I would look into some of the tube specialist forums. I'm sure that there are plenty of various opinions on this.
My take would be to replace all of the aging capacitors, clean all connections as best possible and then re-flow all the joints with a high grade solder ala Wonder, Cardas, WBT, etc.. This will basically maintain the integrity of the existing design while allowing it to function at top performance.
Obviously, you would need to check the tubes and replace accordingly. My one suggestion in this area is that if one output is bad, replace all of them at the same time, etc... Same goes for the driver section. Running tubes of different ages, production runs or brands can contribute to both lower performance and increased wear and tear.
Once again though, i would suggest checking out the various tube resources that are available to you to find out what tubes work best in your specific unit. Sean
Curcioaudio.com is one of the better Dynaco ST70 mod sites. Joe, the owner, knows em cold. All the mods are discussed there and he carries all the parts. No "best configuration."
Some of the ST70s iron is great. They really vary as to condition in other ways. Some even say the phenolic boards cause chemical changes in the electronics and change over time and with it the sound - no two are the same.
If you have an older Series 1 the passive componants probabaly could use to be replaced. They weren't expected to go 50 years. There are lots of versions of these amps over the years so advice about moding is tough. What is the number stamped on the output transformer? A-470, maybe.
Lots of discussion in archives at Audioasylum diy tube section on point.
I agree with Sean's good advice. Also,the ST-70 could definitely benefit from power supply upgrades especially in the filter caps, for increased headroom. Doubling the microfarad amount is not uncommon. Frank Van Alstine has been doing excellent mods/rebuilds on these old Dynaco's for years. He would be the first place I would check. He used to offer complete mod packages, in stages, for reasonable money. Probably still does. He's an old timer and he knows what he's doing, especially with Dynaco.I think his company name is Audio by Van Alstine.
This site has a reprinted article that explains in depth exactly how to rebuild/upgrade a Dynaco St-70. Very long, extremely detailed, step-by-step instrucions. I think you will find it very informative. It's on the Van Alstine website. He is currently no longer modding the St-70, but his does offer a complete re-make of the amp with new circuit board design using your old chassis and transformer.
Leave it Stock.Only upgrade the passive parts.
There's a guy that has done MAJOR re-design of the Dyna PAS-2 and PAS-3 using very advanced computer simulation and a lot of know how. His name is Norman Koren and he's had more than a few articles published in various tube / tweaker mags.
I intend to modify my Dyna pre using his basic suggestions once i get the time. Don't know if he's done anything with the amps, but you can try checking his website. It would at least give you an idea as to what is involved in modifying / upgrading a unit of that nature.
AVA ( Audio by Van Alstine ) may be another good source for parts and schematics but i don't think that his ideas are near as progressive. Sean
I think soundvalves.com points out that even the stock version has some easily correctable mistakes that to do just a parts upgrade would be a bit of a shame. So at the very least/minimum go their route, along with the better parts. Now if you want to take it further others have presented those options too.
If you do nothing else get rid of the selineum rectifier. Replace it with a diode. These things blow up and take other stuff with them and when(not if) they blow they emitt a toxic cloud into your room!
I did the soundvalves mod ($99 many years ago), with great results....more detail, etc. I also replaced the selenium rectifier, beefed up the power supply with more caps, and parallelled all electrolytics with small value Wonder Caps (you would use MIT caps today).
I have modified several ST-70s over the years and the only stock parts I keep are the chassis, transformers, choke, and bias pots. Everything else (RCAs, quadcap, binding posts, etc.) should be replaced with quality parts. I have replaced the 7199 front end with a 12AX7 front end that Jim Aud of Purist Audio came up with several years ago. Considering it's hard to find good 7199s anymore the 12AX7 front end is the way to go and is sonically superior.
Since you know a lot about the ST-70, I will ask you if it normal for them to have a chasis hum, not audible in the speakers but directly from the chasis? I can hear mine from 20 feet away when the music is very soft.
Thanks to all who answered my question about chasis hum in the ST-70. It turns out there was an open circuit in the rectifier, which was fixed by adding a diode in the path. Now it is quiet again and I have sold it.