Troubleshooting the Blue Angel
I loaned my Blue Angel amp to my son a while ago. It was fine when it left my house. When he returned the amp last month, the 6V6 side (the amp is switchable from 2x6V6 to 4xEL84 or all six tubes at once) wasn't working properly. I'd get various odd noises when I took it off standby, the power level would jump up and down, and once in a while I'd get some redplating in one of the 6V6s. If I switched it to the EL84 side (or even the combined setting) then the problem would go away.
The schematic for the output section is here: http://www.tubefreak.com/bluangpw.gif.
The only components unique to just the 6V6 output are the cathode resistor, bypass cap and one pole of the output selector switch. The resistor and switch checked out visually and electrically, and the cap not only looked OK but also survived being prodded with a chopstick without causing any noises. I didn't really think the problem was in the cathode circuit and I didn't want to go subbing components, so I put it away for a while.
I pulled the amp out again tonight to play it. I figured I'd run it on the EL84 side. Again, it was OK until I tried switching to the 6V6s. I did a quick web search and didn't find any evidence of others having this problem. Thinking about the circuit, I reasoned that the PI is always driving all the output tube grids regardless of whether the low end of their cathode resistors are floating or switched to ground. Perhaps something was wrong with the PI, in that it couldn't drive the grids of all six tubes. For a first check, I pulled all four EL84s because they're easier to get to than the PI. There were no more problems with the 6V6 side. Good. So I swapped in a new 12AX7 for the PI and replaced the EL84s.
I thought I had the problem licked. No odd noises on power-up, no power loss... the 6V6 side sounded good... for about ten minutes. Then the power dropped and the amp started to hum. No redplating, but I could tell that one of the 6V6s was running much hotter than the other. I also noticed some microphony right at the onset of the problem, so I quickly gave each output tube a tap and found that one of the EL84s was badly microphonic. I also noticed some bits of foreign material bouncing around inside that tube's envelope. So I pulled that tube and its twin, and tried again. No problems at all. Put back the original PI just to be sure. Still OK.
So, I have a bad EL84 that works fine when it's switched on, but screws up the 6V6 side when it's switched out. My guess is that the bad EL84 is slightly gassy, even though I can't see any flourescence when it's operating. The leakage would pull up the grid, which would pull up the corresponding 6V6 grid, throw the output stage way out of balance, and possibly cause the corresponding 6V6 to draw enough current to redplate. I figure that when the EL84 side is switched on, having the cathodes nearer to ground (they could float up, I suppose, when disconnected) would tend to absorb most of the leakage that was otherwise finding a path through the grid circuit.
At any rate, I'm going to try a fresh quad of EL84s and see whether the problem stays away.
Update: That was the problem, alright. A fresh quad of EL84s, and everything's fine.