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Makes great sense Enzo... I will do this. However, I'm a little concerned as I just have a x10 O-scope probe and a 22?? series Tek scope. Is it safe to measure 500v with this setup? What do you use to scope the high voltages in these amps? I have a high voltage probe but it connects to a fluke DVM.
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Replaced the 22uf/500v caps and that didn't fix it. I still get the horrible screeching on both channels when the Bright switch is turned on (both channels) and the treble pot is increased above 3.
It sounds OK when the bright switch is turned off and the treble is turned low (both channels) Although it is noisy...lots of buzz.
I scoped the B+ and it is a smooth DC. Maybe I'll try the small electrolytics in the pre amp stages, although its got to be one that is common the both channels.
Again, the reverb is turned down to 0 when the symptoms occur.
I'll keep plugging away and eventually I'll get it.
CJComment
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Did you check the DC voltage at the plates? Does it still feedback with V1 pulled? What about with V1 in and V3, V4 and V5 pulled?Comment
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You can't play an amp that has to have the treble turned down.
As Melvin says check the dc voltages especially V1 & V2 (as this seems to affect both channels).
How did the grounds check out, what ohms did your meter read?
Where is the buzz coming from, preamp or output? Pull the PI tube, still buzzing, then it's the output. Pull the rest of the preamp tubes and install one by one from v6, v5, v4 etc...at what point does the buzzing start?
Unlikely to be a layout issue otherwise they'd all do it, grounding is still the prime suspect.Comment
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Just a hunch: pull V4 and measure the resistance between V4 socket pin 7 and the chassis.
Looking for grid leak weirdness there.
Along similar lines, measure resistance from the low side of each volume pot to the chassis.Comment
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The problem has been solved.
The problem was V4. Going on Melvin's recommendation, I performed a systematic series of pre amp tube shuffling. The problem went away after removing V4 and swapping it with V5... then I confirmed this by replacing V4 with a known working 12AX7. I've been running at 5 on all knobs on both channels for about 3 hours. So far it sounds normal.
I'm a little disappointed in myself for not doing this in the beginning, but every time I work on tube amps I learn a little more each time. Like, a systematic approach of tube swapping first, then dive into the circuit and suspect power supply caps, grounding, etc....
Thanks again for everyone's help. It has not gone unnoticed!
CJComment
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