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    ST-70 Clone with KT88's

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    slovell

    Posts : 5
    Join date : 2014-01-28
    Age : 64
    Location : Chesnee, SC

    ST-70 Clone with KT88's

    Post by slovell on Mon Nov 24, 2014 7:36 pm

    I have an ST-70 clone with the VTA board and I have a question. I'm presently using EL34's and I'm considering rolling in some KT88's. How much power would I gain by doing so and what should the bias be for the 88's? Any opinions on KT88 tubes would be appreciated.
    Thanks, Sam

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    Re: ST-70 Clone with KT88's

    Post by Guest on Tue Nov 25, 2014 12:20 am

    what type/model transformers are you using, thats the most important thing, followed by what model rectifier, then we could give you better advise

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST-70 Clone with KT88's

    Post by peterh on Tue Nov 25, 2014 2:44 am

    slovell wrote:I have an ST-70 clone with the VTA board and I have a question. I'm presently using EL34's and I'm considering rolling in some KT88's. How much power would I gain by doing so and what should the bias be for the 88's? Any opinions on KT88 tubes would be appreciated.
    Thanks, Sam  
    If you use the original dynaco mains transformer you might get problem. This transformer
    is on the limit already , kt88 will consume 1.6A filament vs el34 1.5A. This additional 0.4A
    will heat the transformer and might break it.

    As for power i'd say "you won't hear any difference in sound volume". As you use the same
    B+ ( or even less as the transformer is loaded more) the only difference is that kt88 has
    a lower impedance, this may be experienced as a tighter bass responce as the output
    resistance will be lower.

    In general, a power doubling is whats needed for the ear to experience a difference in volume.

    slovell

    Posts : 5
    Join date : 2014-01-28
    Age : 64
    Location : Chesnee, SC

    Re: ST-70 Clone with KT88's

    Post by slovell on Tue Nov 25, 2014 3:42 pm

    I don't know the transformer model numbers. It has Edcor transformers on the right and left with what appears to be an original in the center. I would post photos if I could figure out how to do it. I finally gave up.

    Dave_in_Va

    Posts : 142
    Join date : 2013-04-02

    Re: ST-70 Clone with KT88's

    Post by Dave_in_Va on Tue Nov 25, 2014 3:47 pm

    I've got a VTA ST70 and tried a new quad of Genelex KT88s and the El34s (inexpensive Valve Arts at the time) simply sounded nicer to me. I could tell a slight difference in the sound but it wasn't "better"and it definitely wasn't worth the expense. They look cool though.
    I think the guys here generally use the 88's for the ST120.
    PS I ended up with a NOS quad of Mullard EL34's. I should have done that at first and saved all that tube rolling money.

    slovell

    Posts : 5
    Join date : 2014-01-28
    Age : 64
    Location : Chesnee, SC

    Re: ST-70 Clone with KT88's

    Post by slovell on Tue Nov 25, 2014 3:59 pm

    Thanks for the advice, it pretty much answered my question. I finally figured it out. This is what I have.

    Guest
    Guest

    Re: ST-70 Clone with KT88's

    Post by Guest on Tue Nov 25, 2014 4:19 pm

    slovell wrote:Thanks for the advice, it pretty much answered my question. I finally figured it out. This is what I have.

    looks like you've got the original transformer, which as has already been mentioned, was pushed pretty hard using stock tubes.
    You would be REALLY be pushing it with KT88's, so I would advise against it.
    Option is of course to up grade to the latest power transformer used in current kits. You'll need to be sure the cost of this is worth while, but by going that way, will open up a number of power tube rolling options for you!
    KT88's should be biased at around 0.5V per tube.

    slovell

    Posts : 5
    Join date : 2014-01-28
    Age : 64
    Location : Chesnee, SC

    Re: ST-70 Clone with KT88's

    Post by slovell on Tue Nov 25, 2014 4:32 pm

    Thanks, I'll look into it.

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST-70 Clone with KT88's

    Post by peterh on Tue Nov 25, 2014 4:36 pm

    Looking at the picture, there is no rectifier tube, thus a lot of load has been taken off the
    transformer. In this config i guess it won't overload to use kt88/6550, but the difference
    in power is questionable. ( i won't talk about sound as other does this much better)

    slovell

    Posts : 5
    Join date : 2014-01-28
    Age : 64
    Location : Chesnee, SC

    Re: ST-70 Clone with KT88's

    Post by slovell on Tue Nov 25, 2014 4:49 pm

    Thanks, I've read that the EL34's and their variants do sound better than the 88's. I'm just curious as to what I can do with this amp. I'm not at all unsatisfied with the way it sounds as it is and I don't really need more power as I'm using modified Klipsch KLF-20 speakers. It surely sounds way better for stereo than the Adcoms I use for movies and tv. Just seeing what my options are.
    Thanks, Sam

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