[knotscott]

Hi all - I've been running a pair of Dyna ST70s for over 20 years, and it's time some for updates and possible mods.  They've been running in triode mode, and have only been using half of each amp to get the most from the power supply.  Each amp also had some 20uF pp caps added to the cannister cap, along with some upgraded 0.1uF caps in the main board, but are otherwise all stock.  For simplicity, I'm leaning toward running just one of the amps in stereo, then possibly doing something with the 2nd amp down the road.  I could sell the 2nd amp to fund the mods to the 1st amp, but I'd really prefer to hang on to it if at all possible.  Funds for an upgrade are are in short supply....I could probably put $100-$150 into it at this time. 

My question is, what's the best bang for the buck?   I'm leaning towards a VTA pcb upgrade using the stock PT.  A PT upgrade would pretty much blow my whole budget, and the stock PT seems to be fine.  Am I wasting my time by not upgrading the PT?  I'd really appreciate any thoughts and input about what changes will make the most audible difference.  Thanks in advance!

[Mark Korda]

Hi Knotscott,
  This isn't an answer to your question but maybe a request for someone out there to come up with an article that might be of help to you and of interest to me.
  It was an article in a mag. called Glass Audio in 1990. I'm not sure of the month. It was a (Shoot Out) of the top 5 Dyna Stereo 70 mods of that day.
  I'm still running the Purist Audio Mod in mine which a friend wired. That mod is in there and I never did find the article I once saw to read up about. Mine uses 12AX7 tubes instead of the 7199's which were the stock tubes.
   If anyone can find and post that I would give a big ole Double Thanks from both of us cause I know the knotted Scot would eat this up......thanks....Mark Korda

[avahifi]

I remember that article very well.

It was written by the suppliers of one of the ST-70 modifications tested.

Our Super 70 upgrade was badly panned by that reviewer.

However, the review was a fraud.  We had supplied the Super 70 to the user as a do it yourself kit.  They showed a picture of the Super 70 they reviewed and they could never had listened to it at all!  One of the large power supply capacitors was installed backwards on the mother board.  It would have gone up in smoke if they turned it on.

I contacted the editor of Glass Audio about this obvious issue and conflict of interest and he never responded or corrected the review.

Frank Van Alstine

[rlee8394]

Knotscott,

I can personally recommend the AVA Dyna 70 Upgrade DIY kit. You have the choice of full kit, PCB only (you supply parts), as well as option of new jacks if you wish.
It is rock solid, bulletproof, and amazingly musical. I've been using one for years. Yes, it's that good. Frank will even provide you the construction plans free of charge upon request in case you want to see what's involved in the rebuild. See it here:

https://avahifi.com/collections/dyna-and-hafler-rebuilds/products/dyna-70-upgrade-diy-kit?variant=5834255958046

Go for the full kit if you can swing it. The extra cost is well worth it and saves you the time to cobble together your own parts.

The PT is a nice upgrade but not necessary if your original is working properly. Your money is better spent with the AVA board upgrade. If you have the need for just one amp, sell the other one for the needed funds. It would more than cover the cost of the upgrade. If you wanted another one down the road, you could just purchase the parts from Triode Electronics or Dynakitparts instead of a donor ST-70

Just my $0.02 worth.

-Ron

[knotscott]

Thank you Ron

[OzarkTom]

FVA mod all the way. 30 years ago his modded ST-70 was much better than the highly ranked CJ-75a. I sold Conrad Johnson at that time.

[mick wolfe]

FVA mod all the way. 30 years ago his modded ST-70 was much better than the highly ranked CJ-75a. I sold Conrad Johnson at that time.

Along the same lines, (but different) I sold a CJ MV 75A1 is favor of a Latino ST 70 roughly 9-10 years ago. The Latino still anchors my HT/casual set-up driving a pair of Zu Dirty Weekends.

[Mark Korda]

Hi,
    Even with Frank VanAlstine's explanation of the capacitor being in backwards I'd still like to find that article. This could be a new Guinness World Record for a manufacturer's comment about his entry,.... 28 years later. If Glass Audio didn't correct that I think AudioXpress should. If I'm right Glass Audio morphed into AudioXpress.
  After Franks latest tube amp won awards from Absolute Sound I would think it a must for AudioXpress to tell the real story...Mark K.

[Tyson]

Also, I think Frank's mod is closer to the original circuit of the ST70.  The VTA ST-70 is a different circuit entirely, closer to the VTL ST85.  It's a great sounding circuit (I own two), but definitely different than the original ST70.

[avahifi]

Actually our Ultimate 70 has four regulated power supplies, not even close to any other Dyna ST70 upgrade.

These make a huge difference in dynamic range, transparency, musicality and just simple engagement.

Note that our Ultravalve does have a 30 day satisfaction guarantee, so if you want to hear the real difference, it won't cost you much to listen.

I suggest this is a better approach then just commenting about something you may have never heard.

Frank

[avahifi]

posted in error

[knotscott]

Thanks to all for the replies.  I've opted for the VTA blank PCB, and will supply my own parts.  That looks like a good upgrade and stays within budget, which should also leave enough resistors and caps leftover to update the 2nd ST-70.  Then I can keep both amps.   

This is more of a bad weather project, so with summer around the corner, it will likely take me a while, but I'll post back with some pics and comments when I finish up the first amp. 

