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    New St-70 project

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    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    New St-70 project

    Post by daveshel on Mon Mar 23, 2015 12:13 am

    I bought my first ST-70 about ten years ago on evilauctionsite. I was hot to do the Van Alstine mod. I'd known about him for 30 years and had always wanted to try one out. It was a nice amp but cost like $1250+ even back then, once you buy the chassis and send it to him to do the mod.

    Some years later I came to feel that my solid state system (Dynaco ST-400 with early Van Alstine mod) had better tonal balance than my tube rig. I played around quite a bit with equipment and speaker shuffling and was ready to blame it on my tube preamp - a lovely Van Alstine Super PAS 3 with his black face plate - when I happened onto and rebuilt a Dynaco ST-35. I realized then that the Van Alstine Ultimate-70 sounded like a lumbering aurochs, while the ST-35 (rebuilt boards and the Dave Gillespie EFB mod) sounded like a light and airy sprite with the same mids and a high range that was new to me.

    I sold the amp. Later I read Gillespie's excellent long form analysis on baselining the ST-70 over on that other forum, and I realized that I had never established a baseline for the stock ST-70 before I sent it off to be expensively modified.

    Cut to a few weeks ago, I ran across a local posting for an ST-70 that was pretty much stock, but had been serviced recently. It had new tubes and may have had some other work done, but I wasn't sure.

    I bought it and spent a couple of days listening to it, to try to establish the old baseline. I found that it sounded terrible - muted, shrouded, a boxy soundstage. But the point was to live with the original design for a while and get to know it better then do the VTA mainboard and associated upgrades. Driver boards, input and output connectors...

    So I'm looking for hints and suggestions as to how to get the most of this opportunity. Pics to follow - it's a factory wired unit with those two-hole security screws on the lower chassis cover so I haven't gotten into the unit yet... But I want to start bouncing ideas around.

    peterh

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

    Re: New St-70 project

    Post by peterh on Mon Mar 23, 2015 3:03 am

    What preamp did you use for the st-70? Note that st-70 will be badly affected by DC on
    input as it lacks input cap. A pas2/3 will inject DC and badly affect the sound. st-35 has
    an input cap and is not affected by this.

    As for the old one, tubes are consumed. The original 7199 might have reached their end-of-life and
    needs replacement. New ones is obtainable but using 6U8 with adapter ( or board changes) is way more cost-efficient.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: New St-70 project

    Post by PeterCapo on Mon Mar 23, 2015 4:04 am

    Of course, there are a variety of factors within the original Stereo 70 itself, and also with its interfacing to the rest of the system, that will affect the quality of the sound you hear.  As I understand it, the Gillespie article is predicated upon an original Stereo 70 vetted for proper/peak operating condition interfaced with an appropriate preamp, i.e., a preamp with no DC offset at its output.  

    Dave Gillespie’s Stereo 70 that his test was based-upon was described as being in really nice condition.  Regardless, any given sample of original Stereo 70 would have something like 35 to 55 year old parts inside.  As such, sound-wise it might not compare well with a newer restatement of the Stereo 70 built with new and better parts.  I suspect that in most cases when someone finds an original, even one that is functioning reasonably well, they will likely have to just appreciate it for the classic that it is.  Bringing out the potential of the original design so it can compete with the modded versions is another matter.

    I have owned original Stereo 70s that sound the way you describe.  I also built a new Dynakitparts Stereo 70 not long ago and the difference is night and day.  My newer Stereo 70 for the most part retains the original Dynaco circuit design but with some enhancements, such as a Triode USA 500uF cap board having low ESR electrolytics with polypropylene bypasses, Cree Z-Rec SiC Schottky diode for the bias supply, Dynakitparts independent biasing for each output tube for which I obtained four mil-spec pots, a few "boutique" parts here and there, and a few other things that I hope contribute to its quality.  I think of these things as enhancements (not total changes to the circuit like the mods do) that level the playing field so a proper comparison can be made with other, present-day amps.  The result?  Really quite good.  No complaints, in fact my wife and I feel the quality of its sound within its power output limitations compares very well to our Cary SLI-80 Signature, McIntosh MC275 Mk IV, and Parasound Halo gear.

    IMO, the bottom line is that if someone is to take up Dave Gillespie’s suggestion to establish the sonic capabilities of the original design before making a decision to replace the circuit with a different design, then the playing field has to be level in order to make a valid comparison.  I suggest implementing a one-to-one correspondence between the parts quality advantages available in modded versions and applying them to the original Stereo 70 design, i.e., again, low ESR electrolytics with polypropylene bypasses in the power supply, independent biasing for each power tube like some of the "mods" have, close tolerance metal film resistors, polypropylene coupling caps, larger power transformer, re-do all the solder joints with fresh solder, maybe new tube sockets, etc., etc…  Yes, it ends up being its own rebuild project, but without going this route I don't see how a valid comparison can be made against the different modded designs that employ such improved parts vs. the old parts in an original.   

