Reverb mix resistor change on Fender and the effect I'm getting... Confused.

[PeaveyBandit]

Hi Yall,

Everything is going fine on my new amp but I tried the mod suggested about changing the stock 3.3m resistor down to a 2.2m resistor today hoping it might tone down my reverb mix a little. It almost seems like the decay is getting more in my way now than before. If I didn't know better I'd swear the reverb was stronger or the same. So I thought I'd ask in case I'm doing something stupid or misunderstood the suggestion.

It's an AA1164 and I changed the resistor that's in parallel with the 10pf cap from the stock 3.3m to 2.2m. SEE ATTACHED layout if needed.

P.S. The layout indicates 2m ohm for this resistor but I happened to have a 2.2mb on hand.

[cbarrow7625]

Changing that resistor will do nothing to the reverb itself. The reverb level will be exactly the same. What you are changing is the amount of dry signal mixed back in with the reverb. You have changed the relative dry/wet ratio (more dry with the 2.2M resistor) but the overall reverb level will be exactly the same, just more dry signal will now be present. It also changes the gain structure of the amp for the clean signal, which may not be what you wanted.

If you want to reduce the amount of reverb, you need to reduce the drive level going to the reverb pan (i.e., into the parallelled 12AT7 tube). The quickest 'n' dirtiest way to reduce the drive level is just to remove the 25uF cap from the cathode of the reverb driver tube. A better method would be to replace the 1M pot in front of the reverb driver tube with a 1M pot (or create a voltage divider in place of the 1M resistor). THe pot will allow you to dial in just the right amount of drive level into the reverb pan.

There are other ways too but those two methods can get the job done pretty quickly and actually give you something closer to your desired results than your previous attempt.

Chris

[PeaveyBandit]

Thanks Chris, maybe that's the approach I should take on this. And I understand what your saying. Replace the resistor at the input of the reverb with a pot or resistors and change it from that end.

I also need to study how they add the DWELL control on a Fender standalone reverb unit. That's certainly an effect I love and need. Maybe what you're suggesting is the closest thing I can get to a dwell. Dunno, but your suggestion is appreciated. I'll play with that one day and see.


P.S. Found reverb unit schematic and noticing how they control DWELL. Interesting to see how that's done.

[cbarrow7625]

Replacing the 1M resistor with a pot is basically a poor man's dwell control. The dwell control in the stand alone Fender reverb is just a pot before the drive stage.

In the stand alone, the gain structure is a little different and I think they use a 250K pot instead of a 1M. The 250K pot will keep the drive signal from getting muddy like it might when going through a 1M pot (if you turn it down significantly).