Ampeg B15n--whats the output bias supposed to be?

[marku52]

working on a very nice old B15N (cathode bias on 6L6s--first time I've seen that!), the cathode resistor bypass electrolytic spewed its guts out, blew the mains fuse. I've been powering it up slowly to restore all the electros (hadn't been played in a long time). As I get up to about 80-90V on the variac, it looks as though the output stage is starting to run away (10V across the 250 ohm cathode R means 40mA and still heading north...) I shut it down to think about it. Coupling caps are OK (no DC at the grids of the 6L6s....Cathode R reads 250 ohms like it should. tubes tested OK on my not-terribly-good tube tester.

Anybody know the standard output bias for this beast? Not having any luck there, I'll have to get out a 6L6 set of curves and work out what theory says it should be....

thanks in advance for any help!

m52

[Enzo]

Yes, except there are two 6L6s on that resistor, so 40ma means only 20ma per tube.

I bet it is fine.

Your B+ is what, about 450v? COnsidering you have to subtract the cathode voltage from that to get the voltage acros the tube, I'd probably expect something in the 35-40v range at the cathode.

[marku52]

thank enzo

But I was a little worried because i was at 20ma/tube with only 80-90VAC in, and it seemed to be running away. I guess I could just run it on up to 120VAC and carefully watch the cathode V. Ampeg only has a 50V cap specced there, so I do know what the absolute upper limit is....

[Enzo]

The trending may or may not be troublesome, but you are there to watch it, so find out.