Overheating Power Tube (Mesa 400)

Did you get the bias adjusted when you put those kt88's in? I'm guessing the bias is off and that tube might be running hotter than all the rest so they probably aren't matched very well. Don't use the amp like that, bring it to a tech and get it biased correctly for those kt88's.

 

I also own a Bass 400, but to be honest, I was leery of the idea of a fixed bias amp running both 6L6GC's and 6550's/KT-88's at ideal bias settings. This is why I am having bias trim pots for "push" and "pull" installed in mine. That said, I do think that other players have swapped tubes with no ill effects.

I'm very curious to hear how things work out for you, and I wish you the best.

Take care, Tom.

 

I put JJ's (from Eurotubes) in a Peavey Classic 50 guitar amp, which also has fixed bias...and they are indeed running hot as well (80% of rated dissipation, in watts). I've read JJ's tend to be hotter, apparently Bob at Eurotubes likes them this way. (I was going to put in a trim pot and bias it colder, but there's some Marshalls I believe running EL84's way hotter than the max allowable plate dissipation). Hmmm..

But like Tom B., I wonder how an amp with fixed bias can be biased correctly for both 6550's and 6L6GC's

Hey, I wonder if Psycho Bass Guy has an opinion??

 

Personally, I wonder how any tube vendor can confidently sell you a set of ouput tubes for drop-in replacement without knowing the current operational state of your specific amplifier, even if they're the same variant. Sometimes it's fine, but it seems presumptuous to think it's always OK, no?

 

+100000000000

BTW, it sounds like that single tube may have lost its bias completely through a cold solder joint made worse by the heat generated by the extra current of KT88's.

 

I ordered replacement 6550 Svetlanas for my Bass 400 from Musician's Friend....switched from 6l6's, but one of them was cherryplating also.
I crossed my fingers, and sent them back for an exchange, and the new ones seem to be fine, yet seem to run hotter than the 6l6's I had in there before... about the same as a 60 watt light bulb..??
The 6l6's I had in there before were never hot at all though(you could touch them at any time), and sounded so mushy....the 6550's sound great, even though I didn't have a bias installed...I watch the tubes closely though, and they seem to be OK, although they are a bit microphonic...I shoulda got JJ's instead of Svets...
..oh yeah..I put this here cause I saw the great PsychoBassGuy posted here too! Thanks man..I've been reading your posts for quite a while now.....

 

....oh yeah, sorry to repost, but I asked Tech support at Mesa, and they told me to use a Medium range 6550 in my amp, and that I should be fine without a bias installed, even though I know that a bias is the only way to be 100% sure of peak operation.. I'm just a little lazy... .....

 

That happened in my SVT when one of the tubes died, one of the others ones started burning up to compencate or something

 

I'd look at the socket connection first. It doesn't necessarily mean that there's something wrong with the tube itself, although the possibility exists that it may just be horribly mismatched.