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    Output Power from VTA ST-70??

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    SuperB

    Posts : 15
    Join date : 2012-01-01

    Output Power from VTA ST-70??

    Post by SuperB on Thu Jan 05, 2012 12:39 pm

    Hi!
    I wonder how much output power from VTA in triode mode since I have the amplifier here now?

    I might thinking about building another ST-70 with this VTA driver and running higher operating voltage, what about 600V B+ on the GL reissue KT77 with VTA?

    This is suggestion from my tube builder. Is he correct?

    Thanks!

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Output Power from VTA ST-70??

    Post by Bob Latino on Thu Jan 05, 2012 2:01 pm

    Hi SuperB,

    An ST-70 with with the VTA driver board will do about 45 watts per channel in pentode ultralinear mode and 21 watts in triode mode at low distortion levels. Check the independent test link below for more information ..

    VTA ST-70 kit independent test link

    IMHO 600 volts is too high a voltage for modern KT77 or KT88 tubes to run. Running modern KT77 or KT88's at this high a voltage will result (IMHO) short tube life. The only output tube in a Dynaco type amp that can run at these voltages are the relatively new Tung-Sol KT120 output tubes. If you are considering as you say > "building another ST-70 with this VTA driver and running higher operating voltage" then you should consider the VTA ST-120 amp kit. The VTA ST-120, although conservatively rated at 60 WPC, is really a 70+ WPC amp.

    Bob

    mantha3

    Posts : 298
    Join date : 2010-11-10

    Re: Output Power from VTA ST-70??

    Post by mantha3 on Thu Jan 05, 2012 2:02 pm

    You should look at the ST120 that bob sells. I have one and it is excellent like the ST-70 is. the ST-120 is a beefed up ST-70 and put out 65W per channel if I'm not mistaken.

    SuperB

    Posts : 15
    Join date : 2012-01-01

    Re: Output Power from VTA ST-70??

    Post by SuperB on Thu Jan 05, 2012 7:59 pm

    What about 500V 50ma for a pair of EL34? Is that a proper operating point?

    SuperB

    Posts : 15
    Join date : 2012-01-01

    Re: Output Power from VTA ST-70??

    Post by SuperB on Thu Jan 05, 2012 8:01 pm

    My friend is going to mod his piece (the one he lent me) with regulator. DO you suggest regulate the PS independent for each channel (driver and output)?

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Output Power from VTA ST-70??

    Post by Bob Latino on Thu Jan 05, 2012 8:01 pm

    IMHO > Unless you have some original Mullard EL34's, I would not operate EL34's at 500 VDC. Today's EL34's just won't stand up to 500 VDC for very long ..

    Bob

    baddog1946

    Posts : 304
    Join date : 2010-02-03
    Location : Costa Rica

    Re: Output Power from VTA ST-70??

    Post by baddog1946 on Thu Jan 05, 2012 9:04 pm

    hi:
    IMHO tweaking an old ST-70 with higher voltage is not the best way to get what? better sound? more watts?
    I'm not sure you are on the right track to more power from this route.
    Bob's ST-120 version of this circuit with the VTA board has proven itself to be a very reliable and great sounding amp for the last several years and produces double the power of an ST-70
    If I am not mistaken there are almost 1000 of them out there so far making that point. I haven't heard of any chronic faults to date on the contrary most comments on the reliability and sound are very complimentary and I second that.
    The original ST-70 circuit was designed to run EL34's at much lower voltages trying to get more out of this circuit without considering it as a whole could end up being unsatisfactory not to mention expensive.
    Bob seems to have covered all the bases in his version of this amp and if you read his extensive list of mods to get it right it becomes apparent there may not be a high voltage short cut here worth taking that won't lead to problems.
    My Latino ST-120 has seen an average of 5-8 hours a day for over three years playing every kind of music and is still going strong, that alone proves it is a stable topology.
    It might be cheaper in the long run to get a known quantity like an ST-120 save yourself some headaches and enjoy the music.
    Baddog

    j beede

    Posts : 316
    Join date : 2011-02-07
    Location : California

    Re: Output Power from VTA ST-70??

    Post by j beede on Fri Jan 06, 2012 1:24 am

    My favorite EL-34 amp is my VTL ST-80 which runs a push-pull pair of EL-34 with a plate voltage of 625V--which is still well below the rated max. This amp is rated at 80W per channel. This 10+ year old amp has the factory Russian "Mullards" in place and is truly high fidelity. What other amps run high plate voltages?

    SuperB

    Posts : 15
    Join date : 2012-01-01

    Re: Output Power from VTA ST-70??

    Post by SuperB on Mon Jan 09, 2012 2:18 am

    Dear Bob,
    I have a question for you. Refer to the VTA ST-70 manual, it suggest the B+ voltage for the EL34 in between 410-430V. What is the current going to be if operate at 430V??

    Thanks again!


    Bob Latino wrote:Hi SuperB,

    An ST-70 with with the VTA driver board will do about 45 watts per channel in pentode ultralinear mode and 21 watts in triode mode at low distortion levels. Check the independent test link below for more information ..

    VTA ST-70 kit independent test link

    IMHO 600 volts is too high a voltage for modern KT77 or KT88 tubes to run. Running modern KT77 or KT88's at this high a voltage will result (IMHO) short tube life. The only output tube in a Dynaco type amp that can run at these voltages are the relatively new Tung-Sol KT120 output tubes. If you are considering as you say > "building another ST-70 with this VTA driver and running higher operating voltage" then you should consider the VTA ST-120 amp kit. The VTA ST-120, although conservatively rated at 60 WPC, is really a 70+ WPC amp.

    Bob

    Bob Latino
    Admin

    Posts : 2378
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Output Power from VTA ST-70??

    Post by Bob Latino on Mon Jan 09, 2012 5:45 am

    Hi,

    I don't really know what the current flow will be per tube at that voltage? The next time I have an amp open, I'll see if I can measure it ..

    Bob

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