Dynaco ST70 Design Quality

Tom Bombadil

AK Member
I know the ST70 was designed and built as a mid-cost amplifier and used some average quality parts, but was a respected performer and has certainly stood the test of time.

I see there are still several companies and builders offering new versions of the ST70. Most of them still use circuits very close to the original design, excepting for redesigning the 7199 circuit. I've been surprised by how close most of these offerings are to the original design, many stating that they are basically the same, but using upgraded parts.

Is the ST70 design, still today, considered a first class tube amp design? Or do you think that so many stay close to the original in order to produce a similar, family-style sound to the original?
 
As the old saying goes, if it ain't broke, don't fix it. :) The original ST-70 design was pretty 'high tech' in its day, using then-new printed circuit board technology which was pre-populated with components, saving on assembly time. This, along with their high-quality ultralinear iron, was the genesis behind the whole Dynakit idea. The original phenolic circuit boards were somewhat flimsy; advances in printed circuit board technology since then have helped greatly with the reproductions.

As for the design, I'm not sure it was ever considered truly "first class", whatever the qualifier for that might be. It was a simple design which was quite effective, giving a large amount of 'bang for the buck' while being fairly easy to assemble for the kit-builder. The circuit design itself has withstood the test of time, as Dave Gillespie's baseline testing thread (as linked to in the above reply) has proven. Various modifications and replacement boards are available from numerous sources, but I personally have yet to be bothered to replace the stock circuit board in mine, as it sounds pretty good to my ears. :music:
-Adam
st70_3.jpg
 
I do like the VTA driver board. The 3 tube boards use 12au7's. And with the higher current power transformers available other 12au7 types can be used. Even 6SN7's with adapters or an octal board. The decision between using a clone of the original and the VTA board came down to tube choice for me.
 
The design is straightforward and easy to understand.
If you want to learn and understand power supply, bias supply, driver and output stage design, the ST70 is an excellent platform.
The quality of the iron is good. The layout is textbook. It's an amp that can be fixed forever and passed on to future generations.
What more could you ask for?
 
The only tricky part about it is that variables in the pentode section can mess with phase inverter balance. The fix is either swapping tubes until you get a pair of good ones, or messing with the value of the screen resistor until it gets right.

I would like to see hard data on the various mods to see how the performance stacks up against the original. I know thats not the whole story, but it would be interesting to see.
 
Dave actually mentions it in his baseline testing. To quote

"To produce optimum operating conditions, the standard rule of thumb for phase splitter designs is that 1/3 of the B+ supply
voltage should be dropped across each of: (1) the plate load resistor, (2) the tube, and (3) the cathode load resistor. To correct a
tube example that over-biases the triode section, the screen grid voltage of the pentode needs to be reduced, allowing its plate
voltage to rise, causing the bias to be reduced in the triode section, and optimum performance to return."
 
Dave actually mentions it in his baseline testing. To quote

"To produce optimum operating conditions, the standard rule of thumb for phase splitter designs is that 1/3 of the B+ supply
voltage should be dropped across each of: (1) the plate load resistor, (2) the tube, and (3) the cathode load resistor. To correct a
tube example that over-biases the triode section, the screen grid voltage of the pentode needs to be reduced, allowing its plate
voltage to rise, causing the bias to be reduced in the triode section, and optimum performance to return."
The revised driver used in the SCA-35 was supposed to address this by tying the screen source to the splitter cathode thru a high time constant filter.
 
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Might be worth a look at both the ST-70 and the SCA-35 drivers to see what would be involved to implement that. I like circuits that are not overly sensitive to individual component properties.


I expect part of why the "it varies" note exists in the ST-70 manual is because it would be hard to pin down. The high value resistor feeding the screen plus whatever load a 1960s analog meter presents will give numbers all over the place, plus of course whatever current the screen is drawing. It depends on the tube and on the meter in use.
 
