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Made the biggest difference ever

74.88.152.59
Posted on February 9, 2018 at 08:11:40
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 Just got done with my two year project(mostly in waiting). I built a huge power supply for the ST-70 using my own formulas and doings with my vast but sometimes dumb logic, but things I learned over the years. The only thing I kept stock was the input board and the output tubes. Even the filament supply was over sized with its own filament transformer. I had the HV B+ power tranny custom wound.

The whole project was a huge success but costly. I totally eliminated all the power supply from the original amp and used a power cord from an outside power supply on its own chassis. The filtering on the power supply chassis is a standard R/C with no choke.

I guess, except for the input board, this is not a Dynaco anymore. I call it a "Schwartzaco". Not my main and reference amp as I am not a favorite of push/pull amps but it was an experiment I always wanted to try based on some suggestions from friends and techs I know.

Looking at the amp you can't tell it was modified except for the power cord going from the power supply chassis to the amp chassis. The input board was a new one but the original circuit using updated parts. The capacitance for each stage was increased a small amount. Nothing drastic?

As for the sound? Way more open and detailed than you could ever possibly get from a standard Dynaco. Most of these so-called modifications all deal with everything and anything(input board, caps, tube rolling, etc.) but no one ever addresses the power tranny which is the first and foremost component in anything. And probably the most important.

This all can be put back to stock very easily with about an hours time and a soldering iron. No extra holes were needed to be made.

 

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Congrats! Pics please...., posted on February 11, 2018 at 19:41:32
viridian
 Actually, the Triode and Dynakitparts Power trannies are both higher capacity than any of the OEM transformers, though I am sure that all are compromised by having to fit the original space allotted. I am sure that yours is far better being off chassis, but I have heard very appreciable sonic gains in my Dynas from the replacement trannies.

 

RE: Congrats! Pics please...., posted on February 11, 2018 at 19:51:16
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 I have more than tripled the current output and that alone says it all but the choke that comes with the ST-70 and even the Mk III's and IV's is anemic. It actually slows down the response. In some of my mods I took the choke out totally and some I moved it to after the first stage putting the B+ wires from the output tranny's right on pin 8 of the rectifier and adding 20UF's more capacitance. That in itself made a huge improvement. Made is sound faster.

In some I put tons more capacitance but I had to change the rectifier to SS as not to ruin a rec tube.

With all that said the largest improvement was going to an all film power supply with a huge(and I do mean huge) power tranny.

 

Nice!, posted on February 15, 2018 at 16:53:03
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 An all film power supply would be part of my dream ST-70 too. Nice to hear that all your work paid off so well. Congratulations!

 

RE: Nice!, posted on February 18, 2018 at 14:45:01
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 All film supply can be made putting the caps on another chassis and running an umbilical power cord from the points where the electrolytics are connected to each of the caps on the board. I did that for numerous people over the years and the biggest improvement was still the power tranny. Even the filament supply is way over what is needed.

If you ever choose to do a cap board type of mod just disconnect the ground from each caps and they are out of the circuit totally without removing them. This way if you want to go back to stock it's very easily done. Also, either take out the rear power socket or run a multi conductor wire through the middle of it and solder each connection under the amp to their respective places. No holes to be drilled. On the cap board you can do the same or have a power plug installed. I used to put them on a PCB and put them in a little Hammond or cheap chassis of your choice and have the box next to the amp. Very simple mod and worth it? I'm still on the fence with that. Lytics aren't all that bad. Just a different sound. Better? Maybe! But they are better for bass.

The biggest improvement one can do to that amp is a SS rectifier and getting rid of that anemic choke. Sorta like a VTL power supply. I also used a power tranny with 4-5 times the current output of the original. Worth it? I say yes. Even the upgraded one sold by Triode isn't as good as this one. But this isn't a drop in design. You will have to play with reducing voltages here and there. Mainly on the input board.

Also, with all that said. Increasing the capacitance by more than double opens up another can of worms. You change the TC's(time constants). If you do go more don't go with more than 40UF on the rectifier(pin 8) if you leave it tube. There is a SS drop in you can use if you want to give a SS rectifier a try. It comes as an 8 pin tube socket only there are SS diodes in there. Then you can go to about 100UF or more on the rectifier without doing any damage. Helps immensely if you eliminate the choke.

 

RE: "getting rid of that anemic choke", posted on February 24, 2018 at 05:41:41
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 Everyone seems to agree that the Dynaco C354 choke is under-rated for that application.

And I've also read in previous posts that you've preferred the sound with the choke completely removed, and a dropping resistor substituted.

What I haven't ever read is what a properly rated choke would be in an otherwise stock C,L,C,R,C etc. ST-70 power supply. 300mA has been quoted as a better current rating, but what about Henrys?

Granted, there's no way to include a 5H or 10H, 300mA choke on the stock chassis, but on a separate chassis, connected via umbilical, it would be easy enough.

Has anyone ever calculated (perhaps using the Duncan Amps PSUD2 power supply designer) what the ideal choke for a ST-70 would be?

 

RE: "getting rid of that anemic choke", posted on March 2, 2018 at 19:24:02
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 Maybe post this question on the DIY Tubes section. Those guys are the best. But also the power transformer is way too small for the ST-70 using it for both channels. The ST-70 tranny would be better for either using it for one channel of the ST-70 or using it in the MKIV.

