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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    Too High Bias ST-70

    Bob Latino
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    • Post n°26

    Too High Bias ST-70 - Page 2 Empty Re: Too High Bias ST-70

    Post by Bob Latino Thu Jun 04, 2020 8:54 am

    PeterCapo wrote:
    tubes4hifi wrote:published specs are nearly worthless, as most any piece of "hi-fi" gear has specs so good that they way below listening thresholds.
    I'll just say that Bob and I consistently (and this means like every week of every year for the past 15-20 years)
    get emails from customers saying that our equipment is so far superior to others they have owned and heard.
    This includes McIntosh, Audio Research, Cary, Pass Labs, and others, most of which cost 3-10x as much for equivalent gear.
    I'm not talking about joe-blow customers with a $2000 system, I'm talking about customers with $50K+ systems.
    Good enough for me and our customers !!

    Well, I'll have to respectfully disagree with your statement that formally published specs are nearly worthless.  Performance specifications, verified by disinterested independent lab tests, have long been an integral part of knowing what you're getting - a transparent assurance for those who prefer to be informed consumers.  Apart from this, it's pretty hard to know how to respond to the rather extreme claims you have made over time.

    Peter,

    You make remarks consistently on the forum here about how good the original Dynaco ST-70 sounds without even having heard the VTA version of the amp ! You say you now have a non functional VTA ST-70. Fix this amp and then do an unbiased comparison with an original Dynaco ST-70. Be sure to have not only you but others listen to a comparison of the two amps at the same time with the same music.. I have done the same comparison with audiophile friends in my own living room and most types of music (especially rock music) and the general consensus is that the VTA version of the ST-70 sounds better than the original amp. (stronger and more realistic sounding) Yes - It may not even be fair to compare an amp whose design is over 60 years old with the more modern VTA version of the amp. The older amp will have probably some older parts that are worn and should be replaced. Read the comparison of a stock Dynaco ST-70 with the VTA ST-70 done by a customer and his friend at the link below ..

    Comparison > An original Dynaco ST-70 and a VTA ST-70

    Bob
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    • Post n°27

    Too High Bias ST-70 - Page 2 Empty Re: Too High Bias ST-70

    Post by peterh Thu Jun 04, 2020 9:12 am

    Here is another Peter,
    i have myself measured the VTA-70 in addition to using and listening one for several years.
    And i must say it's a amazingly good amp. Yes, it can be even better, but the amp and
    the supports is outstanding. It comes on top of what i have tried.

    I do miss an Dynaco ST-70 for a future project thus i can not compare the dynaco st70 with vta-70
    at present. One problem with the Dynaco st70 is the 7199 tube, it's beginning to reach end-of-life
    and replacements are somewhat tricky, one has to use band-aid solutions. VTA-70 on the other
    side has all parts as current production and is supposed to be serviceable for at least 50 years to come.

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    • Post n°28

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    Post by Guest Thu Jun 04, 2020 9:28 am

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    Last edited by PeterCapo on Fri Dec 04, 2020 1:36 pm; edited 3 times in total
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    • Post n°29

    Too High Bias ST-70 - Page 2 Empty Re: Too High Bias ST-70

    Post by Guest Thu Jun 04, 2020 9:31 am

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:13 pm; edited 1 time in total
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    • Post n°30

    Too High Bias ST-70 - Page 2 Empty Re: Too High Bias ST-70

    Post by peterh Thu Jun 04, 2020 9:56 am

    PeterCapo wrote:
    peterh wrote:Here is another Peter,
    i have myself measured the VTA-70 in addition to using and listening one for several years.
    And i must say it's a amazingly good amp. Yes, it can be even better, but the amp and
    the supports is outstanding. It comes on top of what i have tried.

    I do miss an Dynaco ST-70 for a future project thus i can not compare the dynaco st70 with vta-70
    at present. One problem with the Dynaco st70 is the 7199 tube, it's beginning to reach end-of-life
    and replacements are somewhat tricky, one has to use band-aid solutions. VTA-70 on the other
    side has all parts as current production and is supposed to be serviceable for at least 50 years to come.


    Adapters to use 6GH8A and  6U8A have been shown to be very good, low-cost, readily available substitutes for the 7199.
    None of the replacements for 7199 are in production, thus we only
    prolong the time we can still support st70.
    How about sending your dysfunctional st70 to me ? I need one for
    a project, i'll return it within a month hopefully repaired.
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    • Post n°31

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    Post by Guest Thu Jun 04, 2020 10:13 am

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 7:13 pm; edited 1 time in total
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    • Post n°32

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    Post by Kennybugs Mon Jun 08, 2020 5:14 pm

    OK, I got the 1N4007 diodes and installed one and disconnected the Se diode. I also installed 2 each CL-80 surgistors which brought down the voltage from 122 volts to 117 volts.

    So, after installing those parts I couldn't get the bias to go high enough with the 10k and 15k resistors paralleled, so I took the 15k out and left the 10k in. It was the Se diode that was causing the bias problem because with the 1N4007 I can adjust the bias just fine with the original 10k resistor.
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    • Post n°33

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    Post by Guest Mon Jun 08, 2020 6:08 pm

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    Last edited by PeterCapo on Mon Nov 30, 2020 7:12 pm; edited 1 time in total
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    • Post n°34

    Too High Bias ST-70 - Page 2 Empty Re too high bias

    Post by serial driller Tue Jun 09, 2020 12:16 am

    Change the two coupling capacitors at pins 1and 8 of the phase inverter. When they leak they will throw off the bias. Also change the one at pins 3 and 6 also. In fact, change the the black cat caps on the board and the bias will fall in line. Been there done that and it works.
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    • Post n°35

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    Post by Kennybugs Tue Jun 09, 2020 7:28 am

    Thanks for the info.
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    • Post n°36

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    Post by rjpjnk Tue Jun 09, 2020 11:08 am

    It is a difficult thing to attempt to rate the sound of one amp over another. It is tempting to revert to measurements because then we can easily make a quantitative comparison, but when all the builds being considered are already in the excellent range numerically, but they still sound "different" how do we measure that? I think it is not a good idea to rely too much on published specs, but it is also not a good idea to claim one design is 10x better than another without evidence to support this. And it is difficult to amass "evidence" from a room full of listeners or emails. The best we can do is listen to them side by side and decide which we prefer best. They we can possibly say something qualitative like "Most listeners preferred the sound of Amp A because it sounded more lifelike to them", or something of the sort. Matters of taste cannot be disputed. Well, we can and do dispute them, but we can't measure them. Peter makes a good point.
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    • Post n°37

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    Post by Guest Tue Jun 09, 2020 11:48 am

    .


    Last edited by PeterCapo on Mon Nov 30, 2020 4:37 pm; edited 1 time in total
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    • Post n°38

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    Post by rjpjnk Tue Jun 09, 2020 1:39 pm

    I agree. Both pieces of equipment should be in perfect condition to be fair. But even so, it is just not possible to quantify the listening experience. Best we can do is take a poll in a controlled listening environment and report the opinions expressed.

    I spent many hours listening to my VTA ST70 and The latest Dynaco ST70 Series 3 side by side with the most critical ear looking for every nuance of difference I could gather. I definitely preferred one amp over the other, but another listener might have gone the other way. But both of them measured extremely well on the bench. I would say identically so.

    What did I hear? Who knows, but it doesn't have a number. All part of the magic and art of audio.
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