1. bscepter

    bscepter Member

    Messages:
    684
    Joined:
    Jul 15, 2005
    Location:
    Omaha, NE
    I have a Hot Rod Deluxe. Recently, I put in a pair of Sino KT66 tubes and biased them at the bias test point at around 62 mA for the pair. I just received a pair of Groove Tube KT66HPs and put those in. With the trim pot in the middle (around where I was for the other pair), the HPs are reading 120 mA! In fact, the coldest I can get them is around 87 mA for the pair.

    What's going on here?
     
  2. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,062
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Go exchange them for a colder pair. The tubes you
    have are too hot for that amp/bias set up. Easier to
    get colder pair then change bias resistors.
     
  3. bscepter

    bscepter Member

    Messages:
    684
    Joined:
    Jul 15, 2005
    Location:
    Omaha, NE
    When you say "colder," in Groove Tube terms, would that be a lower distortion rating? Mine are rated at 7 on their 1-10 scale (lower being earlier breakup).
     
  4. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,062
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Yes, probably a 4 or 5. Your tubes will love you for
    not running them at 44 mA each (87 mA a pair).
    Maybe even a 2 or 3 to get them in the "right"
    bias zone.

    Assuming the following:

    B+ @ 430 then:

    430 * .044 = 18.9 watts

    Assuming the tubes max Pd (Plate dissapation) is 24 watts
    and you want to run them at 75 percent
    that would be....

    24 * .75 = 18.00 watts.

    So at your measured "coldest" setting of 87 mA
    you are slightly above the 75 percent threshold.


    at 70 percent Pd

    24 * .70 = 16.8 watts

    16.8/430 = 0.039 mA (target1)

    - or -

    at 60 percent Pd

    24 * .60 = 14.4 watts

    14.4/430 = 0.033 mA (target2)


    Hope this helps
     
  5. bscepter

    bscepter Member

    Messages:
    684
    Joined:
    Jul 15, 2005
    Location:
    Omaha, NE
    Yes, that helps a lot. I'm trying to get a duet that I can run at 60 mA for the pair at the coldest setting on the trim pot. I've been in conversation with Myles at GT. Hopefully they will allow me to exchange the 7s for 4s or 5s. I'd hate to think that I just wasted $70 on a pair of tubes I can't use.

    Meanwhile, I'm back to the Sino KT66s, which nestle in right at 60 mA dead center in the trim pot's range.

    Harumph. :mad:

    By the way, other than drastically shortening the life of the tubes, can running them at 87 mA for the pair damage the power transformer or any other amp components?
     
  6. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,062
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Electrically, probably fine.

    Temperature wise, it depends on
    how well the amp can drate the heat.
    The bottles are larger, operating at
    higher temp, cranking out more heat,
    that is more watts, less physical space,
    rising through all those plastic components.

    It is probably okay, but ya just never know
     
  7. bscepter

    bscepter Member

    Messages:
    684
    Joined:
    Jul 15, 2005
    Location:
    Omaha, NE
    Exactly. Better safe than sorry.

    I just played a little tonight with my wife on the Hammond L100 organ (a nice built-in tube amp itself). I had the Chinese 66s in, and it sounded really good. Just roll back the volume on the ol' SG and let them P90s fly.

    In any case, I asked Myles if I could exchange the HPs for an earlier-breakup tube. Hopefully, he'll oblige.
     
  8. bscepter

    bscepter Member

    Messages:
    684
    Joined:
    Jul 15, 2005
    Location:
    Omaha, NE
    An L-100? Nice. I've got one of those (an L-102) -- original tubes, sounds great. My wife plays it, too, while I play guitar. A great, warm sound!

    I spoke with Myles this a.m. He said that I should be fine running the KT66HPs (rated "7") at around 42 mA each. In fact, he says that, at that bias setting, they're still in the "cold" range. He also says that the power transformer in my Hot Rod should "easily be able to handle the current draw." In fact, he claims it should be able to handle up to 4 amps. (Now... I know he's a guru and all, but that sounds a bit high to me.)

    However, he was emphatic, so I'm taking a leap of faith. I put 'em in over lunch and biased them at 85 mA for the pair, and they sounded really good. Tons of headroom. We'll see if the tranny can handle the heat, though.

    In all honesty, I'm more worried about the internal speaker wire, which runs right behind the tubes. I may just get a longer wire and run it along the bottom of the cab.

    By the way, I called the Tone Tubby guys (actually, the owner) to ask if their 40w-rated ceramic speaker could handle an amp that's effectively putting out around 50w RMS, and he said, "No problem, dude. In fact, if you blow it, I'll back it!" His exact words. Talk about customer service. (Not to mention, the speaker sounds phenomenal.)
     

Share This Page