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Fixed bias problem, and no, it is not the tubes.

In my newest amp which is SE with fixed bias. The power tubes kt88, kt120 and even kt66, the bias starts drifting after the tubes warm up after 5 minutes. I know it is not the tubes because i use the exact same tubes in another fixed bias SE amp with roughly the same design and they were stable after many hours of use and at the same bias currents.
Both amps are designed for kt120 outputs but with fixed bias i can simply dial down the current for the lower power output tubes. They both B+ voltage of around 350vdc. They both use 6sn7 drivers, but one has cathode resistors while the other uses LEDs to set the driver tube bias. The bias voltage is obtained from a reversed 6.3v filament trans of about 1.0A rating. the AC is full wave bridge rectified with Schottky diodes and filtered by a PI filter for 1 amp and a choke input filter for the other. Both circuits have roughly -60vdc feeding the adjustment pots. Weird thing is i had it playing at the shop with 6l6gc for an hour with no bias drift. I brought it home and problem showed up.
Any help or suggestions are appreciated, thanks in advance, Dak


Edits: 01/11/17
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Topic - Fixed bias problem, and no, it is not the tubes. - DAK 18:00:49 01/11/17 (54)
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