KT88's Redplating

[clyde1]

I built this amp several years ago, gigged extensively, never a problem until recently when one of the power tubes shorted. I installed a new pair and immediately upon trying to bias, either tube will redplate before even getting close to optimum bias. I've disconnected the coupling caps from the PI, no difference. Using the shunt method, once I get much beyond 10ma, 535va, 510vs, -48v bias voltage, the tubes protest by turning red. I'm rather stumped.

Screen resistors have been bumped to 1.2k, fed from a 1k off the choke. Previously they were 100R, fed from a 680R and voltages were over 600 for both plate and screen.

[g1]

Can you try series rather than shunt current measurement? What kind of resistance readings do you get for the OT primary?

[clyde1]

Yes I thought it might be the OPT also, I measured 100R plate to plate and about half that, plate to CT. This seems pretty normal. The choke also measures about 100R. As a rule I always bias using shunt method, never had a problem before.

[clyde1]

Well my trusty Fluke 87 let me down, measuring slightly under 1/10th of what it should, so when it registered 5.8ma, it was actually over 60! I use 2 meters when biasing, the second for plate voltage and when I reversed their roles, it all became clear. The hv measurements are identical between the meters, so it seems to be just the mA that's messed up. Had me scratching my head for a few days and cost me a set of tubes. Why couldn't they have been Sovtek 6L6's?