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    Mixing output tubes on VTA ST70

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    ruffian

    Posts : 43
    Join date : 2010-02-26

    Mixing output tubes on VTA ST70

    Post by ruffian on Wed Jul 18, 2012 2:14 pm

    Is there any reason why I can't try mixing up the output tubes on my ST70? Like a KT88 and an EL34 on each side. Of course each tube would be biased appropriately. I'm thinking it won't matter but I want to be sure.

    Thanks.


    Bob Latino
    Admin

    Posts : 2413
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Mixing output tubes on VTA ST70

    Post by Bob Latino on Wed Jul 18, 2012 2:48 pm

    You won't get the best out of the amp by mixing an EL34 and a KT88 in the same channel (pushing and pulling together). These two tubes have different characteristics and don't "match up" well. In any push-pull amp, you always try to get the two tubes of a pair as closely matched as possible for best sound. This is comparable somewhat to two men on both ends of an old logging saw trying to cut (pushing and pulling) a large log. If both men have the same strength and endurance characteristics, then the log will be cut efficiently and smoothly. You don't want to pair up an experienced logger on one end with an 8 year old boy on the other end of the saw. They just won't cut the log in an efficient manner. The logger will wind up doing more work than the 8 year old boy.

    Bob

    ruffian

    Posts : 43
    Join date : 2010-02-26

    Re: Mixing output tubes on VTA ST70

    Post by ruffian on Wed Jul 18, 2012 5:55 pm

    Actually with a 2 man cross cut saw you only pull, otherwise the blade binds. Wink This is one place where PRAT really matters. It is rather impossible to use one with an 8 yr old, I've tried.

    But I get your point, thanks.

    dunwichamps

    Posts : 36
    Join date : 2011-10-20

    Re: Mixing output tubes on VTA ST70

    Post by dunwichamps on Mon Jul 23, 2012 11:06 pm

    If you had a full quad of output tubes in a monoblock then you could mix 88s and 34s but thats going to generate some harmonic distortion given that the tube wont be matched that closely. It works well for guitar/bass tho! Another thing to consider is that both tubes bias up really differently so you would ne dedicated biasing for each tube in the quad. A better option would be a 6l6GC which is more closely rated to the 34 than a 88 would be.

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