In Reply to: Its not the input tube... posted by Nickel Core on November 10, 2014 at 10:23:43:
I don't know the circuit, but here's a scenario:driver tube has a PSU filter (RC) which phase-shifts its output relative to fluctuations in the PSU voltage by up to 90 degrees.
Driver output goes through a CL high-pass filter (coupling cap and grid choke) giving a further phase shift of up to 180 degrees.
Output tube has a 180 degree phase shift from grid to plate.
Total phase shift is up to 450 degrees, and at 360 degrees you have an enthusiastic oscillator.
Edits: 11/10/14
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Follow Ups
- RE: Its not the input tube... - Paul Joppa 14:32:55 11/10/14 (1)
- RE: Its not the input tube... - Nickel Core 07:56:41 11/11/14 (0)