What to do about dim power tube?

Discussion in 'Amps and Cabs [BG]' started by BassGreaser, Jul 19, 2004.

  1. BassGreaser


    Aug 22, 2002
    Austin, TX
    I have a Bassman 135, and when I check to see how all the power tubes were lighting up I have noticed that two of them are not as bright as the other two. The owner before me put in two 6L6B, and two 6L6S Groove Tubes, and the dim ones are the B's. My question is....what is happening when an amp does this, and should I not play it until I retube it with all matching power tubes? I just bought a matched quad set of SED winged C 6L6GC's. Any info would be much help. :meh:
  2. jokerjkny


    Jan 19, 2002
    NY / NJ / PA
    dont play it. sounds like the previous owner didnt bias the tubes correctly. might not be drawing enough power. if you dont know how to do it, take it to a tech.
  3. Passinwind

    Passinwind I Know Nothing Supporting Member

    Are you talking about the orange (filament) lights? Biasing has no effect on their brightness, and different spec tubes will often have radically different brightnesses. How does the amp sound? Any obvious signs of a problem?

    If you're talking about purple glow, that may mean that your older tubes are more or less used up, although many purple tubes still sound "OK". Again: how does the amp sound?

    Since you have a matched set of new tubes on hand, what are you waiting for? Get them installed and biased properly, and have fun!

    --Charlie Escher
  4. How much light a tube emits is related to its filament (heater) How the tubes are biased shouldn't affect that at all. Some tubes glow brighter than others because of different filament designs. I would guess that it's because of that, but there may be a problem with the filament supply for the dimmer tubes. I'm not sure if that amp has individual filament supplies or not... It probably doesn't. If the amp sounds ok, it probably is ok.
  5. jokerjkny


    Jan 19, 2002
    NY / NJ / PA
    learn something new everyday... thx Mark. :cool:
  6. BassGreaser


    Aug 22, 2002
    Austin, TX
    I waiting for the SED "C" to come in. The amp does sound ok nothin sounds wrong. Can anyone give me a "step by step" on how to bias an amp. The Bassman has an output tube matching dial on the back of the amp
  7. Passinwind

    Passinwind I Know Nothing Supporting Member


    I'd recommend changing the configuration from the "matching" type to the more standard one, and using a matched set of tubes. This is explained if you follow the links in the above FAQ. Be sure to read the safety warning, and that you are comfortable with the tools and techniques required.
  8. Passinwind

    Passinwind I Know Nothing Supporting Member

    I've used the old Groove Tubes version, which works well. Lord Valve sells his own too, IIRC. You can make one for around 5 bucks, if you have a few odds and ends on your bench; assuming you have a bench, eh?

    In any case, the resistive drop measurement systems like this are a good start on doing the job properly.


  9. Thunderfunk


    Mar 27, 2004
    McHenry, IL
    If the filaments are a different brightness it could easily be that in one type of tube the filament is more visible... that's all. If any of the tubes glow purple and the others don't, it's also nothing to worry about. If you have a "bias" pot on the chassis it's probably a "bias balance" pot. You might also be looking at the "hum balance" pot which is different. You adjust it to balance the filament for minuimum hum and this can be done by ear. The bias balance pot might also have been rewired to be an actual bias adjust pot. I think you can still buy a bias probe from WeberVST.com Since you're only going to be looking at one tube at a time it'll take you a while to figure out if it's a bias balance or bias adjust. In summary, it's not that easy to do it right and I'd suggest you have it serviced if you think there's something wrong with the amp, but there probably isn't. Now getting someone to do it right is another problem.

    Being mis-biased won't affect the power output much, but will affect the tone of the amp from hot to cold sounding, and has an effect on symmetrical clipping, which is why balancing the bias or matching the tubes can have a slight affect "power output." The only thing to really worry about is if the tubes glow red. If they do, they're are under biased and are shot. Even if you fix the bias the tubes won't sound right after "going cherry." They will glow more red at idle then when playing.

    Dave Funk
  10. Jerry J

    Jerry J Supporting Member

    Mar 27, 2000
    P-town, OR
    One thing to keep in mind is that working inside a tube amp is akin to playing jacks in a rattle snake cage. There is potentially lethal voltage in there and it is really not for the un-trained. Even with the power off you can get a nice little shock.

    There are quite a few tube amp techs to be found and I highly recommend that direction. If you want to be able to bias your amp I would find a tech that will teach you how to do it. Many are more than willing.

    Also, once your amp is biased for a certain brand of tube you shouldn't have to worry about re-biasing.
  11. Not so.
  12. TheAmpNerd


    Apr 25, 2004
    Dallas, Texas

    Yeah but the question is...

    ...are you the real Dave Funk?
  13. TheAmpNerd


    Apr 25, 2004
    Dallas, Texas
    Ain't that the truth. A buddy of mine was fooling around in
    his Ampeg. After he got slammed against the wall and
    lived to tell about it, he turned it off, cut the power
    cord off and set the amp on the shelf for 20 years.

    And he is semi-electronics saavy.

    I got her now and am working to bringing her back to life.