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EL34 vs. KT88/6550

Discussion in 'The Workbench' started by Papa Lolo, May 6, 2021.

  1. Papa Lolo

    Papa Lolo New Member

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    Has anyone here played a plexi with KT88 or 6550 tubes installed ?
    I have read that Jimmy Page had his amps modded to take these tubes and that Jimi Hendrix also used them but did not mod his amps.
    The reason I ask is not only am I a huge fan of both of them and their sound but also because my son has an Ampeg Lee Jackson amp that is supposed to be based on a Lee Jackson modified Marshall design. It has a switch that allows you to use either EL34's or KT88/6550 tubes and I really like the sound of the big tubes. I dont play hi gain and it seems there is more punch and clarity to the notes with the big tubes. My amp is a 50 watt plexi that has already been modded, it has a Mercury Magnetics transformer which seemed to increase headroom and punch compared to the stock and I was thinking that the tube mod might take it alittle further.
    I have to take the amp in to have the fx loop looked at and was considering the tube mod while it's in the shop.
     
  2. william vogel

    william vogel Well-Known Member

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    KT88’s in a plexi 50 watt are easily swapped. The 220k bias dropping resistor before the diode needs to be changed to 150k and the 2 220k bias feed resistors need to be reduced to 100k. KT88’s give you more headroom and tight bottom end. Power output usually doesn’t change much but the feel of the amp is bigger. Overdriven, the KT88’s are awesome. Also, I bias KT88’s at 28 watts dissipation
     
  3. Papa Lolo

    Papa Lolo New Member

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    Good to know
    Thanks
     
  4. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    I have an Ampeg VL-501 (Lee Jackson 50 watt 1 channel) amp.

    In the VL amp's, 6550 make the amp bigger. I've heard some people say, that, in their Marshall, they didn't like the 6550, because it didn't break up soon enough & made the bass a little muddier.

    IMO, the EL34 in the VL sounded thin & brittle, although, the VL has so much gain, compared to a plexi, or even a JCM amp. I don't think Marshall makes an amp w/ that much gain...

    It has more than the DSL & JVM, by far...

    I like 6550s, but will concede that there may be times where I'd opt for EL34...
     
  5. tallcoolone

    tallcoolone Well-Known Member

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    I've been in Zep tribute bands forever and I've converted 2 50w 1987 clones to KT88s. If you really want to nail the TSRTS tone it is a must IMO.
     
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  6. Pete Farrington

    Pete Farrington Active Member

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    If headroom isn’t power output, what is it?
    https://www.fender.com/articles/tech-talk/keeping-it-clean-what-is-headroom
    The vol control will need turning higher with KT88 because they require a higher bias voltage.

    A 35W design max rating is equivalent to about 30W design centre. So idling KT88 at 28W is equivalent to idling EL34 at nearly 23.5W.
     
  7. william vogel

    william vogel Well-Known Member

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    Just my opinion of the tubes and how they operate in my amps.
     
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