John,

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It could be a cold solder joint in a tube pin.

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If it was an interelectrode short, you could use a DMM

to explore.

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If it was a defective plate blocker it would

show up with a meter, out of the circuit.

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You can run 1 tube at a time, no problem,

and do each separately from each socket.

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If there was a bypass cap short this might also

be seen, as the bypass circuit will mess up on

both tubes at the same time, but only on one socket

at a time. Again, use a DMM to trace.

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When you run 1 tube at a time, note the ammeter

readings at 1 watt, 5 watts, 10 watts, etc., and

compare.

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Hal W4HBM