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I just got done rebuilding a 1967 Sunn 200S with new lytic caps, tube sockets, 3 wire mains mod, new tubes, new voltage divider, transformer mounts and ground rivet replacement. It all came out very nice. The power supply works great and strong, the amp works pretty good as well but there's one thing that is bothering me.
I'm getting instability or oscillation or whatnot on the lower half of the push pull pair just before clipping using a 1Khz sine signal. Attached is a blurry shot of the scope trace with the regions of the instability circled. Sorry about the blurry photo but my camera technique is crap.
Has anyone had this experience and knows what up with this, should I worry about it? The areas circled gyrate radically and that's why there's just a blur in the circled regions of the waveform... it's difficult for me to get a good clean shot. Also the amp goes into clipping and distortion too low (about 4 on the volume, tones centered with a 250mv sine input) in my opinion but it sounds pretty good beyond the clipping point... nice and Sunn like. One other little thing is that I bias with two ma meters, one on each tube and another 1kv meter on the B+, the cathode currents are not perfectly matched but are within 5ma of each other. I've set the bias for 70% or thereabouts.
All the voltages measure good, pretty even from side to side and all the associated resistors from the phase inverter and bias circuit measure within tolerance.
I'm getting instability or oscillation or whatnot on the lower half of the push pull pair just before clipping using a 1Khz sine signal. Attached is a blurry shot of the scope trace with the regions of the instability circled. Sorry about the blurry photo but my camera technique is crap.
Has anyone had this experience and knows what up with this, should I worry about it? The areas circled gyrate radically and that's why there's just a blur in the circled regions of the waveform... it's difficult for me to get a good clean shot. Also the amp goes into clipping and distortion too low (about 4 on the volume, tones centered with a 250mv sine input) in my opinion but it sounds pretty good beyond the clipping point... nice and Sunn like. One other little thing is that I bias with two ma meters, one on each tube and another 1kv meter on the B+, the cathode currents are not perfectly matched but are within 5ma of each other. I've set the bias for 70% or thereabouts.
All the voltages measure good, pretty even from side to side and all the associated resistors from the phase inverter and bias circuit measure within tolerance.