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KT88's in a Sunn 300T?

Discussion in 'Amps and Cabs [BG]' started by Whisper, Aug 23, 2004.

  1. Whisper

    Whisper Supporting Member

    Jun 28, 2001
    New York, New York
    I'm looking at replacing the power tubes in my Sunn 300T with some JJ KT88's. The problem is, they are a physically larger tube, and I'm not sure if they will fit since all 6 power tubes are pretty close together.

    Has anyone tried this, or can someone give me the physical dimensions of the JJ KT88 tube?

  2. They SHOULD fit. There's plenty of real estate in a 300T.
  3. Whisper

    Whisper Supporting Member

    Jun 28, 2001
    New York, New York
    I agree on the real estate part, it's a huge amp. The power tubes don't have a lot of room between them though, even with 6550s in. Anyone else?
  4. Rock&Roll


    Jul 21, 2002
    Perhaps someone here could get some measurements of the tubes for you? More likely, there should be some info on the internet that would show the basic external dimensions.

    The best thing I found in a quick glance HERE was that the KT88 would have roughly a 1/2" larger diameter. Those tube coolers are meant to have some tollerance in either direction of 2" for KT88s. So the best guess so far is that the KT88 would be about 2" in diameter. Get out yer tape measure.
  5. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    You might consider the Svetlana 6550's, the old kind with the "Winged C" logo. Those are some very nice tubes, and to my ear they have a slightly better breakup characteristic than the KT88's (the latter supposedly being mainly hi-fi tubes, although people use them for just about everything). Also they're physically smaller, more like the old GE 6550a package. They're about the same price as JJ's, last time I checked.
  6. Jdyer11


    Jan 13, 2011
    Just curious, what type of tubes did you end up installing? I need to replace mine in my 300t... Are matched sets really important? Thanks
  7. Bill Hughes

    Bill Hughes

    Oct 17, 2013
    They should fit, but the svetlanas were what we designed around.

    SeattleSunn likes this.
  8. What would be the most recommended current production power tube to use in these amps? The winged c tubes are out of production and getting too expensive to use.
  9. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Is the amp designed to run KT88 as an alternative? They draw more heater current so need the transformer made to supply that. Unless th KT88 are actually 6550 in a different shaped bottle.
    BassmanPaul likes this.
  10. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Exactamundo!!! :)
  11. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    I'm pretty sure the OP resolved his issue 10 years ago, but the advice on heater current is spot on.
  12. Still wondering what the most recommended current production tube for these would be, though.
  13. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Shuguangs would be my recommendation, cheap and been fine generally.
  14. Would this be a shuguang 6550 or kt88?
  15. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    That kinda depends on the aforementioned amp stuff. I've used both in amps as appropriate.
  16. Bill Hughes

    Bill Hughes

    Oct 17, 2013
    Over the years I've been of the impression 6550's were 1.6 Ampere, and it looks like KT88's are the same. The SVT filament transformer is 6.3V at 12.05 Ampere. The Sunn 300T is the same current, but 6.6V.

    That increased heater voltage results in more reliable tube life, but not much extra current due to the coefficient of tungsten. Where has anyone found higher heater current for KT88? What brand?

    Most of the losses in the transformer are in the primary. I wouldn't consider another 6 watts in the primary a cause for concern.

    We usually made the heater windings stout to guarantee that the primary fuse will open if the heater winding was shorted.
    Last edited: May 7, 2014
    SeattleSunn likes this.
  17. I thought the same thing, all of my 6550 and KT88 data sheets show 1.6A.
  18. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Only one I've ever seen was an old SED sheet that says 1.65A, otherwise I've only seen the same 1.6A for 6550 or KT88 everywhere else.