Today I was changing out the 4 coupling caps on my personal ST-35 amp to some Russian PIO coupling caps. I did one channel and thought "OK - maybe I will just check out that channel to see if it works OK". So what I did was to put in the driver tube and the two EL84 output tubes for just that (it was the left) channel and check to see if the channel with the changed out caps played OK. I left out the tubes for the right channel. No sense in putting them in because I know that channel plays OK. I turn the amp on and the left channel is playing OK at first but after a minute or so the sound is distorted and I quickly notice that both EL84 output tubes have cherry red plates 
> a sure sign that those two tubes are drawing WAY too much current. What the hell is wrong? And then it dawns on me ... In the ST-35 with the stock bias system all the current for ALL FOUR OUTPUT TUBES is drawn up through a single SHARED 95 ohm bias resistor. The current drawn up through that resistor (maybe 140 to 160 milliamps) is supposed to be shared by FOUR output tubes. Each output tube is supposed to draw about 35 to 40 milliamps apiece. But now, with only TWO output tubes in there, each of the TWO output tubes is getting 70 to 80 milliamps apiece which is DOUBLE what they are supposed to draw. No wonder those two tubes lit up cherry red on the plates. What did Forrest Gump say? > "Stupid is as stupid does .." I was lucky because the tubes did not burn up and were OK after the altercation.
As a side note - this "running a single channel problem" won't happen with a stock ST-70 which has a separate bias resistor for each channel or a VTA boarded ST-70 which has a separate bias control for each output tube.
Bob
> a sure sign that those two tubes are drawing WAY too much current. What the hell is wrong? And then it dawns on me ... In the ST-35 with the stock bias system all the current for ALL FOUR OUTPUT TUBES is drawn up through a single SHARED 95 ohm bias resistor. The current drawn up through that resistor (maybe 140 to 160 milliamps) is supposed to be shared by FOUR output tubes. Each output tube is supposed to draw about 35 to 40 milliamps apiece. But now, with only TWO output tubes in there, each of the TWO output tubes is getting 70 to 80 milliamps apiece which is DOUBLE what they are supposed to draw. No wonder those two tubes lit up cherry red on the plates. What did Forrest Gump say? > "Stupid is as stupid does .." I was lucky because the tubes did not burn up and were OK after the altercation.As a side note - this "running a single channel problem" won't happen with a stock ST-70 which has a separate bias resistor for each channel or a VTA boarded ST-70 which has a separate bias control for each output tube.
Bob
