Marshall 4150 Bias current changes on half pair after 20 min

[kevania]

1) I have a marshall 100w amp with el-34's. I've had it since the late 80's, and used it occasionaly but never found it very inspiring tonally. Thought it might be a decent candidate to change into a 1959 with a metro amp board kit and an additional preamp tube (the 4150 only has two) but haven't got that far yet. I wanted to would check the basics first, such as proper output tubes and biasing.

2) Measured 451v on all plates, -42v at bias pins. Power tubes were a mismatch set of various brand el-34's (2 ruby's and 2 service master's which say made in w.germ). I haven't found much info on the 4150 but i have a schematic and it shows resistor values for el-34's are present in my bias circuit, not for 6550's.

3) Cathode resistor bias measure** with the old tubes was 21ma, 25ma, 9ma and 13ma. after installing a matched set of EH tubes, I measured 24-25ma on all tubes. i'm pretty happy as it seems all the measurements are much more consistent and i believe in the right range.

4) I did turn the bias current up to 28-29ma before pluggin in. It sounded much nicer than with the old tubes, more detail. No funny noises. Plate voltage dropped by about 2 volts after the bias current was adjusted but was still virtually identical accross each tube.

5) After about 15-20 minutes playing on half to three-quarter volume in very low light, I see that the right two tubes are running too hot and starting a small glow spot on the plates in each right side tube. The left pair does not. I shut down right away.

6) I backed off on the bias trimmer a little, Voltages have not changed but now the bias current on the new tubes measures 25-26ma on the left side and 10ma and 15ma on the right. Not too different than with the old tubes! I haven't had a chance to measure any resisters yet but none look burnt.

7) the amp appears all stock.

**I used a 1 ohm 1 watt resistor from ground to pins 1 and 8. I am not entirely sure pin 1 should be connected to pin 8 before going to ground through the cathode resistor but the metro amp 100w assembly manual showed it that way as a revision to the previous metroamp assembly manual and claimed it was more accurate this way.

Any thoughts?

Kevin