decided to design/build myself my own all valve preamp unit for the Rhodes. It was not going to be a copy of the Peterson, Janus or Jordan preamp designs used in the suitcase versions, but rather a preamp based upon the various Fender valve amps that have been used successfully by players over the years.

It would have input and master volume controls, Treble/Middle/Bass controls, Tremolo  (only mono – I dislike stereo tremolo with a passion) and also a selectable overdrive function.

When I have asked various Rhodes players which amp they use (or used) with their Stage Rhodes (as well as reading on various newsgroups) the type of amp mentioned was usually a Fender Silver Faced Twin Reverb, a Fender Tweed Bassman or a Fender Princeton Reverb for small gigs.  Now anyone who knows Fender amps and their design attributes would immediately observe that these three amps are vastly different from each other, so different that I have never seen these three amps grouped together in any discussion about amps, other than the fact that Leo designed them.

If you think about it for a while … the (only) common factor grouping these amps together is that depending upon how they are used, each amp has a reasonable amount of clean headroom before any level of overdrive kicks in.

The Princeton Reverb is a very clean amp with about 12 - 15 watts and only starts to break up with the volume at around 8.5 - 10.  Perfect for a small gig space. 

The SF Twin Reverb certainly can achieve overdrive distortion in levels that will make your ears bleed, but given that it is such a powerful amp, the levels needed for the Rhodes mean that the valves are a long way from really producing a gritty sound when a Rhodes is used with it.

Lastly the Tweed Bassman, an amp associated with a bunch of blues guitar players and also Neil Young’s amp of choice.  An amp known for its overdrive and distortion.  But also like the Twin Reverb, the Tweed Bassman is extremely loud, so the levels needed for a Rhodes are achievable without sounding like Neil Young.

The Princeton Reverb uses a 12AX7 input preamp valve running at about 120V, the SF Twin reverb also a 12AX7 at about 210V and the Tweed Bassman runs a different preamp valve, a 12AY7, at about 150V. All very different input conditions.

So whatever design I come up with, one of the attributes must allow for a decent amount of clean head room.

I also like, in the right situation, a decent amount of warm overdrive with my Rhodes sound … not death metal OD, but nice warm OD. So a decision of where in the circuit the OD control should be placed as well as the method of producing the OD itself needed to be developed.

Lastly the circuit needed to be able to convert the signal (after yet to be defined gain stages) to a voltage and impedance level that will be compatible with LINE OUT specs.

I didn’t think this was that difficult to come up with, but I did know that every new design always has the ability to kick you in the arse when you are in the middle of designing/building it.  [this project would be no different as it turned out].

Circuit Topography
The 1960s “Black Faced” Fender amps and the 1970s “Silver Faced” Fender amps have effectively the same preamp/input topography.  This is:
- input into
- First triode of the first preamp valve (12AX7)
- Fender BF tone stack, known these days at the FMV tone stack
- Second triode of the first preamp valve. (12AX7)

In the larger amps such as the twin reverb a third gain stage occurs  (First half of another 12AX7).

After discussing this project with a friend who also designs valve amps, I decided to try the circuit for creating OD outlined in chapter 6:15 of Merlin "The Valve Wizard" Blencowe’s book “Designing Tube Preamps for Guitar and Bass, 2nd edition”.   This involves a gain stage followed by a DC coupled Cathode Follower, where the size of the preceding (to the CF) grid stopper resistor can achieve different levels of OD. If this grid stopper was a variable resistor the amp would have a OD control.  Mmmm now we are getting somewhere!  Mind you even now as I write this up, I still do not fully understand DC coupling in valve amps … maybe I never will.

I knew that by the time the signal gets past the 3rd gain stage the voltage levels can be in the ballpark range of 150 – 250V. So the Master Volume also needed to be able to dramatically reduce the final output voltage to a level that can be used as LINEOUT.  And finally a circuit was required to enable the output impedance of this PREAMP unit to match those required when plugging direction into a desk, a modern keyboard amp or a Digital Audio Workstation. Some reading of my amps books & on line searches lead me to discover that a “Cathode Follower” is the best way to achieve impedance matching and no loss of the high frequencies for the final stage of this unit.

This is how the topography would look like [Note at this stage of the design I was not even considering the tremolo part of the circuit, just amplifying, having an OD circuit, and a master volume with impedance matching was enough to consider initially]:



Even before I built up the unit or finalised the design, I checked some of my other amps and determined (for a 500mV input) what the rough voltage levels after each of the gain stages, to give me a ball park idea of what I was dealing with.

The final cathode follower, initial thoughts were to use a 12AX7, but ultimately I went with a 12AU7 valve, has effectively unity gain and is purely in the circuit to allow matching of impedances.   I was thinking that the Master Volume would be some sort of voltage divider where I would wash away 90% of the signal, leaving 10 - 14V at the output. The input gain stages and tone stage was already defined via the Fender BF/SF design. All I had to think about (at this stage) was the OD part of the circuit and how could I vary a grid stopper resistor from 5k up to even values of 1M.

