TWIN REVERB SQUEALING.

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Doglover12
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TWIN REVERB SQUEALING.

Post by Doglover12 » Thu Aug 19, 2021 8:19 am

Hi, I have a 1971 Silver face Twin reverb that has very little time on it. I took it in just to get it checked out it has a little crackle once in a great while. Well he replaced some Capacitors and one small tube. Cost me $225. I never had a chance to try it out for a couple months and the other day I felt like playing some guitar and when I turn the reverb up to five it Squeals so load its unbelieve able. I called the guy here in West Fargo ND and he said bring it in and he will look at it and not charge me anything. So I dropped it off and 3 weeks later I still never head anything so I called him and he said pick it up, I can't fix it. I was pretty teed off and thought I would ask this great forum. Thanks evryone.

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jthomas
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Re: TWIN REVERB SQUEALING.

Post by jthomas » Thu Aug 19, 2021 9:04 am

Squealing usually is oscillation in the amp. That can be caused by a bad component (a cap or a resistor) whose failure creates a local feedback loop in the signal path or parasitic coupling between wires. Discovering the cause would likely require an oscilloscope or use of an amplified probe, to track down where in the signal path the oscillation starts. This would require the amp to be on and you would have to have your hands safely in the amp. With all respect and given that you posed this question, that probably should be done by a tech. (maybe not the one that you are currently dealing with).

The other specific possibility here is that your tech accidentally disconnected the negative feedback loop. That seems more likely given that it started after he worked on the amp... but who knows. Here is info about that:

https://robrobinette.com/Silverface_Amp ... e_Feedback

Here is more general info about squealing:
https://robrobinette.com/Tube_Guitar_Am ... ird_Noises

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andy_tchp
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Re: TWIN REVERB SQUEALING.

Post by andy_tchp » Thu Aug 19, 2021 3:04 pm

Does the reverb work normally up until that point?

The 12AT7 reverb driver tube in those cops an absolute hiding.

If it's not a hardy NOS tube it can 'run away' and provide the decidedly unpleasant screeching/squealing as you've described. Last time I saw/heard this happen on my Vibrosonic Reverb (same amp/chassis as your Twin and around the same age) the reverb driver tube lit up in a bright red colour.

I would grab one of these while they're still readily available/reasonably priced:

https://www.thetubestore.com/jan-philips-12at7wc
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Larsongs
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Re: TWIN REVERB SQUEALING.

Post by Larsongs » Thu Aug 19, 2021 8:25 pm

I had a squeal in my Twin & it ended up being a bad Tube. I don’t remember which one though. Sorry..

Might be worthwhile test all the Tubes before spending a bunch of money on a Tech..

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Re: TWIN REVERB SQUEALING.

Post by sookwinder » Fri Aug 20, 2021 7:54 am

When you get it back let us know and you can do the “let’s just have a few valves in and see if it squeals “ test

Then start adding back the valves (safely from a high voltage perspective) and determine which valve / section of the amp is the cause of the problem
relaxing alternative to doing actual work ...

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Re: TWIN REVERB SQUEALING.

Post by øøøøøøø » Fri Aug 20, 2021 4:48 pm

Yep, most likely you’ve got a microphonic reverb driver tube.

This is very very common, as these amps are pretty brutal to that tube

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JSett
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Re: TWIN REVERB SQUEALING.

Post by JSett » Fri Aug 20, 2021 6:26 pm

I had this same thing on my old Silverface twin. It would scream at full volume too which would be horrific.

It was a bad tube.
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Re: TWIN REVERB SQUEALING.

Post by sookwinder » Sat Aug 21, 2021 12:10 am

does the scream go away when you unplug the Reverb cables (the two silver RCA plugged cables going to the reverb tank)?
relaxing alternative to doing actual work ...

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Re: TWIN REVERB SQUEALING.

Post by jthomas » Wed Aug 25, 2021 2:14 pm

Bump

Any fix yet?

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Re: TWIN REVERB SQUEALING.

Post by Doglover12 » Sun Oct 03, 2021 5:08 pm

First off, Thankyou to everyone that is trying to help me out. The tech that worked on it before told me to drop it off again. So I did. two weeks or better weny by and I never heard a word so I called him and he said I don't know whats wrong with it, Come pick it up. I said it worked fine when I droped it off just to get it cleaned ect. he said I don't know it must be a wiring problem that went bad and has a short or something. So I went and picked it up again, Pissed off and let him know it. he is the only guy that works on amplifiers in town here the way it sounds. I called a couple more music stores and they told me they call him to do there work. I mean Fargo ND is no small town there has to be someone else around here. I am goning to check online and see if I can find someone else. Sorry I have not responded to any of you great people for trying to help me and really am thankfull to all of you. If I unplug the reverb tank it squeals the same way when you turn it past 5. I tryed a new reverb tank I happen to have yet and that did not help. The 3rd tube in from the end tube did not light up very much at all so I bought a new tube and put it in and it still makes the noise but is alot brighter. I have been so busy I just put it off untill this evening and happened to come back to this site and could not believe all the great people on here trying to help me out. I will keep checking back with you people and will look for a different tech. I am printing out all the great info to give to a different tech if he wants it. Talk to you people very soon again. Thankyou again to everyone of you that is trying to help me out.

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Re: TWIN REVERB SQUEALING.

Post by Sweetfinger » Sun Oct 03, 2021 8:28 pm

Are you sure it's a '71? I've had parasitic (not microphonic) squealing problems on later Twins where they have the pull boost master volume that takes the boost signal off the reverb driver.

One "squealer" needed a new reverb driver transformer. Other than that, make sure all the reverb tank cables are good, see if moving them around helps. Parasitic problems that "appear" in an older amp, can be a nightmare to deal with because the problem can be aging caps, drifting resistors, or dodgy grounds.

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Re: TWIN REVERB SQUEALING.

Post by Larsongs » Mon Oct 04, 2021 6:51 am

Doglover12 wrote:
Sun Oct 03, 2021 5:08 pm
First off, Thankyou to everyone that is trying to help me out. The tech that worked on it before told me to drop it off again. So I did. two weeks or better weny by and I never heard a word so I called him and he said I don't know whats wrong with it, Come pick it up. I said it worked fine when I droped it off just to get it cleaned ect. he said I don't know it must be a wiring problem that went bad and has a short or something. So I went and picked it up again, Pissed off and let him know it. he is the only guy that works on amplifiers in town here the way it sounds. I called a couple more music stores and they told me they call him to do there work. I mean Fargo ND is no small town there has to be someone else around here. I am goning to check online and see if I can find someone else. Sorry I have not responded to any of you great people for trying to help me and really am thankfull to all of you. If I unplug the reverb tank it squeals the same way when you turn it past 5. I tryed a new reverb tank I happen to have yet and that did not help. The 3rd tube in from the end tube did not light up very much at all so I bought a new tube and put it in and it still makes the noise but is alot brighter. I have been so busy I just put it off untill this evening and happened to come back to this site and could not believe all the great people on here trying to help me out. I will keep checking back with you people and will look for a different tech. I am printing out all the great info to give to a different tech if he wants it. Talk to you people very soon again. Thankyou again to everyone of you that is trying to help me out.
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Re: TWIN REVERB SQUEALING.

Post by MuscleDad420 » Tue Nov 23, 2021 3:12 am

Have you tried a different set of RCA cables from the reverb tank to the amp?

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