glip22":1fosjldt said:
If an amp is designed around an EL34 you should treat any other tube as an EL34 when biasing. I know you are using KT77 so this really doesn't apply. A lot of people think because they install 6550 in their Marshall they should bias the 6550 as you would in a 6550 amp. Not the case. The OT has the impedance for EL34, so biasing a 6550 as you would normally will not mean the tube is operating as it should. You will not get more output either with KT88, 6550, or 6l6 in an amp with an EL34 transformer.
Never heard this before. Especially since as I understand it (Mr. Diezel can elaborate if he wants), amps like Diezels that are designed to accept any tube type you should bias a tube according to its specific design specs and plate dissipation. It's not like the transformer was designed to operate for one tube type. But I do know that some amps—like my Peters Hydra—if you use 6L6 tubes, it is recommended you bias to how the tube should be biased but then half the speaker output impedance. For example if you're running a 16ohm cab, use the 8ohm output. But this isn't for reliability or anything it's just that the sound/feel of the amp will change slightly.
But 6550s should be biased much higher than an EL34. They would sound pretty awful if you biased them at 30mA per tube... Super cold.