ST-70i opinions

Opinions please on the following Craigslist offering:

https://honolulu.craigslist.org/oah/ele/5213015374.html

I am just now finishing a complete update of a SCA-35 using new xformers and Dave's circuit boards so I am on the prowl for my next project. Many thanks in advance.

.

a fair price for that item, but realize you can get a completely new ST70 with our better performing VTA driver and all new tubes for around $300 more than that.
You'll likely spend that much on that amp upgrading it over the next year or two.

tubes4hifi wrote:a fair price for that item, but realize you can get a completely new ST70 with our better performing VTA driver and all new tubes for around $300 more than that.
You'll likely spend that much on that amp upgrading it over the next year or two.

Yes, I do realize that and will probably go the VTA70 route unless I can find a deal on a ST70 locally. Shipping costs to Hawaii can be brutal. What I need is for Bob to sell his kit through Amazon Prime!

.

evoroadster wrote:Shipping costs to Hawaii can be brutal.

Yeah ... real brutal living in Hawaii ... we feel your pain!



We traded that for cheaper shipping? We be dummies!!



I'd take the amp for a test drive if you're close enough to make it a day trip. Seems a reasonable price if the iron's already been upgraded. That tends to be the weakest link in these old timers. Multicap is gone, so the VA production date is probably about as old a cap as you'll find in there. Borderline at 25 years for reliability, but it might surprise you. See if you can talk the seller down some ... try offering $350 and maybe go another $50 at $10 a roll ...

Recently, the small trans under the amp (Triad C24X) was replaced with C354 which has slightly less resistance and inductance than the stock.

One thing I'd put on the to do list - you might want to switch back from the 354 choke to the original spec C24-X ... the 354's were known for bleeding out from heat, and the C24-X tends to run cooler due to the increased current capacity ... real simple, even if you're mechanically declined. Depends on how hard you plan to run the amp mostly.

I had thought about putting some amps and preamps on Amazon. Doesn't make it any cheaper though, and not sure how the shipping would work out
(most likely I myself would be absorbing about $100 worth of shipping . . . )
and of course you can always use PayPal on my website, and get 6 months with zero interest.
I use PayPal to pay for everything, and even get 1% rebate on everything!!

Van Alstine did a pretty good mod - I had the next version, the U-70 - but they can be boomy compared to some other amps. I ended up selling mine once I heard the sweet high end from my ST-35. Not a bad price but it's old enough that some caps and more may be ripe for replacement.

Yes, I fully understand the need to update various components in order to put something like this back into optimum condition. I do have a small hobby bench with the normal type test equipment needed to T/S and repair audio components. My interest was to learn the idiosyncrasies of this particular modded approach since I am relatively new to tubes and the ST-70.

When I first came to this site, I was looking for comparisons of my Van Alstine U-70 with the VTA sound and nobody seemed to have had experience with both. As I heard more amps, particularly my ST-35 rebuild, I became convinced that my U-70 was boomy in the mid-bass and lacking in the high end. I sold mine. I later picked up a nice clean stock ST-70 with a view toward doing the VTA upgrades, but I have yet to pull the trigger for financial reasons - and I may have to abandon the project and sell the ST-70 if things don't look up soon. So I've never actually heard a VTA ST-70.

daveshel wrote:When I first came to this site, I was looking for comparisons of my Van Alstine U-70 with the VTA sound and nobody seemed to have had experience with both. As I heard more amps, particularly my ST-35 rebuild, I became convinced that my U-70 was boomy in the mid-bass and lacking in the high end. I sold mine. I later picked up a nice clean stock ST-70 with a view toward doing the VTA upgrades, but I have yet to pull the trigger for financial reasons - and I may have to abandon the project and sell the ST-70 if things don't look up soon. So I've never actually heard a VTA ST-70.