 

RE: "Those guys are the best", posted on March 3, 2018 at 09:48:51
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 I agree with you, they're very good, and very helpful.

I don't like to wear out my welcome though, so unless I was actually going to build a ST-70 with a proper power supply, necessitating a larger (or separate) chassis, I think I'll save my questions for something I actually need to know, rather than just something I'm curious about.

I really do need to learn how to use that Duncan program correctly, so I can answer my own questions and curiosities. Teach a man to fish, and all that!

Again, thanks for posting your results.

 

RE: "Those guys are the best", posted on March 3, 2018 at 13:45:19
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 They are happy to answer questions pertaining to audio and tubes. Even if it's a curiosity thing. I feel I do make a pain of myself and they are more than helpful.

But as I said in my posting. If you do a "proper" power supply it is not a Dynaco any more. I called mine a "Schwartzaco." Even thought the input board is the original circuit and the rest of the amp is academic. I rather like separate bias pots but to keep the amp in its glory state I kept it at single pot paired biasing. Going to individual biasing didn't do as much as reworking the entire HV B+. But everything helps.

 

RE: "it is not a Dynaco any more", posted on March 4, 2018 at 04:42:02
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 That's very true.

It used to be that people would say, "Who cares? It's just a Dynaco." But the price of stock units has gone up over the years. I'm glad I still have two unmolested ones, and one other that I "un-Dynacoed" years ago.

 

RE: "it is not a Dynaco any more", posted on March 5, 2018 at 16:59:39
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 Hey,

Keeping each of your "unmolested" amps in stereo(do NOT bridge with the switch) use one channel of each amp. That inadvertently doubles the power supply capacity without touching a thing. Even larger than the one used in the MK IV's. Still not as big as the one I built but still it is double the size as if you were using it as a stereo.

 

If I ever run out of projects, posted on March 6, 2018 at 05:59:28
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 ...I'll give running one channel of the two stock ST-70s amps a try. Given that the quad-caps in both are original, and both have selenium rectifiers in the bias circuit, it wouldn't be as easy as just hooking them up for an interesting afternoon experiment though.

I've got two MK-III amps which have been waiting for my attention for far too long. And another project which really interests me, which is building a 6BQ5 amp using a pair of Z-565 transformers I have, but on a custom ST-70 sized chassis, as shown below. (not my amp)





Given the excellent results I had with Dave Gillespie's EFB in my SCA-35, I'd use the same bias scheme for this amp, only I'd change it to individual EFB for each output tube. The added chassis space would allow me to use a choke, and maybe some room for those Wima DC Link caps in the power supply.

The real question is, what input circuit to use? Given the scarcity and price of NOS 7247, I wouldn't go with a ST-35 circuit. I bought a pair of current production JJ 12DW7/7247 and sent them to a guy who has a Fisher SA-100 which uses that driver tube. His take on them was that they were "just OK" (shrunken soundstage and "thin" if I remember correctly) compared to his NOS tubes, so that ended the idea of using the ST-35 input circuit.

 

RE: If I ever run out of projects, posted on March 6, 2018 at 07:24:29
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 I have an EL-84 Lafayette tube amp here that I haven't tried in years. Maybe I should! I redid the front end to it suggested by an old audio friend. It used to use the 6AN8 driver/phase splitter. I now uses the 6N1P and sounds great. The gain is lower but who cares.

 

RE: If I ever run out of projects, posted on March 6, 2018 at 07:28:50
xaudiomanx
Audiophile

Posts: 3647
Joined: August 16, 2004
 That chassis is the same as the one that was sold by Jon Yaeger or it appears to be the same.

 

If I recall correctly, posted on March 6, 2018 at 12:33:00
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 I bought mine from ebay, way back when, but I don't remember who the seller was.

The chassis looks nice, so one drawback will be using the SCA-35 power transformer, which all seem to look like they were pulled out of the La Brea tar pits.

 

Someone gave me a Lafayette , posted on March 6, 2018 at 13:15:04
1973shovel
Audiophile

Posts: 10117
Location: Greenville SC
Joined: February 25, 2007
 A "Stereo 250A" integrated, years ago, but I never brought it to life. It uses EL-86s as the power tubes, driven by 7199s, (with 12AX7s in the preamp section).

I have a fair number of 6AN8, most GE branded, which I used to pick up at hamfests for my pair of MK-III for $1 each.

The 6N1P looks interesting. Perhaps I should consider that tube for my 6BQ5 amp build. Thanks.

I cycle several of my amps into the system occasionally, just to charge up the caps, and reacquaint myself with the sound. You should try your Lafayette again, just for fun (and to reform the electrolytics)!



 

RE: Congrats! Pics please...., posted on July 24, 2019 at 20:00:57
DANIL
Audiophile

Posts: 6
Location: DALLAS
Joined: June 12, 2007
 Hi
If you eliminated choke, what resistor value are you using for tube rectifier? And for diode rectifier what resistor? Value and wattage please.
Thanks, Rady

 

RE: Made the biggest difference ever, posted on March 5, 2021 at 18:45:30
Mushroom Soup
Audiophile

Posts: 310
Location: Western New York State
Joined: November 1, 2003
 I've had a project just like this in boxes for decades. I have most of the stuff I need. Congrats. I dream of a St70 with some punch to it.

 

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