Author Topic: help with faceplate design for an overdrive unit ... throw around some ideas  (Read 794 times)

Offline sookwinder

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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
« Last Edit: March 30, 2017, 02:09:53 AM by sookwinder »
Late 60s Pianet N - Late 70s Pianet T - Ensoniq ESQ1 - Hammond XK2 - Wurli 206A converted to a 200A - 1973 Rhodes Stage 73 - Roland RD150 - Vintage Vibe 64 EP - s**t load of guitars, basses & amps

Offline pnoboy

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Based on my own experience, I would suggest that, unlike the standard Fender tonestack, you should allow the mid control to create a flat frequency response--the Fender unit always has a mid dip--turning up the mid control only reduces it, and doesn't eliminate it.  I have found, by testing with various tone controls and a 9-band graphics equalizer, that the 500 Hz dip provided by the Fender tonestack can be far from optimal.  It really depends on the speakers and type of speaker cabinet you use.  Many times, a dip in the 100 Hz to 200 Hz range is what the doctor ordered.  Other times, no dip sounds right.  In my piano, which I have connected to a single Eminence Ragin' Cajun 10" speaker in a 1 cubic foot sealed enclosure, a 500 Hz dip is all wrong--it produces a sound with no body.

Also, I have never found any overdrive that sounds good with a Rhodes.  If you succeed in that part of your endeavor I would very much like to hear the sounds you get, and if you're willing to share, would like to see the schematic you develop.


Offline voltergeist

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Also, I have never found any overdrive that sounds good with a Rhodes.  If you succeed in that part of your endeavor I would very much like to hear the sounds you get, and if you're willing to share, would like to see the schematic you develop.

I haven't tried this myself, but I'd recommend trying Moog's recent drive pedal(s?).  The overdrive on the Little Phatty is fantastic, and I'm generally not a fan of synth overdrives.  *If* Moog's OD pedals are as good as the Phatty OD, and *if* it works as well on Rhodes as it does on synth (which I would expect it to), it should be a good OD for Rhodes.

Lots of caveats there, but Moog is where I'd go looking for a Rhodes OD.
Restored or Overhauled: '65 A-model Sparkletop, '78 Suitcase 73, early-'75 Satellite 88, '81 MkII Stage 73, two '77 Mk1 Stage 73's, '74 Mk1 Stage 73
In Progress: 1 '78 Suitcase (2nd one), '70 KMC - Customized w/ Peterson 4x12, '77 Wurli 270

Offline sookwinder

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The tone stack is the standard Fender stack, but rather than have the mid control as the fixed 6.8k dropping resistor to earth as seen in the Princeton Reverb or the 10k variable pot as used in the Twin Reverb, I have used a 25k variable pot.

In previous amp design I have built (designs that did not have a variable mids control)  I have used the standard Fender 6.8k mids resistor but with a switch that can add in another 15k.  This brought back (if required) all the mids ... effectively making it a flat response across the spectrum.  Obviously the efficiency of the speaker set up (relative to frequency) needs to be considered, but I know from actually building guitar amps with the ability to add back in the mids via making the mids resistor much larger that it works.

Below are graphs of the two extremes that the tonestack as I am using can achieve when adjusting the mids control independently of the bass/treble controls.

Also I am more than happy to share schematic designs ...
I am a physicist (who works as an engineer) and electronics was my crappiest subject at university.. almost failed ...
But it does seem I am au-fait with value electronics design more than I am with solid state electronics.
Over the past 15 or 20 years I have learnt so much about amp design from many many people (online and in books). 
There really isn't that much new "under the sun" as far as how amps work  .. what is "new" is how people implement designs and design amps for specific instruments/purposes. I am happy the share.

I have already drawn up schematics and layout drawings  (but they are still being "tweaked")
Also below the graphs is a progress shot of the build.

As far as overdrive is concerned ... as keyboard players who have plugged into a guitar amp, we all understand how it can go from creamy to absolute crunch OD with just the addition of an extra note in a chord.  This is partly due to the fact that most valve guitar amps use 12AX7 valves as the preamp valve and secondary gain stage valve. It is these points where the OD occurs too quickly... not what one really wants to hear. 


So ... and it is yet to be empirically confirmed ... I am hoping this use of a DC coupled cathode follower valve design using a 12AU7 triode valve (which is far less sensitive to sudden changes in signal, like adding an extra note to a chord) will allow a smoother transition from creamy to crunch...  well at the very least it is fun designing and building this preamp.




« Last Edit: March 31, 2017, 09:22:38 PM by sookwinder »
Late 60s Pianet N - Late 70s Pianet T - Ensoniq ESQ1 - Hammond XK2 - Wurli 206A converted to a 200A - 1973 Rhodes Stage 73 - Roland RD150 - Vintage Vibe 64 EP - s**t load of guitars, basses & amps

Offline pnoboy

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I am eager to see how your design works out.