Ampeg Rocket bias problem

[tubessence]

I'm working on my friend's Ampeg Rocket, The 6V6's were pretty well burned up so I replaced them with a matched pair of GT 6V6's. I checked the bias and was getting 44.6 mA and 40.0 mA so I switched tubes and got 43.8 mA and 44.0 mA much closer, but way too hot! So I decided to add in 100 ohms to the bias resistor for 350 total which changed my bias current to 35.9 mA and 36.3 mA which seems to be the answer but still concerns me, what would you do?

[tubessence]

I guess I should add that my Ep started at 332v and went up to 348v after mod. Ek went from -22.4 to -25.7 and is supposed to be -18 WTF? Is it just cheap Chinese tubes as opposed to NOS?jeff

[Enzo]

When you reduce the current through the tubes, the B+ supply voltage will rise. That is normal.

You increased the cathode resistance to reduce the current through the tube. The tube bias is develped across that resistor. Increasing the resistor will perforce raise the voltage across it. SO cathode voltage rises as well.

I am hoping your cathode voltages are not really negative as you indicated in post #2.

As always, tube dissipation is based upon the current through the tube and the voltage ACROSS the tube, so the higher B+ is partly compensated by the higher cathode voltage. But you have to recalculate any time you make a change.

All my schematics on the Rockets are for 7591 outputs. I find the R12R at Schematic Heaven with 6V6s, but it is a Joe Piazza drawing, so I don;t know how accurate it might be. None of them have voltages specified, so I don;t know what to say about 18 volts. The voltage is not what is important, the tube dissipation is. SOme tubes run hotter than others.

[mikhett]

my 65 rocket is biased

with 22 volts at the cathode with a 250 ohm cathode resistor. I calculate about 40 ma thru each 6v6. the tube dissipation is 12.84 watts. Its a little high but it doesnt red plate & sounds great. Mike