    So, if you suffer through a period of time listening to your terrible-sounding original as it is now and then switch to a different circuit like the VTA board with the accompanying new parts, then of course you will hear an improvement, but you still will not have heard what the original is capable of.

    Regards,
    Peter

    Pillo69

    Posts : 105
    Join date : 2012-04-11
    Location : Granada (España)

    Re: New St-70 project

    Post by Pillo69 on Mon Mar 23, 2015 7:41 pm

    peterh wrote: A pas2/3 will inject DC and badly affect the sound.

    Is there any way to avoid that DC at the output of PAS ??.
    Modifying tubes4hifi to PC5, TCLA tone controls prevents the DC ??.
    O is for another reason ??.

    peterh

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

    Re: New St-70 project

    Post by peterh on Mon Mar 23, 2015 7:54 pm

    Pillo69 wrote:
    peterh wrote: A pas2/3 will inject DC and badly affect the sound.

    Is there any way to avoid that DC at the output of PAS ??.
    Modifying tubes4hifi to PC5, TCLA tone controls prevents the DC ??.
    O is for another reason ??.
    pas-3X has a cap extra that does this.
    But the other way is to add a cap in the stereo-70 right at the input connectors. othing big
    needed, 0.1 - 0.22uF is big enough, 100V more then enough.

    Pillo69

    Posts : 105
    Join date : 2012-04-11
    Location : Granada (España)

    Re: New St-70 project

    Post by Pillo69 on Mon Mar 23, 2015 8:07 pm

    peterh wrote:
    Pillo69 wrote:
    peterh wrote: A pas2/3 will inject DC and badly affect the sound.

    Is there any way to avoid that DC at the output of PAS ??.
    Modifying tubes4hifi to PC5, TCLA tone controls prevents the DC ??.
    O is for another reason ??.
    pas-3X has a cap extra that does this.
    But the other way is to add a cap in the stereo-70 right at the input connectors. othing big
    needed, 0.1 - 0.22uF is big enough, 100V more then enough.

    You refer to 1.0 uF capacitor which replaces the cable between the bass control and dashboard.
    That means the problem is in the line circuit?.
    Prefer not modify the ST70.

    peterh

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

    Re: New St-70 project

    Post by peterh on Tue Mar 24, 2015 1:21 am

    Pillo69 wrote:
    peterh wrote:
    Pillo69 wrote:
    peterh wrote: A pas2/3 will inject DC and badly affect the sound.

    Is there any way to avoid that DC at the output of PAS ??.
    Modifying tubes4hifi to PC5, TCLA tone controls prevents the DC ??.
    O is for another reason ??.
    pas-3X has a cap extra that does this.
    But the other way is to add a cap in the stereo-70 right at the input connectors. othing big
    needed, 0.1 - 0.22uF is big enough, 100V more then enough.

    You refer to 1.0 uF capacitor which replaces the cable between the bass control and dashboard.
    That means the problem is in the line circuit?.
    Prefer not modify the ST70.
    Well, one cold mount a cap inside the pas right onto the output rca connector.

    daveshel

    Posts : 147
    Join date : 2011-11-06
    Location : Tucson AZ USA

    Re: New St-70 project

    Post by daveshel on Tue Mar 24, 2015 1:57 am

    The preamp is the Van Alstine Super Pas Three. I think it has the caps to block the DC. As I recall, the need for these caps was something Frank Van Alstine wrote about in the Audio Basics magazines back in the day.

    The tubes in the ST-70 were replaced a few years back. The seller didn't know all the details - he was selling it for his son, but recalled that the son had spend several hundred on tubes and other work. So I'm not so sure the lackluster sound is coming from age-related deficiencies.

    With all that has been written about this amp over the years, I believe there are some design deficiencies with the driver board, and I'm disinclined to throw money at replacing parts without also updating the circuit design.

    tubes4hifi
    Admin

    Posts : 1286
    Join date : 2008-11-30

    Re: New St-70 project

    Post by tubes4hifi on Tue Mar 24, 2015 11:54 am

    for just under $100 you can completely transform the sound of an ST70 from a 50 year old dinosaur to a modern hi-end tube amp that will better the MC240 and most other tube amps under $3000 (sure I'm a little biased, but so are THOUSANDS of other users who have bought this mod over the past 25 years, including 90% of the people reading this).

