'72 SF Twin Reverb Circuit Hum

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RJSkypala
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'72 SF Twin Reverb Circuit Hum

Post by RJSkypala »

Alright, finally got my old Sovtek Mig up and running now I have a new project.

My '72 Twin started humming with the reverb turned all the way up. It slowly builds up to a consistent tone that goes away when the reverb is turned down or off. Bought a NOS GE 12AT7 today and swapped it for V3, same problem. Tried a 12AX7 I have lying around, same problem but quieter.

Tried swapping out V4, same problem. Unplugged reverb tank, no problem. I tried all the tubes in V1 of my Sovtek to test for hum and microphonic, they all seem fine.

My powers of deduction lead me to believe that this must be a problem somewhere in the reverb circuit. I am going to open up the amp and put in on my bench tomorrow. Do any of y'all have any suggestions on where to begin looking for this problem?
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JazzGuitarGimp
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Re: '72 SF Twin Reverb Circuit Hum

Post by JazzGuitarGimp »

You might have an acoustic feedback issue. Try taking the tank out of the cabinet and placing it on a pillow.
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Bob S
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Re: '72 SF Twin Reverb Circuit Hum

Post by Bob S »

I've had some similar probs with reverb on a 72 super.
I cleaned up the grounds in the amp & the oxidation on the plugs & it went away.
Good Luck with yours.
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Reeltarded
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Re: '72 SF Twin Reverb Circuit Hum

Post by Reeltarded »

Yah, clean it up and then consider a new tank if that doesn't help. Do you have one you could swap in for a test?
RJSkypala
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Re: '72 SF Twin Reverb Circuit Hum

Post by RJSkypala »

Took the tank out, put it on a pillow and everything seems fine! Popped the original RCA 12AT7 back in and it sounds perfect.

Question is, how do I put the tank back in such that it will not do this anymore?
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Reeltarded
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Re: '72 SF Twin Reverb Circuit Hum

Post by Reeltarded »

In the bag and a little foam under it? Is there an inaudible hum that builds up on the springs then?
RJSkypala
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Re: '72 SF Twin Reverb Circuit Hum

Post by RJSkypala »

Just any type of foam?
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Reeltarded
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Re: '72 SF Twin Reverb Circuit Hum

Post by Reeltarded »

Low density is better if you are trying to isolate very low frequencies. Even folded bubble wrap in those tiny 1/8" or so dots would be a good iso material for lowend.

I might also add small rubber bushings on the bag screws between the bag and amp.. not screwing that down too tight.

Where is the problem coming from though? I wonder if it's about to need filter caps, or you have a power problem at your place suddenly.
RJSkypala
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Re: '72 SF Twin Reverb Circuit Hum

Post by RJSkypala »

It seems to have gone away with taking the tank out of the cab. Not sure about if the amp needs caps or not.

That is something I feel I'm prepared to do and would like to do if it is good for the amp. How can I test for the need of a cap job?
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Reeltarded
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Re: '72 SF Twin Reverb Circuit Hum

Post by Reeltarded »

The obvious first sign is usually 60Hz humming away at idle. How old are the ones in there?

Something must be physically affecting your springs if it builds from nothing until you turn it off..

Did you put it back in and mess with it while making input signal to see if the removal actually was the trick? May have disturbed the real problem at that time...
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Milkmansound
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Re: '72 SF Twin Reverb Circuit Hum

Post by Milkmansound »

there are some other non mechanical things you can try

change the bypass cap on the reverb recovery triode to a lower value to roll off low end

change the 220k to the grid on that same tube to 100k to lower gain (this gives more play on the knob too - I like this value)

remove the bypass cab completely

those kinds of things

Fender amps have a TON of reverb - you can spare a little to lower the noise floor and you never know, you might be able to turn the mix knob up past 2 and get finer adjustment!
tictac
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Re: '72 SF Twin Reverb Circuit Hum

Post by tictac »

Consider the reverb pan position as another factor... I know in amps with the reverb pan closer to the chassis if the input of the reverb pan is closer to the power amp section, it will pickup noise... try rotating the pan both ways and see which way is quieter...

TT
Cliff Schecht
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Re: '72 SF Twin Reverb Circuit Hum

Post by Cliff Schecht »

tictac wrote:Consider the reverb pan position as another factor... I know in amps with the reverb pan closer to the chassis if the input of the reverb pan is closer to the power amp section, it will pickup noise... try rotating the pan both ways and see which way is quieter...

TT
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