Super Reverb Reverb Problem

[jbefumo]

Third try posting this, so if there are multiple copies – sorry.
 
Since a picture is worth a thousand words:
https://youtu.be/L3lIqzJ_cAg
Appearances notwithstanding, this is a fully ‘blackfaced’ AB763 circuit. I didn’t like the look of the black plug, so I returned the Master Volume, but it’s a post-PI cross-line type on a push-pull pot, configured so it’s normally completely out of the circuit (pull to engage).
‘Normal’ channel is fine (as long as the reverb cable is removed – see below).
I’ve already tried replacing all the relevant tubes – Vibrato Channel preamp tube, reverb driver, reverb recovery -- as well as trying three different reverb tanks and a different set of reverb cables.
As shown in the video, with the cable pulled from the ‘Reverb Output’ jack, the problem completely disappears, and the Vibrato Channel sounds as it should (albeit without any reverb).
However, if I leave the cable connected to the ‘Reverb Output’ jack and pull the cable from the ‘Reverb Input’ jack, the problem persists.
Ideas?
 
Thanks!
 
Joe

[VMS]

I wouldn't fix a thing, that sounded awesome   

[VMS]

Little more serious now, does switching the reverb pedal have any affect on the problem?

[jbefumo]

Little more serious now, does switching the reverb pedal have any affect on the problem?

HUH!


Never had one, so didn't even think of it, but I tried with a different pedal and an RCA adapter, and yes indeed (other than noise from the unshielded pedal cable). 

[jbefumo]

The mystery continues ...


Pulled the chassis, put it on the bench, hooked everything up, and .... no noise?!?!?


Well, moving on, while I had it open I replaced the screen resistors (one measured 470R, the other just under 600R), and while in the process of rebiasing, I noticed that adjusting the reverb dial altered the current flowing through both power tubes.  WTF? (I have the amp set up with back-panel BNC ports so I can measure current directly, and have individual bias adjustment for each tube.)


That's when I decided to call it a night, but am about to get back into it after another cup of coffee.


Can't immediately think of any hypothesis to test, though.  Figured I'd start by plugging in the pedal and see if that changes things, then pull the reverb recovery tube and see what happens.

[jbefumo]

First test results:


Turning the reverb knob from zero to 10 causes the plate voltage to drop by 20V, and the cathode current to drop from 40Ma to 18 mA....

[jbefumo]

First test results:


Turning the reverb knob from zero to 10 causes the plate voltage to drop by 20V, and the cathode current to drop from 40Ma to 18 mA....

My mistake, turning up drives current down and voltage up.


Also, shorting the reverb pedal negates the effect.


Seems like this should suggest something relatively simple, but I'm just not seeing it.


(Also tried swapping the reverb recovery tube -- no change.)

[jbefumo]

God, I'm getting flaky:


Turning the reverb control up INCREASES the current flow.


Also, shorting the TREMOLO pedal jack also negates the effect.

First test results:


Turning the reverb knob from zero to 10 causes the plate voltage to drop by 20V, and the cathode current to drop from 40Ma to 18 mA....

My mistake, turning up drives current down and voltage up.


Also, shorting the reverb pedal negates the effect.


Seems like this should suggest something relatively simple, but I'm just not seeing it.


(Also tried swapping the reverb recovery tube -- no change.)

[jbefumo]

I have a sneaking suspicion I'm focusing on the wrong place ...


problem seems to have started when I began messing around with a funky power tube socket.


When I remove the power tubes and tap the signal from input to the phase inverter to an external amp, no problem ....


Actually bought a new pair of sockets that said "drop in for most Fenders" and .... too big for the holes, so have another pair coming, but still need to isolate the problem or I'll be shooting in the dark ....

[jbefumo]

Well, got it sorted out!


It was those damned tube sockets:


http://el34world.com/Forum/index.php?topic=22890