#1
I have a mesa single rectifier (series 2) and it has a fixed bias that i can switch between 6L6 and EL-34 tubes. Am i only limited to those two types of tubes or can i put other tubes like 6550's and 5881's in it too or could that harm the amp?
#2
5881s are 6L6s. Or at least modern production ones are. 6550s could cause problems. They'll probably want to draw too much bias current and possibly too much HT current as well.
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#3
6550s might not even fit in the mesa. The only mesa I know that takes 6550's (at least the mesa 6550's) are the bass amps.
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#4
Oh ya, forgot how narrow the spacing can be in some of their amps. Might be able to cram one in though. Some aren't that much bigger in diameter than a 6L6.
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#5
I've seen people put KT-88s in Rectos, but don't think I've seen 6550s in one. I've also seen KT-66s in them before. From what I've read, it's not really safe to put the 88s in the stock amp in the first place, but I guess people do it anyway
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#6
6550's are more akin to a KT88. However, width wise I don't think there'd be much in it between a KT66 and a 6550. If a KT66 fits I reckon you may get away with it.
Mesa have pretty hefty transformers don't they? I'd be surprised if they didn't work ok. I don't know enough about Mesas to make a definitive comment on it though.

You should be able to run KT66 ok too. Whether you'd want to or not is another question. It's not like you're going to be trying to do beano on a recto.
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#7
^I'm pretty sure the trannies are fine, I think the issue with running the big tubes was in the screen grid resistors?
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#8
Yeah, a 6550 needs a decent screen resistor, up like about 1K. A 6L6 will run ok on a 1K jobbie but I don't know what a recto has in there. Like I said, I don't know Mesas that well.
Might have trouble with the biasing too, in fact how could you not? You'd be wanting to check it properly if you were going to try it.
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#9
1K/2W screen grid resistors on the Rectos. The PT should be able to handle it, but I don't think 6550s would sound very good biased so cold.
#10
kt77s should be a drop in replacement for EL34s.
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