Can I use these tubes?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Gary Brennan, Aug 3, 2008.

  1. Gary Brennan

    Gary Brennan Old cavorting member Gold Supporting Member

    Messages:
    1,617
    Joined:
    Jan 22, 2004
    Location:
    Cooperstown NY
    I have an output tube question. I have a matched set of GE 6L6GCs, I bias them to about 28 mv each (Glaswerks 50 waqtt class AB amp at 460 volts). I also have an old pair of GE 7581As from years ago not knowingly purchased as matched. When I put those after the GCs without re tweaking the bias, they read 19.5 and 12.5. So I assume that these are no longer well matched if they ever were. I can adjust them to both read whatever I want in the 27- 32 range. They sound good.

    For reference, also without rebiasing after the matched GE 6L6GCs, a matched set of TAD 6L6GCs (marked #2) measure 14 and 11.5 mv in the same relative sockets.

    Are these 7581As safe to use?

    gb
     
  2. Faustine Amps

    Faustine Amps Member

    Messages:
    423
    Joined:
    Feb 12, 2007
    Location:
    Southern California
    Hi Gary,

    If your numbers are accurate, you're biasing your tubes very cold! I use a set of matched GE 6L6GCs in one of my amps and bias them each at about 40mA (at 450 volts) of idle plate current. That's about 18 watts per tube, which is 60% of their max plate dissipation (which is 30 watts). Your tubes are running at 13 watts apiece (.028 x 460), which is 43% of their max plate dissipation. Ultimately, if the tubes sound good to you that's all that counts, but you could definitely bias them a lot hotter with no harm.

    As for the 7581As, they are "super 6L6" tubes that are rated at 35 watts max dissipation. If you can only bias them at a maximum of 32 milliamps... that is also a very cold bias setting for these tubes (.032 x 460 = 14.7 watts). And if they are still 7mA apart at that setting (i.e., 32mA and 25mA), that's over 20% mis-matched.

    These aren't good numbers. These numbers don't mean alot if your amp sounds good to you. As for your original question, the 7581A tubes should be safe to use, however, I'm not so sure they are going to sound their best biased that cold.

    But, that's just my two centavos.

    Regards,
    Tim Gregoire
     
  3. Gary Brennan

    Gary Brennan Old cavorting member Gold Supporting Member

    Messages:
    1,617
    Joined:
    Jan 22, 2004
    Location:
    Cooperstown NY
    Hi Tim,

    The dealer I bought the 6l6GEs from (HiTest Guitars) said run them at about 31.5 in my amp at 425 v. and about 28 in the amp with 460, he even wrote these settings on the boxes. So you think I should go much higher?

    GB
     
  4. Faustine Amps

    Faustine Amps Member

    Messages:
    423
    Joined:
    Feb 12, 2007
    Location:
    Southern California
    Hi Gary,

    Well, let me put it like this; it's common practice to bias power tubes between 50% and 70% of their max power dissipation rating. This range is considered a safe bias range. A 6L6GC tube is rated at 30 watts of max plate dissipation. 50% of that is 15 watts, 70% of that is 21 watts. So you could bias a 6L6GC with 460 volts on the plate as high as 45 mA and you would still be in the "safe" range for this tube (460 x .045 = 20.7 watts).

    If you set the idle plate current at 28 mA, the tube is dissipating 12.9 watts. That's 43% of max plate dissipation. There's nothing wrong with biasing on the cold side. If the tube sounds good to you at that setting, then by all means roll with it. I don't know what the seller's intention was in his instructions to bias it so cold but who am I to second-guess him.

    I'm just saying (nudge, nudge), you might want to set the bias a little higher and see if you like the sound any better. It won't harm the tubes as long as you operate them in the "safe" range (50% to 70% of max) and if your amp is rated for 50 watts output, it shouldn't be a problem for the OT or speakers. If there is no improvement to your ears, set them back to 28 mA. They will last longer when you run them on the cold side.

    Regards,
    Tim Gregoire
     
  5. scottl

    scottl Member

    Messages:
    17,041
    Joined:
    Apr 11, 2003
    Location:
    Cherry Hill, NJ
    I think the dealer is a bit silly... You can't make such a blanket generalization. As I PMed you, try a bunch of settings. You can safely run them up more.
     

Share This Page