"Class A" tubes for Boogie MK IV?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AlexT, Jan 25, 2005.

  1. AlexT

    AlexT Guest

    Yeah, I know, they are actually cathode biased....just following Mesa's jargon.

    Here is the question - I assume that any pair of 6L6, 6V6 or EL34 matched tubes can be used in these position (not only the ones selected by Mesa) - Correct?


  2. AlexT

    AlexT Guest

    Old Tele man, not sure I understand your answer - the question is much simpler - Mesa recommends use of its tubes for amps that don't need biasing such as the MK IV.

    The inside pair of tubes are fixed bias, therefore there is no way out unless you make a mod to introduce an adjustment pot.

    The question is about the outer, "Class A" tubes, which I believe are cathode biased.

    Any insight, anybody?

  3. RussB

    RussB low rent hobbyist

    Jul 3, 2004

    You lost us there!

    Alex T,

    First off, the oter pair in a Simul-Class amp are not cathode biased, they are class A/B running in "Triode". A negative voltage is supplied to pin 5, via the bias supply. The resistor from the "tree" to pin 5 is visible. Here's a close-up pic of the outer tube socket in my MKIII,


    You see that pins 1 & 8 are tied together with one lead of the (1meg) resistor, and pins 3 & 4 are tied together with the lead of the diode.

    You can use 6L6's or EL34's in the outer positions with no problems. The tubes can be specifically ordered for their current draw, which will run them hotter or colder. I don't think a 6V6 could be used without modifying the circuit somewhat. It would burn up the way the circuit is currently (no pun intended) wired.

    I use EL34's in my MKIII, some from EuroTubes, and SED's from Lord Valve. Either supplier will ship you tubes chosen to work in your particular amp, and you'll also save a few buck!
  4. AlexT

    AlexT Guest

    I'm completely lost. The MK IV has also a triode/pentode switch that works in both Simulclass and Class A modes. From the MK IV manual:

    "In the CLASS A position only the two end power tubes are active, producing around 30 watts before clipping when operating in TRIODE, or 50 watts when in PENTODE."

    It looks like for the MK III and MK IV the CLASS A has different meaning.

    Any ideas?

    BTW, my issue is if I can use other tubes I already have.

    Thanks to all.

  5. Mark Kane

    Mark Kane Gold Supporting Member

    Jan 4, 2002
    Fort Pierce
    Alex, you may be able to use tubes you have but you would have to have the equipment available to measure them for bias to be sure where you're at with them. If you can't do that I wouldn't mess with what's in there.

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