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Matched Tubes in Mesa 400+?!?

Discussion in 'Amps and Cabs [BG]' started by ratzpaul, Sep 7, 2004.

  1. ratzpaul


    Aug 19, 2004
    Hi guys,

    I'm just making my decission on a used Mesa 400+ but I have two questions I can't answer - also I didn't find something in the search.

    1. I know that you should use matched tubes for the amp, but do I need 12 matched tubes for the power section or are pairs of them sufficient.
    Also stores offer 4 matched tubes, is this someway better than 2 matched ones?

    2. The Mesa has fixed bias - is it in someway possible to adjust it. I'm asking because the seller said that the tubes have been some kind of set up in the amp - so, is it possible that he changed the bias even though it's fixed?

    Thanks a lot guys,
    hope to hear from ya soon.

  2. Passinwind

    Passinwind I Know Nothing Supporting Member

    All 12 tubes should ideally be matched. Matched quartets are better for your purposes than duets, but each quartet could vary substantially from the next one. A good tech can match up sets and give you twelve that are close enough. A good vendor can just sell you what you need. Some larger vendors grade tubes for rough ballpark "hardness" ratings, so you could buy three quartets that at least have the same rating.

    Fixed bias in no way means you can't readjust. If there's no bias adjustment pot, you swap out resistors in the bias circuit. Yes, someone may have already done this. Don't mess with this yourself unless you are accomplished at working on tube amps and have the proper test equipment. Your questions imply otherwise - no offense, OK?

    The seller might just mean that Mesa "approved" tubes in the right general bias range were used in the amp. This is somewhat controversial, as a search will show. If the amp tests at full rated power, sounds good, and the tubes aren't cherry-spotting, most people would be satisfied. But to get the absolute best performance from the amp, and longevity from the tubes, a matched set, biased by someone who knows what they're doing, is the ticket.

    Edit: here's the Mesa philosophy on bias adjustment: http://www.mesaboogie.com/US/Smith/biasadjust.htm
  3. Benjamin Strange

    Benjamin Strange Commercial User

    Dec 25, 2002
    New Orleans, LA
    Owner / Tech: Strange Guitarworks
    I was going to pipe in, but looks like everything's covered here. I'm going bowling.
  4. ratzpaul


    Aug 19, 2004
    Thanks for your answers, it's getting clearer now.

    The reason I ask about the bias adjustment is not that I want to change the settings, it's only that the guy (actually women) who's selling the amp had it at service and she says that new tubes have been inserted and set up for the amp - that's all she is able to say about this topic. Now I'm wondering if the amp got biased or not.

    So, as far as I understand your explainations a bias adjustment on a Mesa 400+ is only possible with some big intervention like soldering and new parts and so on. Did I got it correct?

    Also, are all MesaBoogie 6L6 correct set up for the bias of the amp or do they alter amongst them?

    Thanx a lot gays,
  5. hehehe :eyebrow:

    :D :D
  6. ratzpaul


    Aug 19, 2004

    oh s**t, sorry guys,

    Why is there only so little difference between these two completely different words? In Austria the spelling of these two words are like night and day, but in English it's only one letter deviating.

    Well, as I said this wasn't intendet, don't take it personally.

    Nevertheless I'm looking foreward to an answer of my question :help:

  7. Passinwind

    Passinwind I Know Nothing Supporting Member

    Hi Paul,

    Yes, you've got it correct. But that intervention isn't a huge deal to those of us who do this work. Given the uncertain present state of the amp, it would be prudent to have a tech look at it when it next needs retubing. If it sounds good now, and has a history of reliability, you may not need to worry about that for some time.

    It would be easy enough for a technician to check the bias voltage, and assure that's it's at factory spec. Mesa factory tubes would then work as the company intends. This satisfies many people, and the tubes are guaranteed not to cook spontaneously in a factory spec amp, I believe.

    However, you will then be limited to Mesa's choice of tubes, and will pay their rather high prices. Lots of Mesa owners do that happily. It looks like a retube will run $240 US, just for the power tubes, if you buy from the factory website. A well known US vendor will sell you a 12 tube matched set, spec'd for your amp (no rebiasing needed) for about $140. But, these particular tubes make a little less power then the ones Mesa will sell you. There are intermediate steps, price-wise, as well.

    If you like the sound of that amp, don't let retubing issues deter you from buying it, as far as the technical side. The cost part is totally up to you, of course. Mesa specs a life span of 1-1 1/2 years for an amp played moderately 10-15 hours a week. This varies hugely though, in my experience.