need help mark 3 coli problem

mark 3 coli, simul, eq, running all 6L6 tubes

played the other day for about 30 minutes and shut amp off. Turned back on about 5 minutes later but did not use standby switch (thought amp would still be warm). amp made a funny humming noise, so turned standby switch on and let it warm up for a few minutes. turned standby switch off and guitar had very little volume and increasing hum from amp, then something flashed in the back of amp and fuse blew. unplugged amp. next day replaced fuse, turn on amp and all tubes glow normally. let warm up for a few minutes then switched standby switch off. all tubes start to glow blue and amp starts to hum again. did this twice, only kept it on for a few seconds as hum got worse and afraid of blowing something up.

diagnosis?

thanks!

bad power tube(s) imho.

I agree...

Turn off the lights in the room, and follow the steps you just mentioned once again, this time standing in back of the amp to see which power tube/s flash in an unusual manner.

boogiemon said:

bad power tube(s) imho.

Click to expand...

Yep. I suggest replacing...or selling you Coli to me... 8) :lol:

Elpelotero said:

I agree...

Turn off the lights in the room, and follow the steps you just mentioned once again, this time standing in back of the amp to see which power tube/s flash in an unusual manner.

Click to expand...

... And to be clear, a "blue glow" is not unusual.

can't get tubes to flash, this blue is a alot more than the normal blue.

I looked at it closer with the lights off. The pair of tubes closest to the power cord are the ones glowing blue. Amp has a low hum like a ground noise makes but plays ok. I'm guessing that pair of tubes is bad?

While I think the IIC+ Coli's were wired 3/3 in the power section, I think a different forum member said his Mark III Simulclass was wired with the 4 outer 6L6s wired together, so it's also possible that the far 2 may have or be going bad too. You might want to look into that. If it's "playing ok" now and going in and out, it sounds like you're on the verge of a power tube meltdown. I'd be careful - if they explode it can take out the screen grid resistors with them.