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    Excellent results with VTA70/ST70 build

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    danf

    Posts : 58
    Join date : 2009-01-19

    Excellent results with VTA70/ST70 build

    Post by danf on Mon Jan 19, 2009 2:01 pm

    I am always interested in quantitative information about tube power amps, but mostly find subjective descriptions. I recently put together an ST70 using some A470 (cloth) outputs, a Dynakit PA060 power transformer, and a CE 40-80-30-20 capacitor. I supplemented the capacitance following the choke with a 220 uF cap, for a total of 320 uF filtering the output center tap B+. I used the VTA70 board.

    Here are my test results. I use ARTA with an M-Audio Transit sound card, plus an external Leader LG120B for high frequency THD tests when the M-Audio is used in 96 kHz mode. Bias set for 40 mA/tube. Balance pots adjusted for minimum distortion. One channel driven.

    Frequency response - flat to better than 0.25 db from 20hz to 20 kHz. This is usually stated +/-0.125 db.

    Power output into 8 ohms (pure resistive load) with 120 V AC at 1% distortion - 38 watts at 1 kHz, falling to 30 watts at 30 Hz and 20 kHz. (At 20 kHz, I only measure 2nd harmonic distortion, but who really cares about harmonic components that only bats can hear?)

    IMD - 19 khz and 20 kHz full power - 0.42%

    THD at 1 watt - < 0.1%, rising slightly below 50 Hz and above 20 kHz

    Square wave response - near perfect at 1 kHz and 10 kHz. This is fairly unusual for tube amps that I've tested.

    Power transformer heat rise - 120 F in 70 F ambient after power on for 3 hrs.

    I conclude that Roy Mottram's board extracts all of the performance I would expect to get from the output transformers. The board is very nicely made and even this point to point guy has to say "nice job"! And yes, the amp does sound excellent!

    I can also attest that Roy and Bob Latino are good to deal with. I had some problems with my test set-up at first and they were helpful and patient.

    JunkyJan

    Posts : 103
    Join date : 2008-12-09
    Location : BC, Canada

    Re: Excellent results with VTA70/ST70 build

    Post by JunkyJan on Mon Jan 19, 2009 5:08 pm

    Thanks, danf. Good to know! Far too many people nowadays seem to rely on audiophile magazine listening (subjective) impressions - I'm old-school, I like to see actual measurements too.

    I'm especially impressed with the square wave response test results. I have noticed that this amp sounds incredibly clean on Piano, Trumpet etc - possibly a side effect of the good responses that you measured.

    Not that it would have made (much or any) difference to the test results, but what driver & power tubes (type & brand names) did you use?

    -- JunkyJan

    danf

    Posts : 58
    Join date : 2009-01-19

    Tube types for tests

    Post by danf on Mon Jan 19, 2009 5:29 pm

    The amp uses a quad of Winged C EL34s and three JJ 12AT7s.

    JunkyJan

    Posts : 103
    Join date : 2008-12-09
    Location : BC, Canada

    Re: Excellent results with VTA70/ST70 build

    Post by JunkyJan on Mon Jan 19, 2009 6:22 pm

    Thanks for the info, danf. Very much appreciated.

    -- JunkyJan

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