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differences between 6550 and 6550/kt88?

Discussion in 'Amps and Cabs [BG]' started by jamaica, Apr 18, 2004.

  1. jamaica


    Feb 11, 2003
    Hi all,
    I saw that some tube company(for ex. jj-tesla) makes not the normal 6550 but the 6550/kt88...
    which are the differences between them?
    Are they similar or totally different?
    I have an ampeg svt-av, and when I'll have to change tubes could I put the 6550/kt88 or not?
    If yes, why?
    If no, why?
    Which are the sound differences?

  2. BillyB_from_LZ

    BillyB_from_LZ Supporting Member

    Sep 7, 2000
    I believe KT-88s like a higher (or are rated for a higher) plate voltage than the 6550s. Years ago, I had a Marshall Major head that needed KT88s. IIRC, there was a sticker on the chassis warning against using 6550s...
  3. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Hmmm.... that sounds like one of the later Major's. Mine has a full complement of Svetlana "Winged C" 6550's, they replaced a quad of GE 6550A's. The Svetlana's are especially nice in guitar amps, because of their breakup characteristics, but the GE 6550A's are widely recognized to be the tube of choice for bass amps (including SVT's). For instance they're just about the only tube that'll work in a Fender 300PS or 400PS, 'cause of the extreme electronic conditions in those amps. Anyway, one of the big sonic differences is that the KT-88 tubes break up "roughly", they're not really good for guitar amps (or bass amps). They're good for hi-fi amps though, where breakup is "never" supposed to happen. Another very nice sounding NOS tube is the TungSol 6550 (sometimes branded as RCA). For current production tubes, I'd stick with the Svetlana (now SED) "Winged C". Don't get the new Svetlana tubes distributed by New Sensor, they suck. The Sovtek's suck too, they're boomy and not at all pleasing to the ear (or rather, to "my" ear). Who knows, they might even be the same tube by now (Sovtek and Svetlana). Look for the "Winged C" tubes and those will work fine (and sound great) in an SVT.
  4. I found the opposite with Svetlana versus Sovtek 6550's. I found that the Sovtek 6550WE's have more of a midrange growl to them and have a nice natural compression when pushed hard. The New Sensor Svetlana 6550 is very similar to the Sovtek (same factory? both have three round holes on their plates) The true Svetlana 6550's (with three square holes on the plates) have a nice round warm tone but not that midrange punch that the Sovtek's have, although I am talking about these tubes in my vintage svt so results in a SVT-CL may differ. But my all time favorite would be the GE 6550A which has all the midrange and bass you would ever need in a vintage svt and should last much longer.
  5. New Sensor and Sovtek are the same company, both owned by Mike Matthews. They're the same tubes.

    I have a strong distaste for the currently "SED" formerly "Svetlana" 6550C's. They don't like high voltage. They don't hold stable bias. They don't put out as much power as their counterparts, and they have severe problems with microphonics. ( I say that with authority since I have four sextets and an octet and they ALL behave exactly the same way.) Ask anyone who's had an Aguilar for any amount of time. They are the stock tubes in Aguilar power amps and nearly everyone I've talked to has had them go microphonic.

    As to the original question, the difference between a 6550 and KT88 is that a KT88 has a higher screen rating (and higher voltage ratings) than the original Tungsol 6550, which helps it stay more stable in a amp that runs ultralinear, like a Marshall Major. GE later made the 6550"A" which had similar voltage characteristcs to the KT88 (which used to only be made by MO Valve), but not the higher screen disappation. All three types are beam tetrodes.

    Nowadays, the two types are pretty much interchangeble, but some differences DO exist. I have seen many GE 6550A's labelled as 6550/KT88, which for most hifi applications (the largest market for the types) the differences don't matter, but for big bass amps running 700+ volts on them, the differences are greatly magnifed. A true KT88 will give you slightly more power and more clean headroom, as well as be more shock resistant since thay have better internal bracing.

    The JJ "6550/KT88" isn't a 6550 at all; it's true KT88, and a great one at that. They only label it as a 6550/KT88 to let people know that it can used anywhere a 6550 can. I have a set of them in my Aguilar DB728 and they're killer. I've also had extremely good luck with Sovtek KT88's which appear to be clones of the original Tungsol 6550's, but can handle KT88 operation conditions. EH (which is ALSO a New Sensor brand) also has an excellent 6550 AND KT88. About the ONLY currently made 6550 I don't like is the "SED" I mentioned earlier. Even the Chinese 6550's and KT88's are really nice nowadays.
  6. MrBonex


    Jan 2, 2004
    New Hampshire
    The "KT" of KT88 stands for "Kinkless Tetrode." That was (when it was named) supposed to indicate an exceptionally flat frequency response (hence: "kinkless"). So, yes, it is used by many hi-fi manufacturers for power amps.

    But lest anyone think these puppies don't rock, you have not heard a great British amp (Marshall, Hiwatt, Sound City) loaded with these. I can attest that they make quite beautiful distortion when pushed. Think Jack Bruce (Cream daze).

    My Trace Elliot V6 is also a KT88 amp, and I wouldn't trade it for any amp on the planet. Funky, warm, phat, clear, and, yes, higher-fi than a SVT.

    When I win the lottery, I'm gonna have someone make me a small "Bassman" -- with two KT88s for small gigs.
  7. James Hart

    James Hart

    Feb 1, 2002
    Endorsing Artist: see profile
    just get a Sunn! My Sceptre has a pair of 6550 currently but is about to hit the shop for a cap job and a full set of JJ tubes (pre/power and verb). I'm going with the JJ KT88 for the power section.
  8. Last weekend my Bassman head fried a power switch, and since I didn't have a replacement switch in my shop, I just rebuilt an old gutted homebrew amp of mine into a Bassman copy with a pair of GE 6550A's running at 500V ultralinear into a Hammond output tranny. I used the Ampeg V4B front end, but with no tone controls. Sounds very Ampeg-like.

    12AX7 -> 12AT7 Fender-style driver -> pair of 6550A's ultralinear

    Oh, and the KT refers to a kinkless transfer curve, not a kinkless frequency response.

  9. pff


    Dec 21, 2007
  10. pff


    Dec 21, 2007
  11. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    Look up the spec sheet for the valves you are thinking of buying. That is the best way to see the differences. The variation between modern valves blurs the difference so only proper specs will tell you the important stuff.
  12. pff


    Dec 21, 2007
    I owned SVT's, V4B's, B25B's, B15n and Fender Bassman's. I've always gone back to Sunn with their ultra-linear output trannies using 6550's/ or kt88's output tubes. The Sunn's are clearer then Solid states, and when you crank them they get a deep throatiness and maintain the headroom throughout the frequencies range. I've change from winged"C"s 6550's to JJ KT88's at the advice of my amp tech who said,"the JJ KT88 are the best output tubes on the market today bar-none". I've had 3 Sunn model-T'sand a 200s and a 190B bass amp.... Lov My Sunn's
  13. pff


    Dec 21, 2007

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