Given the voltage levels at this point in the circuit (hundreds of volts), a normal 1/4watt potentiometer would not be sufficient, so after much searching I came up with the idea of using a ceramic 1 pole/6 position rotary switch, which I sourced quite cheaply via ALIEXPRESS from China.  It was perfect, robust, could handle 4 watts and was small enough to fit into the amp chassis I was intending to use.



I have learnt over the years to never rush any amp (electrical) design/build, as mistakes can be made and electrical shocks can occur.  I have been shocked by 240V wall voltage 3 times, now … but none since I realised it is better to take one’s time and live.

A rough outline of the schematic was starting to take shape, with the preamp sections on the left essentially “borrowing” the standard large Fender BF or SF amp, including the addition of a “Bright Switch” and Mids control.


At this point I am compressing 5 or 6 weeks’ worth of emails and discussions with friends and colleagues at work, where the ideas for the master volume and the final cathode follower developed.  I was now at a stage where I could breadboard this design, and for the first time (and I must say THE VERY LAST TIME) I decided to use pre-made turret boards for the first build.  Did I mention I now hate turret boards?

One of the early incarnations. I used a Vibro Champ sized chassis and had planned to use the same type of power transformer that I I had used on the “Reverb Unit” that I had built 18 months back, a Mercury Magnetics PT that developed for new or replacements into Fender style stand-alone Reverb Units.


The filtering circuit in the “dog house” looks like this.  Given the price of electrolytic capacitors is relatively cheap compared to 40 or 50 years ago, my design used enough caps to ensure a quiet power supply. Also given that I was using a solid state rectifier and not a valve rectifier, there was no limitations on the amount of capacitance that the rectifier can “see”.  The section of the photo I have highlighted in yellow I will discuss later.


By this stage I had (stupidly) decided that I knew what I was doing and this “bread boarding” exercise could be also the final version, so I had added in the tremolo section of the circuit.  The tremolo circuit was based on a simple design my fellow amp building friend had used in his amps over the past couple of years: The layout was

Input
12AX7 triode 1  Gain stage 1 – valve 1
FMV Tone stack
12AX7 triode 2  Gain stage 2 – valve 1
12AX7 triode 1  Gain stage 3 – valve 2
12AX7 triode 2  OD  stage2 – valve 2
12DW7 triode 1  Tremolo – valve 3
12DW7 triode 2  Cathode Follower (for line out) – valve 3
The 12DW7 is a hybrid 9 pin valve that combines one 12AX7 triode with a 12AU7 triode.  I could use the 12AX7 half of the 12DW7 for the tremolo circuit and the second half of the 12DW7 (a 12AU7 triode) for the cathode follower.  Perfect.  When one has limited real-estate on amp design, these hybrid valves come in really handy.

27
Forgive me for maybe not understanding, but what is your final desired outcome?

- A Pianet T that has an active preamp  (like the Pianet N)
or
- A Pianet T that sounds like a Pianet N ?

Given that the sound of the Pianet N is more than just the tynes/reeds and the pads, you have the active preamp there as well.

David

28
On top of all the above there is also the phenomenon  where by someone attempts to sell an item at a price based upon what they purchased the item for.  Sometimes the asking price is so far away from reality one wonders whether the sellers have taken their medicine that day.

If the 90s there was a s**t load of vintage instruments (mainly guitars) sold to Japanese buyers, when Japans economy was booming.  Now 20 years later the Japanese economy had been in the doldrums all that time and these sellers are attempting to sell their instruments for 2x, 3x, even 5x what the current market place values them at.

Another thing to consider is that by definition there is a known/limited number of instruments available (that number slowly reducing as time goes by), so economic and availability causes surges in sales.

3 years ago when the Aussie $ was really high against the US $ I knew it was a short window for me to get some of the vintage guitars (electric and acoustic) that I had been  dreaming of.  Along with the depression in the US market place meant that many cost me only 40 - 50% of what they would now cost me ...  if they were even available.

In short, like everything in life, one needs to do one's home work.

29
Parts, Service, Maintenance & Repairs / Re: Grounding Pianet-T keys
« on: May 01, 2017, 02:11:09 AM »
I have both a Pianet N and a T model.
Yes I grounded my Pianet N ... and it is very quiet now after being rather noisy

I have done nothing to my Pianet T as it has always been quiet.

30
Last December I decided I wanted a Rhodes, so I contacted Chicago Electric Piano Co. ... who had done my "mellow yellow" custom paint job and refurbishment on a Wurlitzer EP ...  and requested a quote and some customisation.

Not withstanding that it was completed in the middle of February, but I only just received a couple of days back, here is a fantastic 1973 Fender Rhodes "Stage 73" electric piano. Sounding as great as the refurbishment and restoration looks.

I now have two issues... where to store the lid !   :D
How do I re-arrange the 7 keyboards I have so I do not look like some weird arsed old woman who has 47 cats and piles of newspapers going back 30 years.  :-[

Currently prototyping a preamp for it


Here is a clip that REVERB did of my Rhodes before it was shipped.

David
https://www.youtube.com/watch?v=qNX_34JQ1nU


Pages: [1] 2