As stated in the initial post, I am finishing up on a complete rebuild of a SCA-35 chassis. My work bench is in the garage and the weather here has been horrendous the last month or so. Normally we only get a hand full of 90 degree days but with El Nino conditions we have had weeks of 90+ along with Miami-like humidity. As a result the SCA-35 rebuild has taken a hiatus. I am about 90% done and am looking for another tube project. I currently have several SS amps and receivers that need repair but the lure of the glass bottles are leading me astray. Since the Craigslist is only 4 days old I will sit back and watch it for awhile to see what happens. If still available after a week or so I will look into it. If I can get it for $250 or so I will pull the trigger. If not then a VTA-70 is in my future. Ebay kinda scares me for something like this. Case in point, I am also looking for a Pioneer RT-707 to complement a SA-8100 integrated amp I recently repaired. Nothing like that is available locally and I don't have the option to drive a few hundred miles to check one out and what I see on Craigslist is scary. Oh well, that the Price of Paradise!

that's the same AVA mod that I tested along with 4-5 others back in 1989. Hard to call it a mod, it's the original Dynaco circuit on a new PCB with a couple
of resistors and capacitors added to reduce the bandwidth to just what's necessary for audio. Not much of an improvement over the original circuit at all.
Here's what I had to say back then . . .
"This modification was a personal disappointment to me. VanAlstine has a reputation for very good circuit modifications. However, in this case, it is identical to the original, using two 7199 tubes. The only differences are the addition of "spectrum filtering" consisting of two resistors and two capacitors which limit the input signal to a bandwidth of 20 Hz to 20 KHz, to minimize non-linear distortions created by the feedback signal due to the limited open loop bandwidth of the driver circuit design, plus bigger capacitors for the power supply, and larger interstage coupling capacitors. The parts quality is good but not great (film capacitors rather than polypropylene). I was really expecting a total redesign. Also, the PCB is larger than the original in order to accommadate the larger power supply capacitors. It is mounted 1/2" above the chassis on standoffs, which gives it a rather strange and awkword appearance."
"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.
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."
" Neither of these versions (stock and AVA mod) has a good high end, because of the limitations in the design of the driver circuit. The first impression one has in switching to a tube amp from a "modern" transistor amp is the openness in the sound, the transparency to the music. This was fairly true here, but not completely. The dynamic range of the music sounded compressed and slightly harsh, with a slight veiling of the soundstage (though not as veiled as my backup Carver amp). Music was reproduced with a fair amount of detail and imaging, but it was much apparent in listening to the other mods that these two versions were in a different league than the rest.

Great input. I had already researched what the ST70i mod was and its effect on the "musicality" of the amp but was looking for the opinion of someone who had actually heard both and could make a comparison. So, with all this in mind I will press on with completing my SCA-35 and revisit the ST70 build a little later. Thanks again to all who responded.

Bob Latino wrote:I totally agree with Roy here ... The original Dynaco ST-70 driver circuit with the single 7199 driver tube per channel was designed not for good sound but to allow Dynaco to "save a tube". That circuit was designed as a cost cutting measure. Those 7199 driver tubes back in the early '60's were $1.25 to $1.50 each. Since Dynaco sold about 350,000 ST-70's, they saved ($1.25 X 350,000) = $437,500 in 1959 dollars by using that circuit. $437,500 in dollar value in 1959 (the year that the ST-70 was introduced) would equal $3,552,500 in today's value.

The 7199 driver circuit with the AVA two resistor and two capacitor mod IMHO is a band-aid to limit bandwidth to something that this driver circuit can handle. Its like have an automobile whose front end shakes at any speed above 70 mph and then creating a solution to the problem that equates to placing a governor on the auto so that it can never reach 70 mph. To use the AVA mod and then having an amp with a power bandwidth of 50 to 5500 Hz is not a good thing.

The present VTA CCS circuit is (IMHO) head and shoulders above the original Dynaco 7199 driver circuit or the AVA bandwidth limiting version of this circuit.

In 1974 the Federal Trade Commission came out with standards for amplifiers with respect to power, distortion, bandwidth etc. Below is the page from the Dynaco 1974 brochure where Dynaco now had to rate the power, distortion and bandwidth according to standards set by the FTC. No more 35 WPC - now 20 WPC, bandwidth 50 to 10,000 Hz .. a more realistic rating for the ST-70 amp.



Bob

Hi Bob…

IMHO ….i think the AVA two resistor and two capacitors are there yes to limit band with but to the band with of output transformers, to avoid or prevent saturation of the core of the output transformers.

About the 7199…sure…. there are better driver stage than the Dynaco original circuit like for example two double triode with an intermediate driver stage like the Williamson or the Michelson & Austin or the Conrad-Jonson MV75 each one with different configurations but none of this amplifiers can do a challenge with the quality for complexity of Dynaco circuit.

with regards.

Tiziano