New St-70 project

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.

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.

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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 ??.

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.

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.

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.

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.

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.

.

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.

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