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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Mon Dec 19, 2016 1:19 pm
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Hello Fellas - measured voltages at Pin 5 (with tubes out) pot centeted

(V7) -7.83
(V8) -7.89

What are the chances the bias pot is bad?


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Mon Dec 19, 2016 1:21 pm
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Retroverbial wrote:
Did you inadvertently install the bias-supply cap backwards?

It needs to be wired with the polarity reversed -- that is, with the "+" side connected to chassis ground.

Do both caps need to have the positive soldered to ground? Meaning the one coming off bias pot AND the one closest to the pilot light near the flyback
Diode?

Arjay


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Mon Dec 19, 2016 6:57 pm
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Do you have the amp out of the cab? Need to measure some more voltages in the bias supply. In order to narrow down where is the problem.

Measure VDC at the bias supply filter cap point. If it's still low voltage, around -7.8VDC... you prolly do have a bad diode.

http://ampwares.com/schematics/super_reverb_aa270.pdf

If the voltage measures -44 to 46VDC... you prolly have 1.) a bad or dirty balance pot; 2.) the 15k-ohm dropping resistor, on the pot, is bad; 3.) bad grid stopper resistor on the socket; or 4.) bad socket.

Try cleaning balance pot with good contact cleaner. I like Deoxit D5 or GN5. A little bit, then work the pot (amp off) lock-to-lock several times.

(Amp off) Check the resistance across that 15k-ohm dropping resistor and the 1500-ohm grid stoppers (tubes pulled).

Good luck! Keep us posted.


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Tue Dec 20, 2016 1:37 pm
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Yours truly, didn't installed the bias cap closest to the pilot lamp -negative to ground, the one coming off the pot was correct.

Bias is back! Thank you so much for the input,


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Tue Dec 20, 2016 2:09 pm
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Both should be (+) to ground. Photo is of SF era Vibrolux Reverb bias board. Similar two cap setup as Super Reverb. Glad you found the problem! :)

Image


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Tue Dec 20, 2016 2:13 pm
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Ryan41018 wrote:
Yours truly, didn't installed the bias cap closest to the pilot lamp -negative to ground, the one coming off the pot was correct.

Bias is back! Thank you so much for the input,


Good to see you are back up and running, the SR is a great amp I have a 69, love it, I would love it more if it weren't so loud, can't get it to V3 most of the time and be in the same room, at V4 things rattle off the shelves and sometimes break when they land on the floor. Take it outside and roll it up to V6 and it is heaven and the angles are singing
enjoy
mud


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Tue Dec 20, 2016 3:47 pm
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Ryan41018 wrote:
Yours truly, didn't installed the bias cap closest to the pilot lamp -negative to ground, the one coming off the pot was correct.

Bias is back! Thank you so much for the input,


I thought you said that you checked that when I mentioned it......?

Oh well, no matter. Glad to hear the amp is now operating correctly.

Rawk on!

8)

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Tue Dec 20, 2016 8:16 pm
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Retroverbial wrote:
Ryan41018 wrote:
Yours truly, didn't installed the bias cap closest to the pilot lamp -negative to ground, the one coming off the pot was correct.

Bias is back! Thank you so much for the input,


I thought you said that you checked that when I mentioned it......?

I did check it, I also chimed in and was wondering if "Both" were supposed to be reverse (Positive Mounted to Ground) but perhaps most didn't catch that.

Needless to say, It was fun! When I didn't get an answer, and then rechecked all my soldering on the new filter caps, I looked for pictures on Google and thats where I noticed my error.

Thank you bunches guys, Really appreciate it and your patience too.

Merry Christmas to all and a Happy New Year.

Oh well, no matter. Glad to hear the amp is now operating correctly.

Rawk on!

8)

Arjay


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Post subject: Re: 73 Silverface Super Reverb - LOST BIAS!
Posted: Tue Dec 20, 2016 8:33 pm
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And to you and yours, sir.

Enjoy the holidays.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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