Is there a plate/grid voltage check that would indicate a potential imbalance from the pentode?
Yes. The plate of the pentode, Pin 2, should be as close to 90v as possible. Adjust the screen grid resistor to achieve that and you're done. This is crucial because the plate and triode grid are directly coupled.
 
I don't think it was simply the low cost ($99 for the kit) that made it the most popular amplifier ever sold. Hafler estimated that over 600,000 units were produced and delivered. I love mine. She was "my first". lol I learned a lot abut tube amps. from her and now I can build my own. It did take a bit of courage to work around 425 Vdc though.
 
The Dynaco design is an example of "lean and efficient". Not a unneeded tube anywhere, no complicated circuits, only the straigt road to good sound and fine performance.
The pentod/triod concertine is almost exclusively used, a fine mix of negative and positive feedback taken to an utmost perfection.
Building complicated and expensive amps can many do. Building simple and good - that's an art !
 
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Kind of like the VW of the audio world ... maybe not cutting edge, but certainly popular with the masses.

Biggest beefs over time all had to do with making them affordable. Iron was borderline fully driven, not something that improved over time. My favorite improvements in the newer versions beef that up a bunch, more capacitance for better reserve and recovery, individual bias for each power tube, and CCS on the tail for improved stability in the driver section. The VTA kits also allow you to hot switch between pentode and triode to match the music. Basically stuff that half a million geeky kit builders had been doing for years on their own. ;-}

PS ... did somebody mention using a 6SN7 with adapter? The old Sylvania (Bad Boy) works quite well with a pair of matched 12BH7's here in my custom build. (Not something I'd want to try with the stock iron, especially with a quad of KT120's installed.)

sylvania-drivers.jpg


** Which brings to mind another great feature - the newer boards are built for the 12AU7 family of drivers, which open up a vast multitude of possibilities.
 
Kind of like the VW of the audio world ... maybe not cutting edge, but certainly popular with the masses.

Biggest beefs over time all had to do with making them affordable. Iron was borderline fully driven, not something that improved over time. My favorite improvements in the newer versions beef that up a bunch, more capacitance for better reserve and recovery, individual bias for each power tube, and CCS on the tail for improved stability in the driver section. The VTA kits also allow you to hot switch between pentode and triode to match the music. Basically stuff that half a million geeky kit builders had been doing for years on their own. ;-}

PS ... did somebody mention using a 6SN7 with adapter? The old Sylvania (Bad Boy) works quite well with a pair of matched 12BH7's here in my custom build. (Not something I'd want to try with the stock iron, especially with a quad of KT120's installed.)

sylvania-drivers.jpg


** Which brings to mind another great feature - the newer boards are built for the 12AU7 family of drivers, which open up a vast multitude of possibilities.
Yup. I tried a fist full of 12au7's with the RCA clear tops being my favorite. I also tried E80CC's, ECC90's, 12BH7's and so on. Lots of combo's can be used with the VTA board.
 
Lets not forget the 5814's, CV4003, and 5963's ... LOTS of options.

PS ... for anyone wondering. The standard 6SN7 adapter isn't as tall as my pic shows. I had to put a "socket extender" on mine so it'd clear the hole in the chassis plate. Once again, custom case and some adjustments to make the octal work with it. The washers just pretty it up a bit.

6sn7-adapter2.jpg


Also, the Bad Boy was a bit of a splurge here. Sound is excellent, but the coin base RCAs are real darn close IMHO, and a whole lot cheaper.

6sn7-installed.jpg


Oh. I think I mentioned the quad of KT120's I run here sometimes. NOT something I'd try with the old stock iron! As is, those are currently playing backup to a sweet little set of TAD KT88str's I picked up a while back.

tad-side-view.jpg


Those break in slow, but really bloomed at around 100 hours. They're based on the old British MOV design - heavy glass and evarthang. Be interesting to see how they'd compare head to head with a new set of KT120's - the ones I got are getting a bit long in the tooth.
 
The refurb'ed largely OEM ST-70 "listens" very well within its power limitations, I dont know if any other amps, tube or otherwise, exceed that standard to the point of being worth added complexity and cost.
 
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