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Topics - sookwinder

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1
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







2
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.

3
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


4
Hello everyone.... I am in the middle of designing/building a preamp for a Rhodes (which if I think about it could also be used for any other passive keyboard or even an active keyboard and most certainly for a guitar)
The preamp is based upon a twin reverb preamp set up (using the two halves of a 12AX7), with also a look at the  Princeton reverb preamp set up.
Mids will have a greater variation than the standard Fender tone stack, essentially emulating the Marshall set up.  This maybe more useful for guitar. but given it was just a change of pot size, it was a no brainer.

The second stage gain will use another 12AX7, the first half being for the second gain stage and the second half of the valve for a variable overdrive control, using a DC coupled cathode follower.  This overdrive stage set up should (I repeat ... should) allow for a soft increase in overdrive depending upon where it is set.  I am using a 6 position rotary switch which will allow for various "maximum" levels of OD, refined again by the input volume and tone controls.

Lastly there will be a master volume control to a "line out" output (another cathode follower)

A trem control is also in the design  (mono)

So my question is : As I intend to have a faceplate manufactured for this unit, I need to decide upon the two descriptors I will use on the face plate for the Overdrive control.  My current idea is:

SMOOTH - GRITTY

Obviously the word OVERDRIVE will be above the control.
Does anyone have any alternate ideas to describe the level of overdrive?
Note: I am not having numbers on the controls

thanks
David

5
I have most of my keyboards set up reading to record with. The Pianets have their own covers that kept them safe from dust and the occasional spilt drink.  But the others do not. A year or so back I fell over this website  www.ampcovers.org, have no idea how I got there but it looked interesting.

The guy who runs it [Jordan] sells many things and amongst them are covers for keyboards. 
He had listed covers available for the Roland EP I own, but not the Hammond XK2.
But as luck would have it, he also can manufacture covers for anything, given a drawing with dimensions.

So I sent him the details of the XK2 and ordered a cover for it and the Roland EP. A month or so later they found their way from Hungary to Australia and they fit and function superbly.  They have the typical strong "hiking/sports bag" exterior nylon material and underneath is thin but well padded softer material. Below is a pic of the first two covers.  The colour of "black" is a lot darker in real life... black is hard to photograph.


After getting my 200A  (conversion)  from CEPCo last year I finally got around to taking some pics and measuring the dimensions.
I sent this data to Jordan who produced a magnificent cover for my 200A.  I discovered that I didn't have to have "CUSTOM" on the cover so for the 200A there is no badge.  I think he can also do badges of any names for the covers as well

The cost was US$45 for the cover and US$25 for the shipping (Hungary->Oz)

I can highly recommend this product.  When I get my Rhodes I will order one for that EP as well
Obviously no affiliation with the seller other than being a customer

David




6
Can anyone give me a "ball park" idea of the output voltages (pk-pk) of a stage Rhodes Mark1  for say two handed "square fingered" playing ?  i.e 6 or 8 keys played heavily.... i.e. what is the maximum output one would expect from a Mark1 stage Rhodes ?

I have read/ believe the output of each key pickup has a lower output than say a typical guitar single coil pickup (strat or tele), but have no real notion of the actual maximum output level.

Can anyone help with this question?

I am currently designing my preamp and would like an idea of the maximum output to use to determine the voltage divider part of the circuit.

7
G'day there,
I recently bought a preamp unit/pedal/box (what ever one calls it) from Reußenzehn in Germany. 
It is for Stage Rhodes (and obviously can be used with other passive vintage keyboards) and is meant to duplicate the sound, the OD, the tremolo of the Suitcase preamp.  It can do mono or stereo tremolo output.

The version I bought also has reverb.  My intention was to use this when recording in my home studio, when mic'ing up a full size valve amp is somewhat prohibitive.

http://www.reussenzehn.de/de/musiker/orgel-und-keyboard-preamps/suitcase-piano-preamp-reverb.php

The unit utilises a 12AX7 and a 12AT7. I fitted NOS 1960s valves and I am very happy with the sound. The overdrive function , having a volume input as well as a volume output (and bass/mids/treble) controls, can be varied to achieve just the right sound.

What I am a little concerned about (although there is nothing I can do) is 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.

So my question for those with Suitcase models (which I have never had a chance to play) is:
 - does the tremolo on a suitcase model also "click" ?

I am wondering whether Reußenzehn in their desire to simulate exactly the sound of the 70s Suitcase preamp, have designed in the "click".  Alternately there could be something wrong with the unit I have purchased, but given that everything else works perfectly this is a low chance.  The other possibility is that it is just a bad design trem circuit.

Large Fender amps that have tremolo circuits that use an a light dependant resistor (LDR) in the opto-isolator (AKA the  the 'roach' ) have a "click" when the tremolo is used. 
But the type of tremolo I prefer and have used in the amps I have design/built myself use the extremely simple tremolo design from the Fender VibroChamp. There is no "roach" in that design, so there is no clicking.

So before I go back to the manufacturer, I would like to have an understanding as to whether the original Suitcase models had "clicking" when the tremolo was used.



8
Hello every one,  just received the Wurlitzer 206A conversion to a 200A below, from Chicago Electric Piano Co.

The following link is to a thread I have written up for a guitar website I help moderate.
http://www.offsetguitars.com/forums/viewtopic.php?f=32&t=98874

I gotta say a big "Thank you" to Max at CEPCo.  Much appreciate his expertise and his willingness to ship EPs such as this outside of the USA. Many others wouldn't.

Now back to the wurly ..  :):D ;D




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