Fender Yale Reverb mod

[tgs]

I have an old Fender Yale Reverb that I don't really use for anything so I figured I'd play around with some mods. I wanted first of all to try to get it to distort a bit more. If I dime the volume and gain knob I get a pretty good starting point, but I want to push it a bit more. I found schematics here:

https://dokumen.tips/documents/fender-yale-reverb-schematic.html?page=1
I figured at first that I'd do something around R23 (after the gain pot so I tried adding diode clipping in parallel with R23. It did nothing other than possibly dropping the level a bit.

So next I looked at the diode clipping stage D3-D4. I figured I'd put a resistor in parallel with R9 to increase the feedback voltage. Did the opposite of what I though, brings the distortion down. So I guess I'm getting it all backwards.

I'll be very clear here that I'm a total novice and I'm just experimenting a bit to try to learn. So what I'm looking for is mostly to get some ideas for starting points for experimentation. The circuit looks a bit strange to me, I'd expect to find the diodes around the gain pot so I guess there are some basics here that I'm missing. Anyway, if anyone has any tips on this I can try out, I'm all ears. Thanks for your attention!

 

[musipol]

I haven't modified one of these myself, so take with a grain of salt.
If I'm not mistaken, most of the clipping is coming from IC4 which s/b a 4007 CMOS IC that runs at ~ 6volt rail. This looks like a Rivera era design. You might try increasing R23.

 

[tgs]

I haven't modified one of these myself, so take with a grain of salt.
If I'm not mistaken, most of the clipping is coming from IC4 which s/b a 4007 CMOS IC that runs at ~ 6volt rail. This looks like a Rivera era design. You might try increasing R23.

Interesting! Looking at the 4007 pin-out now and comparing with the schematics. To get a basic idea: the offset variable resistor R27 would be part of a voltage divider for the +15 coming in and increasing/decreasing the voltage at 2/11/14, making clipping asymmetric?

Trying to understand how increasing R23 would push more signal into the 4007. Would that be due to increased input at IC1 pin 6?

 

[musipol]

Increasing R23 should increase the gain of the op amp feeding the 4007 chip. By the way, the offset pot(R27) is only receiving 6.2V(not 15V) because of zener D7. You could adjust it for asymmetry, but the effect will be static. As I said, I have not played with this amp, but I have built a similar circuit for distorted guitar usage(using CD4007 or CD4049 in a similar fashion). If you are interested, I'll see if I can find the schematic for it.

 

[tgs]

Thanks Musipol! Yes, I got that the 4007 only receives 6V, thought I thought it was due to a voltage divider setup. But the zener makes sense. I saw it in the schematics but thought it was a protection for over-voltage.

I tried out different resistors in place of R23 yesterday. Maybe a bit extreme but it sounds great with a 680k, get a bit of a Big Muff effect. My plan now is to add in a switch so I can jump between the original state and the extra distortion.

 

[tgs]

Here's the result.