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Hi all...so I been working on a friends SF Twin reverb...it was all but toast when I got it. I rebuilt the whole board to AB763 specs, got new tubes and plugged her in, it had sound (positive!) the reverb and vibrato worked again (double positive!) BUTTT...it doesn't break up properly..(negatttiivve) here are the symptoms:
On volume 1-2 it sounds ok, a little soft but clean. At 3 instead of good old fender breakup, the amp produces an ugly broken speaker type fizz at the clipping point, a clean undertone is still present beneath the fizz. at 5-6 the amp reaches maximum volume, not really much louder than 3, you can hear where the amp is breaking up, but like before instead of nice solid crunch it clips out with a fizz, not a very loud fizz either. Again there is an almost separated clean undertone underneath, the clean is slightly louder than the fizz. At volume 10 its exactly the same as 6. Even at 10 the amp really isn't that loud, comparable to like 3-4 on my 50W bassman....
Here's what I think it's not:
Pre-amp circuits, it happens on both channels.
PI section, all values and voltages check out.
Speakers, I played through a separate cab.
Power tubes? I dont think so, I had brand new JJ's and older GT's what I am gonna do it try them 2 tubes at a time...
I was thinking output transformer, but on reading a few posts here it sound like it might be that parasitic osscillation thingee....
Any thoughts? Its really driving me nuts trying to figger it out!!
Thanks!!
On volume 1-2 it sounds ok, a little soft but clean. At 3 instead of good old fender breakup, the amp produces an ugly broken speaker type fizz at the clipping point, a clean undertone is still present beneath the fizz. at 5-6 the amp reaches maximum volume, not really much louder than 3, you can hear where the amp is breaking up, but like before instead of nice solid crunch it clips out with a fizz, not a very loud fizz either. Again there is an almost separated clean undertone underneath, the clean is slightly louder than the fizz. At volume 10 its exactly the same as 6. Even at 10 the amp really isn't that loud, comparable to like 3-4 on my 50W bassman....
Here's what I think it's not:
Pre-amp circuits, it happens on both channels.
PI section, all values and voltages check out.
Speakers, I played through a separate cab.
Power tubes? I dont think so, I had brand new JJ's and older GT's what I am gonna do it try them 2 tubes at a time...
I was thinking output transformer, but on reading a few posts here it sound like it might be that parasitic osscillation thingee....
Any thoughts? Its really driving me nuts trying to figger it out!!
Thanks!!
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