AB763 split cathode conundrum

[Randall]

Same Super Reverb as before with reverb mod. This time the second cathodes of V1 (Normal) and V2 (Vibrato) are split, with V1 staying where it was (including keeping the 820R bias resistor), and V2 going straight to ground. The vibrato channel has less gain and less fullness. And I think it is somehow leaking approx - 0.3v onto the volume pot with it full on, and it goes to 0.5v in standby mode. This is with all three tone stack caps lifted. Same with different tubes. New volume pot has a crunchy spot at about 1 - 2 on the dial.

so, looking at this drawing, imagine V1 with the 820R/ 25uf, and V1 cathode to ground. Customer asked to leave any mods in place. My question is, what would be the purpose of this mod?

[nickb]

It would be much clearer if you were to post a marked up schematic. There is not enough clarity in your description V1? V2? a? or b? I have no idea what you mean.

If a cathode is grounded that's a really bad idea and would tend to make that stage distort rather easily. I wonder if the person doing the mod mistook the square box around node "A" (normal channel) for ground and so wired it up wrong?

63_(AA763)_Super_Reverb_Schematic.pdf

[Steve A.]

The "guy with the soldering iron" (certainly not an amp tech!) presumably heard about splitting the cathodes on the second stages of both channels on a BF amp and really butchered it up as nickb surmised.

To split the cathodes you would replace the common 820R resistor with a 1k5 resistor to ground on each cathode. For the cathode resistor bypass caps you could use two 25uF ecaps - or experiment with lower values. A .68uF poly cap for the Normal channel can give you a pseudo-Marshall tone (use a 2k7 cathode resistor for bonus points.)

I always thought 22uF bypass caps were too large on BF/SF amps... 2.2uF is the biggest one I usually use (with bonus points for poly caps!)

Steve A.

P.S. Maybe I'm missing something here but I don't see how ANY signal would reach the volume pot if all 3 tone caps were lifted. DC voltage on the volume pot is often coming from a leaky treble cap but if the amp still has the original eyelet board there could be leakage from that.

[Randall]

Edited post #1 to add drawing and better explaination.

[g1]

Steve: I think Randall lifted the tone caps for troubleshooting purposes.

Randall, you're kind of stuck in a jam here with some butchery that the owner considers 'mods'. The cathode to ground is a definite no-no, and probably the cause of that DC problem, and maybe also causing the pot glitch. The other cathode resistor should get a value change like Steve mentioned.
The other reverb mod, as mentioned in the other thread, looks like a resistor change trying to undo the result of a cap change, rather than just going back to stock. Again, the 390K is a no-no.
Now how do you tell someone that what they think are mods are actually mistakes?
Maybe you just offer it back as is, or offer to put it back to stock, I don't know.
I had an amp with the OT all miswired with secondary windings put in parallel. I just undid it and did it up right. Then the owner said it didn't sound the same and could I put it back? No, I said, I will not put it back wrong. Sorry. Not to mention I did not document the error so I couldn't put it back anyway, but I don't think that is improper. I'm not going to put bad parts back in a refused repair either.

[Randall]

Correct, the lifted caps were to see where the - dc was coming from. It is not the original board. This amp went under water with the recent hurricane, so I put a new birch baffle to replace the destroyed particle board, new reverb tank, built a new board on glass epoxy, etc, etc. So there is no going back in this case. I have made an email plea to lose the crappy mods and put it back to stock. Plus, I will have put in a large amount of un-billable time and effort into this when it is over and done, just because I want to see a BF SR come back from the dead, and I can do it and have the time.

I think I am hearing that the grounded cathode is a definite no-no, and that the reverb mod is crap. I think I may just put it all back to original and take my chances.

[Enzo]

If you ground the cathode, the stage becomes a grid leak biased stage, so it is natural for some negative to build up on grid. That of course winds up across the volume control. Which makes it noisy.

If customer knew ther were mods and asked to leave them in, it might mean he likes the way the amp sounds as it was. I would leave them, and contact the guy to tell him the amp was repaired but you had concerns the mods were not in line with good circuit practice, and it would be a matter of a few cheap resistors and a cap to restore it to stock. Let him decide.

If you leave the mods, and a month later he decides "you know I think I WOULD like it stock", then you now have a new repair order. Don't include the later restore to stock work as part of the old ticket.

[Randall]

I get your drift Enzo, but in this case, "leave them" means "reproduce them". The customer told me the mods were done by "a really good amp tech in Germany". First glance at the work tells me that is not the case. I am awaiting response to see what he wants to do. My vote is send it out fixed, not fucked. Reputation and all.

[Steve A.]

I get your drift Enzo, but in this case, "leave them" means "reproduce them". The customer told me the mods were done by "a really good amp tech in Germany". First glance at the work tells me that is not the case. I am awaiting response to see what he wants to do. My vote is send it out fixed, not fucked. Reputation and all.

I would correct the miswiring of the V1 & V2 cathodes*** but leave the weird wiring in the reverb circuit since he likes it.
As to the scratchy volume control you did mention that you replaced the original eyelet board, right? So there is no way that there is DC leaking on the board. My guess is that the new pot you installed was damaged by DC leakage, possibly from a bad tone cap. I would wire up the tone caps to the plate of the initial gain stage and then check for any DC voltage on the opposite leads which are NOT connected to the pots. (I guess you could also check for leakage by temporarily connecting one end of the caps to B+.)

Steve A.

*** In explaining why the Normal channel sounds better now you could say that you had to replace V1 and leave it at that. If he insists that it sounded better before then you could ground the damn cathode...

P.S. When I was an HVAC service tech I would explain to the customer exactly what needed to be done before starting the repairs to get him onboard. In your case with 20-20 hindsight you could have explained that V2B was not wired correctly and for liability reasons you must correct the cathode wiring. Tell him that if after testing the rebuilt amp he liked it better before you could ground the cathode but with the caveat that it could cause scratchiness in the volume control that will not be covered under your warranty.
I'd probably add that I checked with top-notch amp techs on-line who strongly recommended that I correct the wiring. "If you liked it better before I can change it back... no problem-o."

As I suggested all of that belongs in the coulda/shoulda/woulda file and that ship has already sailed. Good luck!

[pdf64]

If a stage has its cathode at 0V, it will push its grid current up, creating a voltage across any resistance between its grid and 0V.
Look up 'grid leak bias'.
If there's a pot there, it will be scratchy due to that dc, and likely have a 'thump' too if it's rotated across its range quickly, due to the operating point shifting.
I guess that stage's plate voltage will be rather low, and so the plate load resistor's dissipation may be higher than normal.

If you want to keep the grid leak bias but get rid of the scratch etc, then a blocking cap and grid leak resistor (somewhere in the range 1M to 10M) could be inserted between vol pot wiper and the grid.

[Randall]

"I guess that stage's plate voltage will be rather low, and so the plate load resistor's dissipation may be higher than normal."

Bingo! This is also true.

[Randall]

I successfully convinced the customer to remove both the cathode and reverb mod, and put it back to stock. Now the restoration is complete and a great success. Everything functions as it should, all voltages are good, and with the new birch baffle, glass epoxy board, and biased to 55% it sounds just fantastic!