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Help, Ampeg V-4 weirdness (a wee bit long)

8/14/1999 6:14 PM
Help, Ampeg V-4 weirdness (a wee bit long)
Got a 1973 Ampeg V-4 (Magnavox, Tennessee made with some weird symptoms originating in the V210 - 6K11 triple triodes stages. When I received the amp it had a non-sinusoidal oscillation of a few Hertz that appeared in the output stage. Once I opened the beast up I noted that C208 (high pass cap from plate of 6CG7 reverb driver to ground) had incinerated itself and scorched C209 (reverb drive coupler). I replaced these (still don't know why they fried) and then started working on the oscillation. Found out that the bias was off on the second 6K11 stage due to a leaky blocking cap, C202, which cut down on the oscillation greatly - but it's still there. Have pulled or replaced all of the caps w/o change, have also changed the 6K11. Funny part, when I temporarily pulled C201 from circuit (DC blocking from 12AX7 input to first 6K11 stage) I got a really nice "squeal" type oscillation. I replaced the cap and I now get a "charge up" oscillation as the B+ builds - same thing with the original cap back in circuit (remember, this wasn't an original symptom). Also, I've tried decoupling stages with a temporary electolytic filter (80 uf, 500v) across the B+ feed and across various parts of the circuit with no effect on the original symptom nor the new squeal. I was able to quench the new squeal by hooking a 0.22 up cap across the second 6K11 stage to ground but this would also kill the normal signal when in use.  
So, I'm frustrated and I've resorted to just pulling/ replacing parts - a sloppy technique that my pride just doesn't easily swallow, and still no change.  
Ampageurs - I'm at your mercy, got any ideas?  
8/21/1999 5:08 AM

I have a V-4 on my bench in for outpt tube replacement, mains switch replcmnt, full cap job- somebody had V-4B-ized it... I had a badly oscillating VG-22 in here a while back, it would seem fixed and then when bolted back in to the cabinet it would do it again on one channel I dont like the flimsy ampeg construction.. My EKO AC-30ish amp needed a 4700 picofarad cap across the 1st gain stage plate load resistor to keep it from freaking out maybe yer V-4 needs something like that...

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