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Do I need to have tube head setup/biased if I replace tubes?

Discussion in 'Amps and Cabs [BG]' started by myke.nytemare, Dec 29, 2006.

  1. Well after buying a Peavey 400 and it having tubes broken in transit, I tested it out, just to find out that while it did work for a few moments, all the tubes are shot and I need to get 8 new power tubes.

    My question is can I just buy 4 sets of matched tubes and plug them in and use the amp safely for a while till I have the cash to bring it to a tech (spent too much after christmas!)....or do new tubes mean settings/tolerances need to be readjusted?
  2. mvw356


    Mar 2, 2006
    i think you will have toi get 8 matched power tubes (ouch) and then have it rebiased.
  3. i know i need tubes... (at least) $200 for 4 matched sets..ouch..but necessary

  4. SubMonkey


    May 3, 2004
    Denver, CO
    I'd really save your pennies an plan to take it to a tech before running it....

    worst case, your head/new tubes are far enough out of bias that you cause damage (either the tubes or more expensive internals).

    Since you're already looking at 8 new 6550/KT88s ($200-ish if you find some good deals) the additional bench charge for a bias job is not that much more, and would be money well spent as insurance on your $200 worth of tubes IMNSHO.

    As much as that sucks, the pain of the expense will go away when you have a stable trustworthy amp. (My first Classic 400 came with some faulty power tubes, so I know this from experience) That said, I like this amp enough that I now have 2 of them...

  5. BbbyBld


    Oct 13, 2005
    Meridian, MS
    If you are switching from the factory tubes to new ones you will to have the bias set.

    You said it worked for a while...what happened?
  6. Well, I let the three unbroken sets in the amp and followed the procedure you gave me (BbbyBld) and noticed right away that more tubes were not lit, but there was one set lit so I flipped the switch and played the bass..it worked. Suddenly a fizz/pop...shut the amp off instantly and now there are no working tubes!

    Its ok though...poor shipping...got $100 back towards tubes from the seller.

    HOWEVER...does peavey sell factory matched tubes? Got some 46,47 and even 48 in mine (i think)...if I got those I wouldnt need a re-biasing then...hmm...how much are sets if you sell them?!?! ;-)
  7. Some sites will sell you a matched octet of tubes. If all the tubes are matched when you replace them, you will not damage the amp. Biasing makes sure that the plate voltage of the amp matches that of the tubes and ensures the amp runs efficiently. You should have the amp re-biased when you replace the tubes at some point though.

    Go to:


    Stay away from Chinese tubes. Russian & Eastern European tubes are the best. Look into Sovtek or Electro-Harmonix 6550's and if you order them online be sure to note that you need all 8 matched. Check the preamp tubes to make sure they're ok too.

    Also, downloading the manual for your Peavey would be a good idea too!
  8. BbbyBld


    Oct 13, 2005
    Meridian, MS
    We don't have those old tubes anymore...I think we carry JJ KT88s now...I'm on vacation now. Give me a few days and I'll see what the prices are.

    Once you get new tubes I still think you should have the thing looked at by a tech.
  9. No. "Matching" just means that the tubes match each other. They could still require more bias current to remain stable than the amp is now set to provide and would toast themselves and it.

    No. Biasing sets the amount of negative current applied to the tubes' control grid.

    It should be done EVERY time you replace power tubes.

    China is making some great 6550's now, and their KT88's have always been good. Matter of fact, my Peavey Classic 400 has Chinese KT88's as the stock tubes complete with "Peavey 6550" painted on them. The only Chinese 6550's to avoid are the coke bottle shaped ones, which inicidentally, were the OTHER stock tube for that amp. There are good examples of them, ( I have a couple now) but the vast majority, IME, are fireworks waiting to go off.
  10. Thanks for all your fact checking.

    I'm sure Chinese tube plants make a great product nowadays but based on my past experiences with them I'll likely never try them again. Groove tubes tend to be "graded" chinese tubes and most of the time, JUNK. I got that bit of info from a tube amp expert. You're just paying for the name. I maintain that Russian and Eastern European tubes are still the best. I have a sextet of Svetlana 6550's in my SVT and I could'nt be happier.
  11. I understand where you're coming from. I never would have given any of them a chance until I got a set of Chinese KT88's in an amp ages ago and they sounded good. The problem with Chinese tubes is quality control, which has improved drastically in the past five years along with the Chinese aquiring old US tooling. Literally, the machines that made used to make 6550A's in Owensboro, KY for GE are now making them in China.

    GT rebrands a wide variety of tubes, not just Chinese. You'll also have to look VERY hard to find anyone more opposed to GT's sales tactics AND products than myself; just search my past posts here. What you're talking about is barely the tip of the iceberg.

    I wouldn't say that in EVERY case, but generally, yes, that's true. Right now, New Sensor and JJ Electronics are making some great 6550's and KT88's. Ei is out of business, BTW.

    I'm glad they work for you. I have had tons of bad experience with Svets. They don't put out rated power, require far too much bias to be stable compared to other 6550's and become microphonic very easily. I have nearly 100 Svetlana 6550C's that won't work in an SVT for me, your mileage, obviously, varies to mine.
  12. okabass


    Mar 19, 2005
    PBG quote: "Biasing sets the amount of negative current applied to the tubes' control grid."

    You mean surely voltage not current.
    Yes, there is current (usually very little). But the main thing in bias procedure is setting a proper negative voltage (against cathode) to power tubes control grids; that voltage sets correct cathode current thru the tubes.
    When you're turning the bias pot you're setting just that negative voltage.
  13. Yes. I just ran two things togther: bias voltage and emission current. I need to think slower and type faster! :D

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