Silverface Twin Problem...help!

wingobingo

Member
Messages
274
Before I start changing tubes, would like some expertise. 1972 Twin with intermittent problem...the kind that drive you nuts. Amp sounds good. At least once a night, the amp just starts letting out a loud continuous shriek....usually in the middle of a song. If I flip the standby switch on and off it goes away immediately. Usually happens in response.....maybe just coincidently...to me switching something on/off on my pedalboard. It's not the board as I use multiple amps/different projects..the Twin is the only amp that does this. Amp was "black faced" by George Alessandro a few years back.
Any help would be great..
 
Last edited:

TweeDLX

Member
Messages
3,756
First thing to check is V2. Swap it out for a tube you know is good. You can use V1 just to check. It might be getting microphonic. Second option involves tech work: You may have a shorted connection in the circuit. They can sound "shrieky". Third, try a different speaker cab. I've had speakers do that shriek thing.
 

wingobingo

Member
Messages
274
First thing to check is V2. Swap it out for a tube you know is good. You can use V1 just to check. It might be getting microphonic. Second option involves tech work: You may have a shorted connection in the circuit. They can sound "shrieky". Third, try a different speaker cab. I've had speakers do that shriek thing.
Thanks Mike...much appreciated. Which preamp tube sockets are v1 and v2 in this amp? And I assume both take 12ax7s.
 
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4,819
Looking at the back of the amp, V1 is to the far right. V1 is the input preamp tube for the Normal ch. V2 is next to it and is the input preamp tube for the Vib ch. V3, a 12AT7, is the reverb driver tube. V4 contains one triode that is the 3rd gain stage for the Vib ch and one triode that is the reverb return tube. V5 is the ttrem tube. V6, another 12AT7, is the phase inverter. V7-V10 are you 6L6 power tubes.
 

TweeDLX

Member
Messages
3,756
Thanks Mike...much appreciated. Which preamp tube sockets are v1 and v2 in this amp? And I assume both take 12ax7s.
You're welcome. You might want to check the tubes (V3/V4) for the reverb as well as the connections from the amp to the reverb tank. Those can develop issues.
 

wingobingo

Member
Messages
274
Thanks everyone. Swapped in a good 12ax7 and all is good.One question...the tube that removed was a 7025. The chart in the amp also specifies 7025. Is the substitution of a 12ax7 ok?
 

TweeDLX

Member
Messages
3,756
Thanks everyone. Swapped in a good 12ax7 and all is good.One question...the tube that removed was a 7025. The chart in the amp also specifies 7025. Is the substitution of a 12ax7 ok?
Yes. A 12AX7 is a perfect replacement for a 7025. The 7025 is simply a "quieter" version of the 12AX7.
 




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