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Hi, I have this great amp a friend wired up for me in 1990.I traded him 2 Dyna PAS's with 1 coming back modified by him and one for me to keep.Also in the deal was he modified my Dyna ST 70 into a Purist Audio Mod by jim Aud in Texas.It uses 12AX7"s instead of the 7199 original tubes and bias's at .75 volts instead of !.56 like the original ST-70.I've never changed an output tube because they don't get hot,warm but can be touched.The mod was a winner in a 5 amp ST 70 shootout in one of my old stereo mags.I noticed one day a small hum coming from the amp.When I looked closer and the output and power transformers had missing screw rods and nuts where holes were drilled in the bell housings.If you look at a lot of pictures of the Stereo 70 these holes are vacant.While looking for new ones on the net I did a little experiment.I put a shiv,a little wedge of wood carpenters use for hanging doors ect, under the power transformer.The noise reduction was like night and day! I saw this clip on McIntosh amps where they poor tar into the transformer bell submersing the primary and secondary in black gunk.My idea is the iron plates in the Dyna transformers must corrode because of their age and pressure from the securing screws is important to keep out corrosion.Less screws hense....the Poor Man's McIntosh.Just my observations,I could be dead wrong.The amp still plays beautifully....Mark Korda
Follow Ups:
Sometimes it pays off to disassemble things down to the individual components before modding/restoring something like a ST-70. Here's the one I did many years ago, using the Triode Electronics driver board and 6L6GC output tubes. I probably regret selling this amp more than anything else I've let go of over the past 50+ years.
that's a lot of work so I'm jealous of its obvious beauty
Yup. Unbeknownst to Glass Audio, Roy Mottram, the author, owned/owns Tubes4hifi. When Ed Dell, the publisher, found out he printed a retraction/apology.
Hi Shovel, that was a great find but it's not the correct article.Thanks for looking that up.I'm going to save that. I emailed the guy who modded my amp to the Purist Audio mod Bob Gates. Bob and Jim Aud were in the Vietnam conflict working on and around Phantom jets, or the jets of that era.I kind of taunted them a little to join this forum and back up my story. Bob also modded my Dynaco PAS 2 following the Last PAS article by David Vorhis. I made a little square of brass covering the tone controls and gave him a stock PAS 2 to keep. We met thru the old Audiomart before the net. Recently I stripped down the PAS and made a new faceplate with wood and poly and my name covering the selector switch which is now on the bottom left.I have a gold plated switch there, cd or LP. I'll send a picture to see if you like it.I'm hoping Bob or Jim will get back to me on this before I enter The Audio Asylum...-take care...-Mark Korda
I recall us discussing this modded PAS once before, but I seem to remember you saying that he didn't regulate the B+ in yours. From your picture it looks like you don't have the external cap bank for the filament supply either, as called for in the build article.
Here's a section of the article regarding the heater supply. My 'Vorhis' has the regulated B+, and even so, I found impressive improvements to be had by adding large amounts of capacitance to the heater supply. If you've got room, I'd strongly suggest paralleling extra capacitance to yours.
The phono stage of the Last PAS always left me somewhat wanting, but I still find the line stage particularly good.
Hi Shovel, the guy who modded one of the PAS 2's I had took out the tone controls and added more caps.It worked and sounded great but he left more stuff in that I took out including the rotary control. You can see the extra caps he installed. The only components I took out were 2 resistors in the phono section.I put 2 5 position lugs to reinstall the phono resistors in the phono section. I have not retired it yet but have a picture of what my friend did inside on the circuit boards.I also will be putting in a Sheldon Stokes cap board which will replace the selenium rectifier area. Here is a picture...-..Mark
...actually reduces filament supply capacitance over even the stock PAS. Dynaco used two 2,000 uF caps, which the Stokes board reduces to 1,800 uF each, which is a step in the wrong direction. I'm going to assume the capacitance was reduced due to space constraints on the SDS board.Nichicon makes 12,000 uF @ 25 VDC caps (link below), which in my experience would be a sonic improvement, but you may have trouble making them fit on that board.
Another thing to note, the SDS board uses 1N4007 diodes, which is what I originally built my 'Vorhis' with. I later changed them to 31DQ10 Schottky diodes, and heard a distinct improvement over the 1N4007s [in the filament supply].
Edited to clarify that the 31DQ10 Schottky diodes are only to be used in the filament supply. I also upgraded the 1N4007s in the B+, using UF4007s per Eli Duttman, but there I heard no improvement.
