Twin Reverb.. last issue.

[Sea Chief]

Hi chaps. With the other noisey TR thread 14 pages ( huge thanks) best to do a new thread for last bits. Partial success with that main issue: POPS seem to be 90% better but not irradicated entirely, & bad general noise remains. I don't think I can get this any bettter. So onto this other hopefully easier issue:

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I have a high pitched, piercing noise with reverb knob over 4. Question: I've been advised the 1st thing to look at is possible "oscillation due to lead dress".

What I don't get is how it just suddenly appeared during the POP noise attempted fix. All leads around v3 & v4 are rigid & all are as carefully dressed as when I ( re) built it. But this noise suddenly just starts completely out the blue. It is the most infuriating amp I just want to fix & get rid of now.

I've swapped in/ out v3 & v4 for alternate tubes, enough times to say a bad tube confidently ruled out. I can't see lead dress being possibly the cause if the leads around v3, v4 have not moved an iota. The tank I removed from its original thick bag, checked all internal connections & springs. The rca plugs & sockets Ive cleaned thoughrally.

Would anyone lend another hand? I guess there are only so many possibles.. & it being a more common fault, I ( desperately) hope fairly quick to find the cause.

Thanks, SC

[pdf64]

The tank needs to be in its bag; that helps stop it feeding back (typically at a lowish frequency).
Maybe V4 cathode bypass cap is bad, so try replacing it or just tacking another similar cap across it.

[Sea Chief]

The tank needs to be in its bag; that helps stop it feeding back (typically at a lowish frequency).
Maybe V4 cathode bypass cap is bad, so try replacing it or just tacking another similar cap across it.

Hiya pdf. Very kind to offer your advice again.

Yup had put the tank back in it's thick bag/ only took it out to check inside connections ok. I will try your cap idea.

How can a towel or cloth ( 'put that on the tank' presumably on a tank if the original bag his awol, as some www info suggests) or even this bag provide any "electronic shielding" to be considered a possible fix for this issue? Seems strange to me.

[pdf64]

The reverb tank bag etc are to reduce acoustic / mechanical coupling between speakers and tank.

[Sea Chief]

The reverb tank bag etc are to reduce acoustic / mechanical coupling between speakers and tank.

Aha ok thanks.

Is the cathode bypass cap of v4 the pin 7 one? I guess the bypass word is relevant to the tagged on resistor ontop of the cap.. I seem to think this piggyback affair is seen on all the "cathode bypass" cap/ resistor partnerships on circuits Ive seen.

[Dave H]

Is the cathode bypass cap of v4 the pin 7 one?.

No, pin 7 is the grid. V4A and V4B cathodes (pins 3 and 8) are linked so the cap is the one connected to pins 3,8. It's 25u 25V

[pdf64]

7 should be a grid?
Cathodes are 3&8, should be linked together and connected to a resistor and bypass cap.

[Sea Chief]

No, pin 7 is the grid. V4A and V4B cathodes (pins 3 and 8) are linked so the cap is the one connected to pins 3,8. It's 25u 25V

Hi Dave H. Ah got it yes I just counted wrong pin, thought 3 & 7 were joined. 3 & 8 then thanks.

Its a newish cap so I very much doubt bad. There is an original bigass waxy film cap next to it tho, goes to pin 6.

[g1]

How close is the tank to the speakers, what kind of speakers? If you move the tank away from them does it make a difference?

[Sea Chief]

How close is the tank to the speakers, what kind of speakers? If you move the tank away from them does it make a difference?

hi g1. Well the tank & spkrs are as a twin's are, in the cabinet. Jensen italian c12's.

I tagged a 25/25 over the cathode bypass (edit: at v4..... pin 8) & no change.

I swapped a different tube in v3 tho, & the piercing whine seems awol now. But.. there was also a heap of noise behind it, that remains. Louder as knob rotates, nil at zero on knob. Guitar seems extra 'groundy' than usual.

The noise now, is a general uniform BRRRZZZZ on the reverb. The reverb works behind it ok tho.

. Thanks SC

[pdf64]

Post 1 noted that the possibility of the issue being due to a bad valve in V3 and V4 had been eliminated

I assume that the new circuit board is TR AB763 spec, and so has got rid of the pull boost feature of its 135W original design?

[Greg_L]

How do you have the plates connected on V3?

[Sea Chief]

Post 1 noted that the possibility of the issue being due to a bad valve in V3 and V4 had been eliminated

I assume that the new circuit board is TR AB763 spec, and so has got rid of the pull boost feature of its 135W original design?

Hi pdf, yes it is a AB763 board, & yes indeed it got away with this MV pullboost thing.

But again the insinuation here, forgive me, is back on another new red herring avenue.. that my build isn't correct/ that my ( slight ) reworking of the UL135 w into this new ( slightly ) different guise, is now risen to th3 top of the list as 'possible if not likely culprit/ to look at his build'.

I repeat, the amp worked very well when fired up innitially. It had the usual testing, done by myself & Rift walking me through step by step.

If it worked as well as any newly built amp, or newly bought amp, for a year..... then this prooves that the build was good ( & therefore still is if I haven't changed anything). Even if it is slightly different now, to originaly, this doesn't mean its bad. The year working well prooves it isn't badly or incorrectly made.

IF the amp had these symptoms, from the moment I first fired her up after tests, then YES you have logic/ rationale to persue this avenue.

But it worked, fine.

The innitial consensus, the firm ( seemed almost literally) consensus was, collectively here from opinions, WAS a conductive board. I questioned even bucked this trend using logic to question this appraisal. I was deemed correct.. NOT (& with respect) the now proven incorrect almost dead-certain diagnosis from knowledgeable folks that it was a conductive board: if I found a resistor pair shot... changed them... & POPPING has reduced by 98%.... then I was right ( a bad component) & the consensus was wrong.

That is not to say I know more. Of course not. I know nothing. But my hunch, & logic, cannot be dismissed.



Thanks. SC

[Sea Chief]

How do you have the plates connected on V3?

Hi Greg,

can you be more specific? Apart from " using solder" Im not sure how to answer this question.

Thanks, SC

[Sea Chief]

Post 1 noted that the possibility of the issue being due to a bad valve in V3 and V4 had been eliminated

I assume that the new circuit board is TR AB763 spec, and so has got rid of the pull boost feature of its 135W original design?

Is the first sentence here questioning whether i have, or have not, ruled out a bad v3 & v4?

If so, as you hint at, indeed in #1 I said/ stated this.

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I would only start a thread like this ONCE I had done all I could, with my limited knowledge, to rule out things first/ before creating the thread.

I said in 1 that I had done this. Or rather, I had swapped in/ out known tubes & the results ( at the time ) gave reasonable proof the tubes were ok.

I have a series of strange things happening currently: if I take out my ( i only have one jj 12 at7) old jj12at7 yesterday & put in another 12at7 into v3... &when i turn on........ the high pitched piercing noise has gone......... then it SEEMS like there is a correlation. So I report this on here.

But today I repeat the test, put the old jj12at7 back in, to show myself that this had to be the cause, ot rather something to do with the cause. I expect nothing other than the high pitched noise to be there again. But no. I get different results: the high pitched noise is awol again.

So, how do now I consider this jj 12at7? Good? Bad? Nothing tells me its bad? Everything tells me its bad?