Not long ago, I was at rehearsal. I'd brought my 200S along to play through my 200S bottom I keep at the studio. Imagine my distress when I noticed that the power tubes were red plating! I grabbed a screwdriver and cranked the bias up, which helped, but the plates were still glowing. I shut it down and used the Fender Bassman.
Finally got around to looking at the 200S a couple of days ago. Voltages all looked normal, compared to the voltages I measured last time I had it open. I looked at the schematic, but didn't see anything particularly out of line, except that all of the voltages were higher than on the schematic. That's to be expected, given today's higher line voltages and the mods I performed a few years ago, upgrading the power supply capacitors and replacing the rectifier tube with diodes.
That got me to thinking, though. The amp was originally designed to use KT88 power tubes. Later, Sunn went to 6550 tubes. That led me to take a look at the tube specs.
KT88 tubes have higher voltage specs than 6550's do. In fact, the maximum voltage for the KT88 plate is 800 volts, while for the 6550 it's 600V. With the mods, the plate voltage with the tubes removed is over 600V, and quite close to it even with the tubes in place. When I did the mods, I had KT88's in the amp. I pulled those when my Model T took a dive and one of the tubes broke. I replaced the KT88's in the 200S with some old 6550's I had laying around. So I brought home two of the KT88's and put them into the 200S. No red plates. Amp appears to be working fine.
Moral of the story? Sunn was running the 6550's pretty close to their max ratings even with lower line voltages and rectifier tubes. With rectifier tubes, I imagine that 6550's would still be fine. But if, like me, you opt for solid state rectification, KT88's are the only way to go.
