Altering the bias changes the standing current in the output tubes. If this changes the *level* of the buzz, then that indicates a power supply problem, most probably - but not necessarily, given the on-going work that's been done around the rectifier circuitry - to do with the filter caps. If altering the bias pot actually does affect *pitch* - ie frequency - of the buzz, then you've got a problem, as this would indicate that something is oscillating at low frequency ...
Arfa
It changes the frequency not the amplitude. Changed out *all* caps in the power supply, replaced the sarkes tarzian units with 5R4 tubes, double checked the replacement silicon diodes - they're fine, all power outputs are spec at the amp under load. It's gotta be something in or around the 6DR7/0A2 part of the circuit. Used a 2.0uf Auricap for C20 but C21 is an electrolytic. Maybe replacing C20 with a
2.0@450 would do the trick... Checked all resistors in and around the power amp circuit and all test good and within tolerance.
Low frequency oscillation, once called motorboating, might be caused by feedback along the supply path. Try a fat capacitor added to the supply to see whether that occures. Also check other de-coupling caps, shunt them with a bigger one to test.
Been troubleshooting this with an oscilloscope and I see AC on both
12.6V lines - as well as on the 600V line. But - if I place a 300-600 ohm resistor across the monitor amp output (15/16 on the amp schematic on the term. strip) the buzz goes away and both power amp and monitor amp work perfectly. The -38 from the PS checks clean and the 6.3 AC sources are, well, AC... but I can't see why I'm getting AC taint in the 12.6 lines and the 600 plate supply. It just doesn't make sense. The 80/450 caps are NEW and have been lifted, checked, tested, etc. and they're good - I also replaced all the other caps in the circuit and STILL get the motorboating noise when the HV relay closes. If I ground certain points in the circuit - particularly the .047 uf caps in the monitor amp circuit the buzz also goes away (using a 10 uf 600V to the chassis), so I'm thinking the real problem is in the amp somewhere in the monitor circuit where it connects to the EQ card - when the card is out, everything works fine but when it's plugged in the buzz comes back. This one's got everybody stumped I guess...Have something to add? Share your thoughts — no account required.
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