Boogie Mk IV Problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by veldt666, Feb 18, 2005.


  1. veldt666

    veldt666 Member

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    I realise that Boogie's dont have variable bias / balance pots fitted, but I'm trying to revalve a Mk IV combo, & not much success.

    On Installing a set of Sovtek 6L6GB's, I switched on & heard High pitched Oscilation. Re installed old tubes, switch on, & no problem.

    I then Installed a set of Phillips 7581's, switched on, no problem, although while checking the amp with the 7581's Insalled, a few "crack"s & squeals appeared. I suspect something in the amp is unstable? Any suggestions from anyone?,(Mr Phillips perhaps?)
     
  2. AlexT

    AlexT Guest

    There are basically four ways to go:

    - Buy Mesa tubes (that's what I do, with luck so far)

    - Buy tubes that are screened OK for Mesa amps

    - Install an adjustable bias mod

    - Cross your fingers

    Personally, I wouldn't risk frying tubes.

    Alex
     
  3. John Phillips

    John Phillips Member

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    You forgot...

    Buy decent tubes and don't worry about it.

    I've never had to rebias a Mesa amp, ever - or one of the Fenders without adjustable bias, some later BFs and most if not all SFs - if you use good-quality, old-production US-made tubes. The reason is very simple - those tubes conform to the correct specs for the tube type.

    That's all you need to do - Randall Smith is absolutely right, provided you follow through his thinking completely. Mesa screen their own branded tubes, mainly to ensure that they fall within the right range too. There's no need to fit an adjustable bias... unless you want to run tubes which aren't ideal for the amp anyway.

    FWIW, I installed 7581As in my DC-5 and didn't need to rebias it either.


    I think the problem with the MkIV may be different - it sounds like a poor contact in the tube socket, or possibly a cracked grid-stopper or screen resistor.
     
  4. veldt666

    veldt666 Member

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    Thanks for the advice John, that's kind of my way of thinking generally.
    In my other amps, I've been using "TAD" 6L6GC's, as I need to find a "consistant" production 6L6, (for obvious reasons), & these "TAD's seem to be quite good, but they ain't 7581's, which I've used for years, but now my stock is running low.

    Would the "TAD" valves fall within the spec required for the Boogie?
     
  5. John Phillips

    John Phillips Member

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    I don't know. What manufacturer are they?

    Mesa can obviously screen Chinese and Russian 6L6s to get enough that are usable, so I'd imagine it's possible with most tubes. It's just that the quality control and tolerances on a lot of new-production tubes aren't great, so you're bound to get some that are outside the right range.

    I once tested about 40 old Philips/Sylvania 6L6s, plus about 10 GEs and a couple of RCA blackplates in my Mesa BTW - not a single one was outside the right range. The 7581As were dead in the middle too.
     
  6. veldt666

    veldt666 Member

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    I think the "TAD"'s are Chinese, bulk bought by "The Tube Amp doctor" in Germany, (see website), for exstensive testing, & grading, similar to Groove tubes.
    Watford valves now stock them.
    I was recomended them for my Fuchs amps, by a freind who now uses them in his Dumble.
    They do sound fine, the obvious question is the life span. I have seen some 7581's last for years, even @ 12 to 15 gig's a month.

    The current quartet has been installed for about 4 months, & seem fine.

    Have you tried Harma's, JJ, or Svetlana's in your Boogie?, If so, which one performed best, or survived?
     
  7. John Phillips

    John Phillips Member

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    No... only old-stock tubes - apart from the stock Mesa Chinese ones, which were crap. One went rattly after about a month, and the replacement went the same way even quicker, so after that I just ignored the warranty and fitted decent tubes :). I would have done anyway, except that I wanted to get value for money out of the shop... ;)

    I never buy tubes except for customers BTW! I use old, tested, used tubes in my own amps. The tone of the oldies is just so much better... and their life expectancy is too, although I think the JJs at least may be getting better in that respect. But I've run Sylvania and GE tubes in amps for years, even though they were old when I got them.

    My 7581As I got from a Fender Concert I bought and resold - the previous owner must have fitted them. I have no idea how many hours they have on them, but they sound great, and put out significantly more measured power than the stock Mesa tubes... as did the Sylvania 6L6s I had in it before. They are just better tubes, period.
     
  8. veldt666

    veldt666 Member

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    That's exactly what I have experienced over many years.
    I've used 7581's in any amp requiring 6l6's, Rivera Concert's, Twin II, Music man, etc, they just have the biggest "voice" & headroom, period.
    Now my ear is attuned to that, everything else I try is a let down, so I'm having to compromise big time.
    Yet, most players I know,(who bother about tubes, etc), prefer a 6L6 with an earlier distortion threshhold, or even convert their amps to use EL34's. If you discover a 6L6 displaying the 7581 vibe, or even something close, I would appreciate a shout.

    In the 6V6 camp, which make would you say is the 7581 of that tube? I currently use NOS Brimar CV???, or Standard's.
     

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