[serialdriller]

Hi Knotscott,
  This isn't an answer to your question but maybe a request for someone out there to come up with an article that might be of help to you and of interest to me.
  It was an article in a mag. called Glass Audio in 1990. I'm not sure of the month. It was a (Shoot Out) of the top 5 Dyna Stereo 70 mods of that day.
  I'm still running the Purist Audio Mod in mine which a friend wired. That mod is in there and I never did find the article I once saw to read up about. Mine uses 12AX7 tubes instead of the 7199's which were the stock tubes.
   If anyone can find and post that I would give a big ole Double Thanks from both of us cause I know the knotted Scot would eat this up......thanks....Mark Korda

[LesterSleepsIn]

I think you’re referring to the Roy Mottram article from Volume 2 Issue #1 1990 Glass Audio magazine:

http://www.tubes4hifi.com/st70mods.htm

Cheers,
LesterSleepsin

[CometCKO]

Two more ST-70 mods that deserve a mention.  One is the now-defunct Ron Welborne version, which uses a new driver board with 6U8A and 5751 input tubes, plus totally new power supply and bias supply.  I built that one and still think it is the best-sounding ST-70 that I've heard.  I received my first ST-70 as a birthday present in 1967, I think.  My dad bought it factory-built as he had no confidence that I would not burn the house down using my soldering iron.  That unit later became the basis for the Welborne revision 30 years later.  I still have it and use it in my bedroom where a Dynaco FM3 tuner pulls in local classical FM.

The other one I've more recently experienced is the K&K Audio version, which uses transformer-coupling instead of capacitors.  Not too surprising, given that Kevin's main business is supplying audio transformers.  But this is a sweet unit, and well worth resurrecting a beater ST-70 to a new glory. 

What an amazing story the Dynaco amplifier is.  It's amazing that it can still serve as an entry-level tube amp, with great support from folks like AVA all these years later.

[dB Cooper]

Thanks to all for the replies.  I've opted for the VTA blank PCB, and will supply my own parts.  That looks like a good upgrade and stays within budget, which should also leave enough resistors and caps leftover to update the 2nd ST-70.  Then I can keep both amps.   

This is more of a bad weather project, so with summer around the corner, it will likely take me a while, but I'll post back with some pics and comments when I finish up the first amp.

If $100-150 is the budget, I'd opt to make the AVA-recommended changes to the stock circuit (input filter and interstage couplers) and re-cap it. Not being familiar with some of the other mods you've been considering, I can't comment on them, but AVA's track record on this amp, and in this business, speaks for itself.

An intersting experiment might be to do the mod you're considering, do a different mod like the AVA on the second one, and have a shootout.

One wonders what thoughts were going through the author's head as he was making up imaginary listening impressions to mislead his readers with. Unfortunately this is not the only example of questionable audio journalism out there. There was a B&K amp that was given a rating by Stereophile two levels higher than the AVA amp it was a direct copy of. The designer there blew the whistle. Stereophile didn't have the integrity to do the right thing either. Guess they didn't want their reviewer (and, by extension, themselves) to look like fools. The company in question is out of business but AVA is still kicking. Transcendent Sound designer/owner Bruce Rozenblit has a point when he says "The only thing you need to be an audio expert today is a keyboard."

I personally think rlee has the best solution- sell one and go whole hog on the other one.

[avahifi]

Our Ultimate 70 rebuild for the original Dyna ST-70 is still available.

The complete rebuild plan set with schematic, parts list, step by step build instructions and all is available free of charge as a .pdf file.  Just email me at frank@avahifi.com and ask for it.

If you just want the new mother board (holds the audio circuits and most of the power supply including four regulated power supplies, this is available for $75.00.

If you want our complete parts kit including new mother board and all parts and the needed 6GH8A tubes, the price is $445.00

After you build it you won't look back.  Nobody else offers a small vacuum tube power amplifier with multiple regulated power supplies.

Frank

[Unison845]

Best Mod for Dynaco ST70 is replacing EL34 tubes with Russian GU50. An awesome rugged, higher power and great



sounding tube!

[knotscott]

Hi all - Sorry for abandoning the post for so long.  It took me until this spring to finish up all the rebuilds and tweaks, but it's all finished now.

As mentioned previously, I had two Dynaco ST-70s, one a kit that my uncle built in 1964, and the other a factory wired version made in 1961.  I purchased two of the blank VTA PCBs, then used a combination of new purchased parts and some excellent Convergent caps I've had on hand a while.   In all, new tubes throughout, new quad caps, and new inputs and outputs.  I finished the first amp near the end of January 2020...it took me a couple of weeks to work out some soldering bugs and to burn in all the components before making any serious evaluations.  At about that time I also decided to add the toggle switches so I could flip back and forth between triode and ultralinear mode.  (I ended up with a preference for triode mode, but at least now I have easy options to switch)  In a nutshell, I was extremely encouraged by the results I was getting, so before getting started on the 2nd amp, I decide to scrap the junk CCA speaker wire I was using and replace it with some more appropriate 4 conductor OFC 12 awg.  Then got started on the my 2nd ST-70 with the same mods.  I found the amp updates to be fairly straightforward for someone with basic circuit knowledge and some soldering skills.  Bob Latino offered great support through his website...much appreciated.

As fate would have it, once the 2nd amp was done and working as expected, my 25 year old Lazarus Cascade Basic preamp starting making some impolite noises, so I ended up recapping that, replacing the power supply resistors, and adding 4 new 12AX7s ...that made the whole thing a bigger project than originally anticipated, but thanks to Covid-19, I had time!

Per Bob Latino's comments, I hooked everything up in a vertical bi-amp mode with separate amps for each side, and was really pleased with the outcome.  Enough so that I upgraded the stylus on my TT, and added a more modern DAC to my CD.  All-in-all the speakers are about the only part of the chain that's unchanged, and I'm very, very pleased with the sound I'm getting from the system.  A great way to spend some of the quarantine time.