Bias mA values for 6550s in marshalls?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by dewman, Aug 8, 2006.


  1. dewman

    dewman Gold Supporting Member

    Messages:
    5,749
    Joined:
    Nov 23, 2005
    Location:
    NC
    Looking for a range of mA current draw values for 6550s in Marshalls, preferentially, or Germinos. Anyone swappin them EL34s for 6550s and can let me know where I need to be in mA values? Thanks-
    D
     
  2. Dave C

    Dave C Gold Supporting Member

    Messages:
    1,359
    Joined:
    Jun 10, 2005
    Location:
    Waterford Ct
    The biasing target should be approx 60% to a max 70% of max plate dissipation of the tube. A 6550 is a 35w tube so that would be 21-24.5 watts per tube. The current can be calculated by dividing by the plate voltage. So as an example if your plate voltage is 450v, 24.5/450=.0544 A or 54.4ma for 70% setting. YMMV with different plate voltage.

    You will probably have to increase the range of the bias circuit by about 10 volts or so since the 6550's require more bias voltage than el34's. I don't know if you knew that or not. Hope this helps.
    Dave C
     
  3. dewman

    dewman Gold Supporting Member

    Messages:
    5,749
    Joined:
    Nov 23, 2005
    Location:
    NC
    I am thinking about trying them in my Germino CLub 40, which, according to Greg Germino, can take 6550s with no adjustments to the bias circuitry. Guess it is all built in to the design, which is different than early Marshalls. I just didnt know where to begin in the dial in preparation for dropping the tubes in. Plate voltage is about 415V on the amp...thanks for the advice..
     
  4. Dave C

    Dave C Gold Supporting Member

    Messages:
    1,359
    Joined:
    Jun 10, 2005
    Location:
    Waterford Ct
    Yeah I've never worked on one of Greg's amps , but all the Marshalls I've converted to 6550's had to have the bias supply increased. With 415v on the plates I would set bias at 50-59ma on it.
    Dave C
     
  5. dewman

    dewman Gold Supporting Member

    Messages:
    5,749
    Joined:
    Nov 23, 2005
    Location:
    NC
    tried them last night....nice...biased at 55 mA using Weber's bias rite. You can feel the bottom end tightens up relative to the EL34 Svetlanas I had in it before. I haven't raised the volume up at all, but will soon. SO far so good- I could see if nothing else, these tubes will make for some nice recording sounds.
     
  6. AdmiralB

    AdmiralB Silver Supporting Member

    Messages:
    3,074
    Joined:
    Aug 24, 2004
    Location:
    Central Indiana
    Couple things:

    1. Only the original 6550 spec is 35W. The 6550A is 42W, and the spec sheets from Svetlana, et al indicate that they expect their tubes to be held to that spec.

    2. A 100W Marshall is, well, a 100W amp. Biasing 6550s (or whatever) to 25W per tube is fine as far as the tube is concerned, but the amp isn't designed to deliver that kind of idle current. At 25W per tube you're dissipating 100W at idle, once you start playing you're pushing the PT beyond its design. Unicord didn't bias the 6550 Marshalls in the '70s any differently than the EL34s.
     
  7. Dave C

    Dave C Gold Supporting Member

    Messages:
    1,359
    Joined:
    Jun 10, 2005
    Location:
    Waterford Ct
    Hi AdmiralB and welcome to TGP, you bring up a couple of good points.....I gave specs for the 6550 , not the "A" which wasn't mentioned and I agree it is different. I probably should have mentioned that.
    100w Marshall is rated at 100w at the onset of clipping, it'll put out a good bit more than that wide open. El34's idle at 70w or so , so a jump to 100 is not a huge stretch , but I do agree it definitely stresses PT as well as OT more than the 34's do. I have noted Pt's running warmer with 6550's but nothing I would call excessive.The Marshall's I've seen and worked on have had pretty rugged and reliable tranny's and I can't think of any offhand that have had failures that I could directly attribute to 6550 use.....but hey, I could be wrong.....I just haven't seen a problem, but like I mentioned ....it's still a good point worth considering. All bets are off concerning the new HW stuff and the failures they've had.
    Dave C
     
  8. AdmiralB

    AdmiralB Silver Supporting Member

    Messages:
    3,074
    Joined:
    Aug 24, 2004
    Location:
    Central Indiana
    Thanks. I've been around for quite a while, actually.

    My point is, you're setting the amp up to draw almost half-again the idle current for which it was designed. It follows that when you play it, the 'headroom' as far as current capacity you had with EL34s vanishes pretty quick.

    Most people wouldn't put a quad of EL34s in a Twin Reverb for the same reason applied to the filament supply; I wouldn't do it here either. I have used KT88s and 6550s in Marshalls-a-plenty but I don't bias them any differently than EL34s.
     

Share This Page