Overdrive in Fenderīs reverb

[chocopower]

Iīm trying this mod (change reverb pan for a 470 K resistor).
It make a good, but hardly controler, booster. It cut to much basses and is really hard to pass from 1-2 in the dial

Any way to make it more bassiest and controlable?

[stokes]

For more bass,try bypassing the resistor with a .1 cap.Perhaps a larger resistor in place of the 470k will give you a little more out of the dial.Since you are now using the reverb circuit as a gain stage you could tweak the output stage,but if you wanted to go back to the reverb,you would have to modify that stage back to original spec.

[The Radium King]

fender reverb circuits are voiced to only pass treble signals. if you look at your schematic, you'll see a couple of caps in the signal path (one on the driver triodes and one on the return triode) that are only passing high frequencies. i've tried to figure a way to rig it that was externally reversable, but haven't been able to come up with anything. a larger resistor will reduce the amount of signal getting to the return stage and make the gain a bit more controllable, but otherwise you are stuck changing capacitor values. note that you want to increase these cap values to pass more bass, and capacitance increases in parallel, so it's easy enough to add caps to shape your signal, and perhaps put them on a switch. also note that, if you only want a bit of gain, you can bypass the reverb driver triodes and internally jumper straight to the reverb return; this gives you one gain stage with footswitch control.

[chocopower]

...If you only want a bit of gain, you can bypass the reverb driver triodes and internally jumper straight to the reverb return; this gives you one gain stage with footswitch control.

Yes....a bit of gain is enought for me.... (just for boost the solos )

Could you explain it a little more?

[The Radium King]

sure - tell me what you have (amp, year) and i'll dig up a schematic and try to help out. ps, if you have a two channel amp, you can split the signal at the output to the reverb tank - send some through the resistor to the reverb retrun for a footswitch-controllable lead boost, and send some to the reverb tank and patch the return through your clean channel (this gives you tmb control of your reverb, but no footswitchability).

[chocopower]

Hi!!

The amp is Guyatone GA-1050. Is one of the closest copy i have ever seen of a Fender Silverface-blackface.
Is a 2 6L6 Twin Reverb.....or a Pro-reverb whith diode rectifier

In really interesting in this mod...

[The Radium King]

can't find guyatone schematics anywhere. depending on your circuit, you may have a phase reversal issue if you bypass the reverb driver (my bad). try a larger resistor as stokes noted to drop the gain of the channel. parallel caps to increase the size of your coupling caps in the reverb circuit will let more signal through. the cap to ground as stokes noted will bleed off treble; depending on the sizing of the various caps there may not be much signal left - perhaps some higher mids - might be ok for leads.

[chocopower]

can't find guyatone schematics anywhere. depending on your circuit, you may have a phase reversal issue if you bypass the reverb driver (my bad). try a larger resistor as stokes noted to drop the gain of the channel. parallel caps to increase the size of your coupling caps in the reverb circuit will let more signal through. the cap to ground as stokes noted will bleed off treble; depending on the sizing of the various caps there may not be much signal left - perhaps some higher mids - might be ok for leads.

Oh..... my amp is a Pro-Reverb clon, but whith diode rectif.
I,m realy intersting in this other mod:

"if you only want a bit of gain, you can bypass the reverb driver triodes and internally jumper straight to the reverb return; this gives you one gain stage with footswitch control"

Could you use a Pro-Reverb lyout to explain this mod?


Best regards from Galicia!

[The Radium King]

yeah, you will have phase inversion issues if you only use one gain stage from the reverb return; you'd have to use both; this would mean a big reconfiguration of your vibrato channel. also, if you bypass the reverb driver, you'll also have to relocate the footswitch connection to the cathode of your gain stage. the easiest solution is to use a larger resistor as was advised in the first post. here's what you have to think about:

original configuration:

in - gain1 - tone - gain2 - a - 3.3M resistor - b - gain3 - 220k resistor - out

(note that each gain stage has a coupling capacitor on the output)

in this configuration, the reverb signal is taken from 'a' before the 3.3M resistor and re-inserted at 'b'. the reverb circuit looks like:

in - gain4 - transformer - reverb tank - switch - gain5 - out

with your current boost circuit, you are putting a 470k resistor in place of the reverb tank, so that you now have several gain stages that get switched in and out of the main circuit. this is a lot of gain, and the circuit is voiced (using capacitors) to only pass treble frequencies. increasing the size of the resistor that replaces the reverb tank will attenuate the signal and reduce the gain. increasing the capacitor size will let more frequencies through, make it less bright and increase the gain.

note tha gain4 is two gain stages in parallel (you need lots of power to smack a reverb tank properly). i've never done it, put you could probably reduce gain by disconnecting one triode of the parallel reverb driver tube. try disconnecting the wire that goes to pin 8 and see what happens.

if you acknowledge the fact that each gain stage inverts the signal, you need 2 gain stages so that the gain signal is the same phase at 'a' as it is at 'b'. if you bypass the reverb driver (ie, run a wire from 'a' to the input of gain5) you will have to move the footswitch to the cathode of gain5 (use the 'cathode lift' approach to turn the tube on and off) otherwise you will be sending your entire signal to ground when you activate your switch. further, you only have gain4 and your signal will be out of phase (gain5 is after the summing node at 'b' and is required to compensate for the attenuation due to the 3.3M resistor).

at this point things start to get more complicated. you have other options - 3 way switching to introduce the gain stages in series instead of parallel (this way phase is not an issue). you can also try running a wire from before gain2 to the input of gain5 (don't forget to move your switch to the cathode) this brings gain5 in as a parallel gain stage to gain2; things won't get much louder (your voltage won't increase) but your tone will thicken (especially the treble in this case, if you don't revoice the circuit); it may be what you are looking for.

etc etc etc. think of each gain stage as a lego block. remember phase inversion and to always have a coupling cap on the output and you can do a lot; the standard two channel fender preamp with reverb and vibrato has a lot of gain stages and foot switches that can be configured to do just about anything.