AB763 reverb mods

[EVMode]

I have a custom built fender amp which I’d like to tweak the reverb characteristics. The reverb is too intense even when the mix control is very low. I also own a DRRI and the reverb sounds better. They both use the same spring tank so there must be differences within the circuit.

I have tried using lower gain tubes for the reverb send/return, this definitely helps by reducing the gain but it still sounds different.

I’d like to trace the circuit on my custom amp and tweak it to be as close to the drri as possible. This custom amp is very close to an original AB763 circuit but has only one channel and does not have a tremolo circuit. I’m thinking this amp’s reverb circuit has too much gain and maybe not designed for this reverb tank size.

I’ve very handy with soldering iron and able to drain the power caps. What I’m not good at is figuring out which component values I should be checking and which ones could be modified to get me closer to a DRRI reverb circuit.

Any help is appreciated.

20200930_152242.jpg

[statorvane]

See the 470K resistor after the 100K reverb potentiometer in the linked schematic (bottom center) ? Try adjusting the value of this resistor (I have seen from 220K to 470K there) to a higher value to attenuate the reverb signal.
https://wiremystique.com/wp-content/ima ... c-gpln.jpg

[pdf64]

Or just swap to an audio taper reverb pot

[Stevem]

On a Fender type circuit the tremolo sucks down the signal level, so when you remove it the condition your experiencing with the reverb could be why, in short your simply driving the pan too hard I believe.

[stratomaster]

On a Fender type circuit the tremolo sucks down the signal level, so when you remove it the condition your experiencing with the reverb could be why, in short your simply driving the pan too hard I believe.

Tremolo loading occurs way after the Reverb send tap.

The reverb will be louder, but in the same proportion to the dry signal.

This assumes this build didn't run a 47k-51k to ground to simulate the effect of the Intensity pot loading.

Edit: I did a quick trace of the circuit. It looks like standard AB763 fare except there was no attempt at stimulating the loading of the tremolo or the normal channel at the mix resistor.

If you're wanting to try to get the two amps to sound the same then you'll need to load the signal before the PI. You can start with this:

Screenshot_20260109-164556_Brave.png

Ignore the stray red line at the top left. Note: you're going to lose a significant amount of overall gain. But the noise floor will drop as will the perceived reverb tail, so it's going to sound/feel different.

You can put the 51k on a switch and have that as a "boost" option. This is/was a popular Blackface mod for those that didn't use the tremolo.

[lonote]

Another option is to add a dwell or reverb send control, which variably attenuates the drive signal of the reverb circuit.

It is as simple as swapping in a 1M pot in place of the 1M resistor to ground that is connected to the reverb driver grid. You could even temp it in to see if you like it. I include the dwell on all the amps I build these days.

Dwell Control Fender Reverb.jpg

[GTSjoe]

A bit late to the discussion but, this works well.Cheers.

[Smitty]

Another option is to add a dwell or reverb send control, which variably attenuates the drive signal of the reverb circuit.

I do this all the time on old Fender amps. I use the reverb footswitch hole. Reversible and non destructive.