    Pillo69, if you haven't upgraded your PAS3, all of the Tubes4HiFi boards have output coupling caps right on the PCBs, so no DC to any amp.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: New St-70 project

    Post by PeterCapo on Tue Mar 24, 2015 12:15 pm

    From the Gillespie article, page 9: "...based purely on a model of performance CAPABILITY, the original driver board is still a very hard design to beat."

    The two main concerns he cites about the driver board are the DC coupling of the inputs, and the potential for bad 7199 tubes.  The DC coupling thing is easy to address: either use a preamp with no DC on its outputs, or add DC blocking capacitors to the Stereo 70's inputs as he described in the article.  No biggie.

    As far as the 7199s are concerned, well you'd need to have a good pair of them, or use one of the substitute tubes that have become popular: https://youtu.be/hol6ZEv7fR4

    The popular criticism about the value of the coupling caps on the original driver board has also been addressed, and I discussed it a bit with him in an AK thread http://www.audiokarma.org/forums/showthread.php?p=8400580#post8400580   It doesn't seem to be as much of an issue as it has sometimes been made out to be.

    As Gillespie rightly points out, other than a lot of subjective enthusiasm, there is relatively little information available about the various replacement boards, certainly not to the level of scrutiny that the original Dynaco driver has been subjected to by independent parties.

    Of course the lack of objective test data about the replacement boards does not prove anything, as you can't really make much of an argument from silence.  At the end of the day I suspect that it would boil down to a matter of personal taste.  But, I do feel, as Dave Gillespie seems to feel, that the original design doesn't get a fair shake these days.  My $0.02.

    tubes4hifi
    Admin

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    Re: New St-70 project

    Post by tubes4hifi on Tue Mar 24, 2015 12:30 pm

    here's some test data from tests I did back in 1988 . . .

    Dynaco ST70 stock, and Audio by VanAlstine mod - two 7199 tubes are used, one per channel. This tube has a triode voltage amplifier and a pentode current driver output.
    Open loop response was a shocker. Bandwidth is only from 50 Hz to 5500 Hz, with good square waves from 100 Hz to 2000 Hz. Open loop gain is only 16.  ~ 20 db feedback.
    The only way this circuit is getting by is thru heavy feedback. Closed loop response does extend from 5 Hz to 22 KHz, with good square waves from 100 Hz to 5 KHz. The only difference between stock and AVA is that the AVA is purposely bandwidth limited to keep the circuit operating within its useful range. Low end response is rolled off below 20 Hz, and the high end rolled off above 25 KHz (which the circuit didn't reach with the tubes used).
    The stock amp maximum power output was 32 watts per channel, but was up to 35 watts per channel with the AVA, probably due to the improved power supply.

    VTA70 - three 12AT7 tubes are used (the new version uses three 12AU7s, and a CCS). One half of the first tube is used for each channel as a voltage amplifier.
    One each of the other tubes is used per channel as a combined phase splitter/driver in a long-tailed pair configuration. 13 db total loop feedback is used.
    There is an individual bias pot for each output tube, as well as a signal balance pot for the phase splitters. The bias is set for 40 ma, about 20% less than stock
    to reduce heat and conserve tube life.   Open loop response of this design is incredible. It has flat frequency response from 7 Hz to 70 KHz, with good square waves
    from 7 Hz to 30 KHz. Open loop gain is 40.    Max power output is 40 wpc.

    PeterCapo

    Posts : 386
    Join date : 2008-12-05

    Re: New St-70 project

    Post by PeterCapo on Tue Mar 24, 2015 12:47 pm

    This test was discussed later in the same AK thread by Dave Gillespie: http://www.audiokarma.org/forums/showthread.php?p=8407058#post8407058

    And, just as a reminder, I am not trying to denigrate any of the various "mods" out there.  As I said, I think the original Dynaco design is not treated fairly.  And, test data is probably best verified via disinterested independent party.



    Last edited by PeterCapo on Tue Mar 24, 2015 4:35 pm; edited 1 time in total (Reason for editing : Clarification.)

    PeterCapo

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    Join date : 2008-12-05

    Re: New St-70 project

    Post by PeterCapo on Tue Mar 24, 2015 2:08 pm

    daveshel wrote:So I'm not so sure the lackluster sound is coming from age-related deficiencies.

    Does your Stereo 70 have any of the original parts, then?  If so, here's an interesting thought...  If you end up removing the original Dynaco circuit in favor of one of the various replacement circuits available, if present try harvesting the original, 50 year old parts from the Stereo 70 (or another source of well used, 50 year old parts) and install them into the new replacement circuit.  For instance, from the Dynaco you might be able to harvest the carbon comp resistors, Black Cat coupling caps, the original quad cap and bias supply electrolytics, original tube sockets, choke, and connectors.  Would be interesting to see how the replacement circuit would fare with well used, 50 year old parts installed.

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