Edits: 10/10/24
Hi Shovel, thanks for all that info. Even though I have had no problem in the past with the selenium rectifier rig wouldn't the Sheldon Stokes board best the old selenium set up? D Lab does an installation You Tube video on the Stokes board but he installs it upside down.Like Vinny Barbarino...-.I'm so confused! Thanks for taking the time writing that detailed letter. I had no clue about the quality of diodes...-take care...-Mark Korda
Hi Shovel, thanks for all that info. Even though I have had no problem in the past with the selenium rectifier rig wouldn't the Sheldon Stokes board best the old selenium set up? D Lab does an installation You Tube video on the Stokes board but he installs it upside down.Like Vinny Barbarino...-.I'm so confused! Thanks for taking the time writing that detailed letter. I had no clue about the quality of diodes...-take care...-Mark Korda
Yes, if for nothing else, a decreased voltage drop and more importantly, to potentially avoid the rancid fumes produced by the selenium rectifier when they 'go'.
I wasn't suggesting that you keep the selenium rectifier, but I would suggest using caps much greater in capacitance than the Stokes board's 1,800 uF. You have posted that you have a Last PAS, but if you still have the selenium rectifier, stock value caps in the heater supply, and are using the 12X4 rectifier tube instead of regulated B+, then you really don't have a 'Vorhis' Last PAS, in my opinion.
The improvement in adding significant capacitance to the heater supply is easy to hear. Using your current set-up, get yourself two 25 V or greater electrolytic caps of substantially higher capacitance than what is already in your heater supply, along with some alligator clip leads. With the preamp off, attach the caps in parallel to those already in place, being sure to observe proper polarity.
Turn the preamp on, and listen for a while. Then, without the need to power the preamp down, carefully disconnect the clip leads from the stock supply, and listen again. As long as you're careful not to discharge the external caps, you can reconnect and disconnect them at will. I think you'll like what you hear with the added capacitance.
All this already is detailed in The Last PAS article by Dave Vorhis. Email me through the Asylum if you don't have a copy.
Hi Shovel...-..
"bias's at .75 volts instead of !.56"
That's 24ma per tube of idle current vs. the stock 50ma.
It's no wonder the output tubes last a long time.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
...so easily. One point though. You used a shim, not a shiv. If it were a shiv, then that ST-70 might be just the thing to have if someone had to do any serious time. It would be pretty difficult to hide a ST-70 in their prison wallet though.
You can have a personal music player unless you are under restrictions.
You find a warden who's audiophile-friendly you likely could get that inside. Guy next to me has some nice Wharfdales.
/ optimally proportioned triangles are our friends
.
/ optimally proportioned triangles are our friends
Hi, the Dynaco ST 70 shootout is in Glass Audio magazine.I 'm looking for it in my stack of stuff but it's In the 1988 to 1990 range...-.In the Purist Audio Design link I sent there is a last line on the bottom if clicked will give you the history of Jim Aud and Purist Audio Design...-.Mark.
The only mods covered were the stock amp with parts upgrade, the Sound Values mod, the Van Alstine mod, and the Enlightened Audio mod by Randy Nachtrieb.
The overwhelming best mod was Randy Nachtrieb's mod, but the best value was the parts upgrade mod.
I remember the article. The writer turned out own the company whose modification won the competition.
This one? (Link below) If so, there's no mention of mark.korda@myfairpoint.net's Purist Audio Design version. The GSI mentioned used 12AX7s, so perhaps PAD changed their name from GSI at some point, unless Glass Audio did another ST-70 "Shoot-Out" article.
Hi Spelling Police, my mistake! In my defense though I'm from Maine. There are several books on Down East Lingo where there are words spoken you guys wouldn't even understand. A long time ago I worked in a fisherman supply store.There were young lobstermen coming in for supplies I couldn't even understand. Down East is not really down east.It refers to the islands and small towns that are north of say Portland. Ayuh means yes. A guy named Marshall Dodge made this lingo kind of famous on a comedy record called Bert and I. Tre, I have the schematics and info on this amp.The guy who came up with this design, Jim Aud had started a company in Clute Texas called Purist Audio. While no longer involved with the Stereo 70, he was selling these esoteric cables which one is encased in H2, I can't type a small 2. It is not H2O but something like (Heavy Water) which the Germans were experimenting with in WW2. P.S., I use Siemens EL34 power tubes in this baby. P.S. again,I did mean Shim but it went right over my head. If I still had my copier I would send you guys the mod instructions. If your interested I will make you a copy and send it to you for nothing...-.take care...-.Mark Korda
Hi, Purist Audio Design is still kicking and has a web page.
https://www.puristaudiodesign.com/index.html Here is the link...Mark
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