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Thread: KT150 tubes in CJ amps

  1. #1
    Member Sponsor Senior Member Marcus's Avatar
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    Oct 2012
    On the sunny side of the Alps

    KT150 tubes in CJ amps

    Years ago I read a thread on some other audio furum how one guy substituted KT120 tubes for KT150 in his Premier 12 amp and his report was extremely positive. Then a friend of mine did the same with his Premier 12 and was equally satisfied and now reading microstrip’s answer in the ”Jadis JA200 mk2 amp” thread that he did this change in his cj LP275 amp too (post #185), I’m wondering, is this practice still considered questionable or not any more? I know there are some technical reasons against this practice and cj does not recommend doing it too but it seems more and more cj users do this anyway and nobody reports any negatives whatsoever.

    Anybody else here did this swap?

    Would be nice to read micro’s technical explanation on this and his listening impressions would be welcome too...
    Last edited by Marcus; 01-12-2018 at 05:45 AM.
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  2. #2
    Addicted to Best! DaveyF's Avatar
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    Jul 2010
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    Marcus, I think it is a mistake for people to assume that you can easily swap in the KT150 tubes in place of stock KT120's or 6550's, KT88's etc. While it may work on the face of it, the KT150 draws a LOT more current than those other tubes, as a consequence of this, there is a much bigger demand on the transformer. If the transformer is not up to the additional draw, you are asking for trouble, IMHO. BTW, the reason (one of them) that the new Jadis line of amps is called the Mk2, is because they were re-designed to take the new tubes ( KT120's and KT150'...but particularly the KT150's). The transformer's on the Mk2's were specifically redesigned for this tube.
    Best to determine from the manufacturer before hand if the amp can support the new KT150's....they would be the authority i would rely upon....and nobody else. YMMV.
    Last edited by DaveyF; 01-12-2018 at 07:10 PM.

  3. #3
    VIP/Donor [VIP/Donor] microstrip's Avatar
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    May 2010
    Quote Originally Posted by Marcus View Post
    (...) Would be nice to read micro’s technical explanation on this and his listening impressions would be welcome too...
    In order to clarify the KT150 / KT120 / 6550 subject we must stay away of gossip and subjective generalizations, and look at tube specifications and curves.

    1. Filament current 6550 1.6A +/-5% , KT120 1.7-1.95A, KT150 1.7-2A

    Considering designer margins, the difference in power is very small, and in most cases will be marginal considering the whole transformer power.

    2. Plate current versus grid 2 voltage - the KT150 needs a significantly lower negative voltage for the same plate current.

    IMHO it is why cj and some other manufacturers openly say NO. As this curve varies from tube to tube we risk inserting a tube that is out of range of bias adjustment at any point, having excessive plate current, risking ruining the tube and the output transformer before the user notices it. In order to use KT150's safely the resistor in series with the adjustment potentiometer should be changed. Because we have success with one set of tubes does not assure the next one will work. Or even worse, during burn-in you risk going out of range if you start on the limit.

    In my case I had to turn the KT150 pots of the LP275m's almost fully anti clockwise to get the correct plate current, but it worked perfect. One KT150 tube could not bias correctly in the Audio Research's.

    3. If the amplifier complies with 2, then in all other characteristics, except dimensions, IMHO the KT150 is usually a direct replacement of the KT120, but with very similar although not equal curves. Its maximum specifications outperform the KT120 ones.

    Jadis uses fixed bias , the cathode resistors must be changed when moving from KT150 to K120/6550 or vice /versa. Some amplifiers also need to change the power transformer to use KT150's, I do not know exactly what is the change in them.

    It is late here, I will go on subjective opinions and a few facts on Jadis mk1/mk2 in another post - curious that the truth about them is different in french and english!
    Last edited by microstrip; 01-12-2018 at 06:01 PM.
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