Tube Ratings - if I just give them the number can I...

Discussion in 'Amps and Cabs [BG]' started by markorbit, Feb 14, 2006.

  1. markorbit


    Apr 16, 2004
    I want to change the power tubes in my V4BH to a different brand but I would like to avoid the hassle of taking the amp to a tech if I can. On the stock tubes it has a sticker of '08' on all 4. It means nothing to me but if I were to give that info to a dealer would I be able to replace the tubes without needing a rebias? Are ratings universal?
  2. You would more than likely need a rebias anyway, because stock tubes arent all born the same :)
  3. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    Learn to properly rebias so you never need to take it into a tech for that.
  4. THB_bassman


    Oct 11, 2005
    Is there a particular book, or some other source that you'd recommend?
  5. Fuzzhead


    Sep 26, 2005
    Buy this and never pay a tech again ...well, unless something drastic happens! This is much safer too, you just need to know how many milliamps to set the bias to for your particular amp/tube combo.

    EDIT: Ummm, you do need adjustable bias trimpots to use this! Sorry...
  6. Fuzzhead


    Sep 26, 2005
  7. THB_bassman


    Oct 11, 2005
    That looks cool. Of the many options available, which would I need for a (73) SVT?
  8. Fuzzhead


    Sep 26, 2005
    Psycho is the guy to ask, I think Sovtek KT88s might work. Best to do a search or PM Psycho Bass Guy. Do a search for SVT and 6550 tube.
  9. BbbyBld


    Oct 13, 2005
    Meridian, MS
    If the amp is not already biased hot, you probably won't hurt it by running a set of quality tubes in it. It just may not give you the performance you want. You could try it.

    The problem with a book is that you'd have to learn some basic electronics before you could even understand it and properly use test equipment.

    You would need a multimeter (at least), a waveform generator, I like to use a clamp-on ammeter, maybe a dummy load, and an oscilloscope if you want to do it right.

    It would be best to have someone show you how to do it.
  10. It doesn't matter. Plate current biasing (what you are describing) is pretty much useless for bass amps.

    Tubes ALWAYS need to have their bias checked when replaced, if for nothing else, than to see if there may be problems OTHER Than bias. ALL the "rating/ tube grades" are BS.

  11. Fuzzhead


    Sep 26, 2005
    Really? It works ok on guitar amps. Not as perfect as scoping out the crossover distortion, but close enough. There's a fair range you can bias them over for tonal preference Vs tube life. I think 5150's come out of production biased at 13mA, which is real cold for 6L6's.

    So is the only decent way to bias a bass amp with a scope then? Is that because of the greater power ouput/higher plate voltage? What's the go with SVT's bias points etc? In the V6 technical manual (thanks Psycho), they actually endorse the plate current method under biasing procedure. Then of course they say to check it with a scope and tone generator.
  12. Skel


    Jun 19, 2005
    Boulder, Colorado
    I just happened to be looking at Sovtek 5881/6L6's on "TheTubeStoreDOTcom" today for my V4BH. I was told the V-4BH comes stock with Sovtek 6L6 WXT's, but this site has Sovtek 5881 WXT's and/or 6L6 WXT+ tubes, so I'm not sure which of these 2 comes stock, but the 6L6 WXT+ was rated higher. Incidentally, they are both rated as 30 watt tubes - not 25 watts like I thought. Anyway, TheTubeStore claims if you get tubes with their number and get your amp biased, then the next time you order tubes, you just specify the number you used - eliminating the need for rebiasing. I guess this is not true, and I think it's pretty cheap and easy with one of the kits, once you did it a few times. Plus even if you did get their tubes, you'd still have to bias it once, and by the time the tubes wore out, you would probably have sold the amp anyway. I have a kit, and have been too lazy to learn how to use it. But I want 4 matching WXT+ tubes, so I'm going to learn.

  13. Guitar amps don't push as much current as bass amps, and the idle 'guesstimates' for them aren't going to vary as much under use. In an amp like an SVT, if you bias it using plate current, the first time you try to get loud on your low strings, the tubes will begin to cherry out and you'll probably pop your mains fuse.

    Higher plate voltage is a factor, but the main aspect is that bass has a higher current content than guitar. Lower frequencies have greater current content.

    The bias varies with the quality of the output tubes more than anything.

    That's funny. When Trace was still in business the first time and I wrote to get a manual for the VR400, their tech support guy was adamant that plate current biasing was bad.
  14. markorbit


    Apr 16, 2004
    Skel - I just checked and my V4BH has 5881 Sovtek WXTs. Not bad but I suspect I can improve on them, I'm thinking of ordering a set of SED/Svetlana Winged C.
  15. BTW, Svetlana/SED/Flying C, etc. are NOT good tubes, IME.
  16. Fuzzhead


    Sep 26, 2005
    Well, there you go. So how come SVT's and V8's have bias test points? Can't you just bias a bit colder to be safe?

    BTW here's what that V6 manual says:

    ISSUE 1 PS 31/8/99
    1) Connect ‘BIAS KING’ valve plug/sockets to rear middle two valve
    sockets on amplifier chassis.
    2) Insert matched set of KT88’s into all positions.
    3) Switch unit to Standby mode and allow valves to warm up for at
    least 1 minute.
    4) During this time connect unit to appropriate load and set all
    controls to minimum.
    5) Switch unit from Standby to fully On.
    6) By observing the current read out on the LCD display of the BIAS
    KING and switching between the two valves, adjust the quiescent
    cathode current to give 30mA (+/- 2mA) on each valve. The trim
    pots are accessible from the top of the chassis and can be
    adjusted using a trimmer tool.
    7) Switch unit to Standby and then Off.
    8) When cool enough remove BIAS KING from between valves and
    re-insert them into bases.
    9) Turn to Standby again, warm up and then switch on.
    10) Connect a signal generator to the INPUT and set up controls to
    give maximum power output.
    11) Check that this is an acceptable power for the unit in question
    and that there is no excessive distortion.
    12) Gently tap each valve with a screwdriver, signal on scope
    should remain stable.
  17. ricplyrz


    Dec 26, 2005
    Speaking of tubes, I just replaced a noisy Sovtek 12AX7WA with a GT-12AX7-M. A very big difference. I may of had a bad Sovtek, but the replacement has increased CLARITY and POWER as compared to the Sovtek. The Sovtek was not even 3 mos old and it started to make a "scratchy" sound. I reseated it. Still the same. New tube...hello dynamics and clarity!!!
  18. Fuzzhead


    Sep 26, 2005
    Ahh, that's GT's Mullard copy isn't it? I had a look here and it sounds unreliable. What amp did you put it in?
  19. ricplyrz


    Dec 26, 2005
    SVt CL...replaced a scratchy sounding Sovtek. Well, I hope it works out well for me...those reviews were not to reasurring.....:meh: