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In Reply to: Re: Help! What has happened? posted by Try-ode on April 5, 2007 at 08:40:30:
Seems like your EH tubes need a different bias setting. From your description, they're drawing far too much current. It's not unusual to have to readjust the bias when changing output tubes and it doesn't necessarily mean that either set of tubes is bad, they're just different.The safest method is to turn the bias controls to minimum current setting, then increase the current slowly for each tube until it seems about right, leave it running for an hour or two, then readjust if necessary.
Follow Ups:
With multiple parallel pairs, you need to be sure one tube isn't hogging the current. So adjust each one carefully to spec, and re-check when you're done.
Hey guys,Sorry for the delay. The tubes are not single bias, they get biased via one adjustment on the back.
and you expect it to be stable? looks like your amps should have been a bit more expensive.
Whatever.....
you have 3 tubes hanging off each grid resistor. These resistors are evidently connected to a single pot. There's be no circuit penalty for using a pot per side of the PP pair( just cost ). If you treat all 6 tubes the same and they're not, what do you expect to happen?Building a driver stage that can drive a 3x parallel set of grid circuits(needed for proper idividual adjustment ) is not going to be as easy, or cheap. You get what you pay for.
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