Overheating Power Tube (Mesa 400)

Discussion in 'Amps and Cabs [BG]' started by chaosMK, Nov 26, 2005.

  1. chaosMK


    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    I was noticing that I wasnt getting as much clean power as about a week ago, after our last gig. I checked the back of my Bass 400 while it was on and saw that one of the tubes (left center) looked like it was overheating. Most of the interior of this particular tube was orange-hot, while the rest of the tubes just had orange glowing filaments, like they are supposed to.

    I recently retubed my 6L6's to JJ's KT-88's and they have definately been running a lot hotter than my last set. I have my amp in a rack case and suspect that the built in fan, which is pretty strong, may not have been dissipating heat too well on the opposite end of the case.

    Any ideas?

    Does this sound like something wrong with the tube, or something wrong with the amp?
  2. timv

    timv Supporting Member

    Jun 7, 2000
    Chandler, AZ
    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.
  3. chaosMK


    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    This amp has a fixed bias, so I didnt adjust it. I ordered the tubes from Eurotubes and counted on them (Bob) picking out the correct ones for a fixed-bias Bass 400.

    I have a bias tester but hadnt used it because the amp functioned perfectly for the first week (until a gig, where it was pushed more), and also I havent managed to find out what the specs are for it. If anyone knows where I can find this info, please pass them on!

    Would switching out the troubled pair of KT-88's for some 6L6 tubes be a possible solution?
  4. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    North central Ohio
    Editor-in-Chief, Bass Gear Magazine
    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.
  5. 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?? :D
  6. Passinwind

    Passinwind I Know Nothing Supporting Member

    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. :meh: Sometimes it's fine, but it seems presumptuous to think it's always OK, no?

  7. +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.
  8. 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.....
  9. ....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... :eyebrow: .....
  10. That happened in my SVT when one of the tubes died, one of the others ones started burning up to compencate or something
  11. chaosMK


    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    So it looks like a problem with the tube, and also potentially with that solder joint? Are there any ways I can test things out to isolate the problem?
  12. 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.
  13. chaosMK


    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp
    Thanks for the guidance Psycho Bass. Ill check out the socket. In the past, I've blown a few fuses due to a loose power tube and it was always this particular socket (not sure if that is related).

    I explained my problem to Bob (Eurotubes) and he recommended looking at the socket as well. Props to Bob for writing me back on a Sunday.
  14. chaosMK


    May 26, 2005
    Albuquerque, NM
    Hi-fi into an old tube amp

    As I was trying to take a bunch of readings to figure out how my amp was operating, I used some contact cleaner and shot some into all the tube sockets.

    I noticed that my Overheating Tube is no longer overheating. I am hoping this was due to contact cleaning and reseting it.

    Head seems to work fine powering the 2x10 Avatar cabinet I have at home at different volumes. I am sure it will be drawing out a lot more current with the 2 4x10s that I rehearse with. I ran the Bass 400 and Avatar for about 2-3 hours today, practicing, and there were no bad signs.

    The tube still has a minutely brighter filament than the others.

    Any thoughts? Do the TB tube experts think contact cleaning might have been a fix?