Fender Deluxe Reverb AB763 reverb pot adding gain/mids?

[acorkos]

I can't figure out why the reverb pot adds gain/mids when turned up. Tubes are ok, tank is ok, cables are ok, reverb effect seems to be working normally. Amp has a solid state rectifier plug, the voltages are all up higher than the schematic. Any ideas of where to look? All the resistors in the reverb circuit are within spec. Thanks for your help.

http://www.thevintagesound.com/ffg/s...763_layout.gif

[nickb]

I can't figure out why the reverb pot adds gain/mids when turned up. Tubes are ok, tank is ok, cables are ok, reverb effect seems to be working normally. Amp has a solid state rectifier plug, the voltages are all up higher than the schematic. Any ideas of where to look? All the resistors in the reverb circuit are within spec. Thanks for your help.

http://www.thevintagesound.com/ffg/s...763_layout.gif

Did this just suddenly start happening?

Check the wiring around reverb pot and also that the 470K resistor is in circuit and the correct value.

[acorkos]

I'm not sure if it just started happening...i'll ask the customer. 470k is correct and in circuit.

[acorkos]

ok this is interesting

460v on pins 1 and 6 of v5
-43v on pins 2 and 7

I don't think that's right

[nickb]

ok this is interesting

460v on pins 1 and 6 of v5
-43v on pins 2 and 7

I don't think that's right

Assuming you are looking at one of the 6V6's my guess is you have confused real pin 4 with pin 1 and so miscounted the pins. (ps /edit: If it's the 'vibrato' oscillator then that is normal).

With the amp off, turn the reverb pot all the way to zero and measure the resistance from the grid the mixing node to ground. By that I mean the junction of the 3.3M, the G1 of the 7025, 470K and the 200k. Repeat for reverb all the way up. Report your findings.

Do you have a scope?

[pdf64]

V5 is the trem oscillator and buffer; how does it measure up if the trem is switched on?
Regarding the reverb issue, strange things can happen if the V4 cathode bypass cap goes bad, as in excessive ESR, due to a positive feedback loop forming.
By the same effect, it could also be a similarly failing HT decoupling cap.
As the positive feedback level is increased, there will be an increase in gain prior to oscillation.

[Steve A.]

Are you sure that reverb circuit was not modified to add gain? It could be done with just a resistor and cap (or two.) I had a "blue ice" module that plugged into the reverb jacks to add distortion to my Pro Reverb. There was a deluxe module that allowed you to connect the reverb tank as well.

Steve Ahola

[acorkos]

With the amp off, turn the reverb pot all the way to zero and measure the resistance from the grid the mixing node to ground. By that I mean the junction of the 3.3M, the G1 of the 7025, 470K and the 200k. Repeat for reverb all the way up. Report your findings.

Do you have a scope?

155k ohms with the reverb pot at zero, up to 165k with pot at 10
and yes, I have a scope
thanks for your help

[acorkos]

Regarding the reverb issue, strange things can happen if the V4 cathode bypass cap goes bad, as in excessive ESR, due to a positive feedback loop forming. By the same effect, it could also be a similarly failing HT decoupling cap. As the positive feedback level is increased, there will be an increase in gain prior to oscillation.

which one is the HT decoupling cap?

[g1]

The filter cap at point marked "D" on schematic.

[acorkos]

thanks
I just subbed in new coupling caps and cathode bypass caps on tubes v3 and v4....no change.
what about the reverb transformer? I have a spare, I guess I should swap it in next?

[52 Bill]

what about the reverb transformer? I have a spare, I guess I should swap it in next?

Please stop subbing in parts to try and find the problem.

Is this an old one or a reissue?

If you unplug the reverb tank is the problem still there? How about if you ground the reverb return jack?

[nickb]

155k ohms with the reverb pot at zero, up to 165k with pot at 10
and yes, I have a scope
thanks for your help

Sorry for slow reply - for some reason I stopped getting email notifications.

Well, those numbers look normal and means that it's likely wired up OK. Can you please check for DC at the top of the reverb pot? I'm wondering if a small amount of DC here is modulating the gain of the reverb recovery stage as you vary the reverb level.

[acorkos]

.8mV on the reverb pot

[acorkos]

mid 70s silverface, no master volume or boosts
unplugging the reverb tank and grounding the reverb return jack eliminates the problem...same as turning the reverb pot to zero