This amp blew up on me a while back. I replaced the shorted diodes, and I've done the Cathode resistor mod to ease in the Bias reading. I've got the amp on, but the measurement I'm getting is unstable and jumps all over. These are NOT new tubes so I'm thinking that could be the problem. Right now I've got the bias control in about the middle.

Plate voltage reads- 458
These are JJ KT77 tubes.
Can anyone shed some light on this dilemma?
what you're trying to measure, is the voltage between the two lugs of your cathode resistor (i'm hoping it's 1ohm or something similar). it should be about 15~50mV (millivolts), and you set the bias with that.

if your measurement is unstable, you probably have the input wide open. plug into your guitar, and put the guitar's volume knob to 1. that should make it stable.
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Thanks Shot!
The input was wide open. I also had the bias way too low even though it was near the middle. I've got it set to 44mv now and it is sounding great so far. Might not need a tube change after all.