Tube Amp Archive

Unsolder the brown wire from the Reverb pot wiper. Measure the resistance to ground of that wire. it should be about 700K. If the 470K resistor is shorted you'll get 220K. That would allow the condition you describe. The amp should work fine with the reverb cables unplugged. Just no reverb. Unplug ...

Have you validated this condition or is this what the owner is reporting? Your last pic of the chassis does not show that the reverb mixer has been modified. I can't see the pot, but if it is connected according to the schematic it would be very difficult for it to act as a master volume--even if i...

D'oh! I sent the Twin Reverb Amp head home with its owner, only to discover the Reverb control acting like a Master Volume control. My first thought is Reverb Tank - but I have no idea. Does anybody have any knowledge as to what's causing this? Reverb driver? Reverb recovery? How do I fix it? Thanks...

Looks great. Nice work. Where did you end up with idle current? Thank you, Smitty. I appreciate the acknowledgement. :D As for idle current, I did NOT re-bias this amp. Ill-advised move? Yes, indeed. Why didn't I? Allow me to explain... TL; DR. :) The amp's "liaison" - the gentleman that brought th...

'69 Fender Twin Reverb Amp head repair job completed. :D https://sodacity-amp-repair.com/Media/20250307_093656.png https://sodacity-amp-repair.com/Media/20250307_093612.png https://sodacity-amp-repair.com/Media/20250306_221657.png Thank you, all, for your admonition and assistance. - sssmoka

Your work looks good. As best I can tell Fender switched from 25uF to 5uF after the ac568 but before the AA270. Either value will work (or anything in between for that matter). I don't think a 25uF is wrong. I think that Fender probably decided that with a 100K cathode resistor a 5uF capacitor woul...

Referencing schematics, the AB763 has a 25uf/25v Vibrato cathode bypass cap while the AC568 has a 5uf/25v Vibrato cathode bypass cap. What's on my bench has a 25uf/25v white plastic Mallory that matches the rest of the cathode bypass caps. Going to replace them all, just want to get the value right....

F&T AC Filter capacitors installed.





- sssmoka

Opened the doghouse this morning. It's time for caps... https://sodacity-amp-repair.com/Media/20250226_072529.png At least one is leaking electrolyte! As such, I'm rather sure it, and possibly others, are leaking current, as well... https://sodacity-amp-repair.com/Media/20250226_072511.png I'll repo...

Do you have a method for checking for leakage? I do NOT have an easy or simple method to check for cap leakage - as far as I know, I have to remove the doghouse, discharge the caps, disconnect a cap lead and test the cap for leakage. I'm sure there's a simpler way I'm unaware of. Your knowledge her...

This looks to be an AC568 circuit. It's got the funky output tube cathode network. I'm going to guess it's a 1969. Marking on the chassis is a hint. Here is a link to the schematic. http://www.prowessamplifiers.com/schematics/fender/twinreverb_ac568_schem.pdf I recommend replacing all of the mallor...

Hello, all. I do hope this memo find everybody alive and well as the alternative seems rather void. I am in need of some advice... I've been fortunate enough to have the opportunity to service an original '69 blackface Twin Reverb Amp that has been converted to a head at some point in time during it...

So, whilst I was probing and scoping gain stages, I noticed that my scope was displaying odd readings (intermodulated frequency included) at V1A pin 7 during my signal generator input - before I even cut the amplifier on. What was in line between input jack 1 and V1A pin 7? Nothing...except a 1 M O...

So, whilst I was probing and scoping gain stages, I noticed that my scope was displaying odd readings (intermodulated frequency included) at V1A pin 7 during my signal generator input - before I even cut the amplifier on. What was in line between input jack 1 and V1A pin 7? Nothing...except a 1 M O...

As a resolve, I took two stereo jacks and wired them accordingly to the schematic... A stereo jack is functionally different from the jacks on the schematic. They are not interchangeable just because they each have three lugs. Thank you for pointing that out, sluckey. Perhaps, for clarity's sake, I...