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Re: Finally got my OTL apart to check suspected bias shift in one channel

If this is an Atma-Sphere style circuit, the output section bias is derived from the cathode voltage of the driver tube; 12AU7 in the old days, 6SN7 today.

Compare plate, grid and cathode voltages between the driver tubes in each channel - mainly the grid voltages. Take some notes, then swap drivers between channels and compare the results.

It's a rather simple circuit, so if voltages are way off (20V or so in your case) it's fairly simple to track it down; bad resistors, bad coupling caps, etc. And of course, the condition of the driver tube is critical to maintaining stable bias.

Bill


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