Thank for the info.They'll both run into the same load and they both have similar plate characteristic curves so the bias supply should not need to be altered. The main thing to watch for is that the screen voltage under load does not exceed 500V. If it does, it would not be a good idea to install 6550s in that amp unless you drop the power supply voltage.
The voltage was about 480, so I put in the 6550s, biased, and it sounded great. This is a 2204 clone I bought and it came with KT88s, but I want to modify it for EL34. I already have a 2204 clone with 6550s. I was afraid it was built with a transformer for KT88s, but it doesn't seem to be the case...
From what I read for EL34s it should be: 220k bias, 220k supply, 56k bias pot. Is that correct?
It's a little funny how this amp is set up. The resistor by the bias pot is 68k, the bias resistors are 100k and the bias supply is 220k. It looks like it's a little bit of everything in there...