Identify the Driver Board on a Modded ST-70

I've already sent images of the board to Bob, but thought you all might like to see the inside of one of my ST-70.  I acquired it along with four others and a Pilot - this one was modded by a real great guy and a tube amp genius/mentor and friend.  Bob hasn't confirmed this, but he believes, from my description, that the board is an earlier version from the 1990's.

It did not have the tubes so I wanted to be sure which version it could be.  The 12AU7 version, or the 12AT7 version.  I had a suspicion from the part values...but good to know for sure.

Thanks for looking into it Bob. Enjoy.

That's amazing Bob. Thanks a bunch. There is a lot going on inside this thing, and I will enjoy/go mad working it all out. Any chance of a schematic being available? Lots of things I haven't seen...not saying a whole lot...and certainly unique to my original ST-70's.

Again, many thanks to yourself and Mr. Mottram for sharing your valuable time.

Best, David

yes, that's a board from 1989 or early 90s, using 12AT7 tubes, my original design

Honored that you took the time to confirm it and doubly cool to meet the designer.  Kudos to this fantastic site!

Ever since I cobbled up a my first guitar amp as a kid, I have been fascinated with all of this stuff. Especially tubes of all kinds. Since, have built many guitar amplifiers, etc.

Only recently, due to the fantastic gifts from a dear friend and mentor, have I started to dive deep into hifi designs.  I love the cross training!

If I might ask while I have this unique opportunity, is it pretty much the same circuit or have there been improvements to it as the years have gone on?  It sounds great and I will enjoy its uniqueness.

Again, your time and attention is greatly appreciated.

Best,
David

welcome David,
the circuit you have is schematically identical to what was offered up until about 3-4 years ago (no improvements really necessary!!)
The only real change the past couple years was to change a few components to use lower gain 12AU7 tubes and to add a CCS circuit for each phase splitter.
From what I could see in the photos, there is a pretty good chance that I actually stuffed the parts on your board,
and punched the holes in the chassis for the 3 large capacitors.
Roy Mottram - Vacuum Tube Audio - tubes4hifi

Even more cool to know you might have stuffed it. Not sure how my friend acquired all of these Dynaco units I now have, but I do know he would appreciate; and likely knows full well, your expertise. I do appreciate this very much.

Now I got to dig into several PAS preamps (3,4's), FM-3's etc. to put together a kick-butt system.

Have a fantastic day.

Sincere regards,

David

Hi Roy,
Been meaning to ask...if there is any documentation available from the time this board was new. Build instructions, bias tips, etc. I'd love to have it to go with the unit. Totally enjoying it. Thank you again for sharing your knowledge.

Welcome to the board Mando. Plan to get stuck in here--it's a friendly corner of the web to learn, get help, and to enjoy the company of other Dynaco loving, fiscally restrained audio philiacs.

the oldest documentation I have is around 5 years old, but the circuit should be nearly identical to what you have.
send me an email at info@tubes4hifi.com and I will send you an old PDF file

See, this is the kinda customer service you get around here--you're not even a customer yet, and you get service direct from the man himself!

Couldn't be more happy to be here and look forward to contributing.

Best...