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Point-to-point wired Dynaco ST-70

EliFisk

EliFisk

Active Member
Had NOS Valves convert my ST-70 into a point-to-point amp. Interestingly, Craig used the same basic front end circuit as in the VRDs, with a single 12AX7 flanked by a pair of 12AU7s.

From the photos, you can see that the stock Dynaco board was replaced with a turret board with discrete components wired up beneath. Upgraded power transformer, power supply and coupling caps as well.

Awesome hi-fi for (relatively) small coin and quite possibly the ultimate ST-70 mod.

:D
 

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There's a reason why...

The ST-70 is the best-selling tube amp of all time - they sound good.

Do you have the original printed circuit board?
 
yes I do. I got the amp from the orig builder. he was an HP eng when he built it and did good wireing. I replaced the Sel. rect, can cap and bias caps plus new resistors on bd. He put in amperex tubes from HP stock and TRW caps..
and bias test pts. I did.

a better view
ST700014.jpg
 
Last edited:
EliFisk said:
Had NOS Valves convert my ST-70 into a point-to-point amp. Interestingly, Craig used the same basic front end circuit as in the VRDs, with a single 12AX7 flanked by a pair of 12AU7s.

From the photos, you can see that the stock Dynaco board was replaced with a turret board with discrete components wired up beneath. Upgraded power transformer, power supply and coupling caps as well.

Awesome hi-fi for (relatively) small coin and quite possibly the ultimate ST-70 mod.

:D
Click to expand...
Unfortently he did not install individual bias . That should be the next mod
i think.

But shure it looks very nice and friendly!
 
EliFisk - what does "point to point" do for you vs. the original, stock setup? Obviously it's a cleaner look but I am sure there were sonic differences. I am a bit of a noob at the tube game but have a 70 w/Craig right now. Waiting to discuss and decide on his mod recommendations. Thanks.
 
I never get tired of looking at ST-70's ... even though I am heavy on tube amps right now, (two sets of monoblocks) I'm tempted to pick one up just to play with.
 
I would like to rebuild one with the circuit board mounted from above and all connections done with connectors.
By changing the amp in this way experimentation and upgrades would be much
easier and only require a screwdriver.
 
peterh said:
I would like to rebuild one with the circuit board mounted from above and all connections done with connectors.
By changing the amp in this way experimentation and upgrades would be much
easier and only require a screwdriver.
Click to expand...

But it would not look as clean as the OP's amp, which is one good looking ST70.
Possibly the cleanest example I've seen. Very nice!
 
I had thought about doing one like this.

Antique Electronic Supply has a great triple-tube-socket turret board, exact fit for the ST70 board location.
IIRC- they also have a dual-tube-socket board that fits the MKII/MKIII amps...

Regards,
Gordon.
 
peterh said:
Unfortently he did not install individual bias . That should be the next mod
i think.

But shure it looks very nice and friendly!
Click to expand...

It has bias controls per tube. I used modern multi turn pots so you have to look close to see them. The two full size pots are now AC signal balance controls.
 
MRL_Audio said:
EliFisk - what does "point to point" do for you vs. the original, stock setup? Obviously it's a cleaner look but I am sure there were sonic differences. I am a bit of a noob at the tube game but have a 70 w/Craig right now. Waiting to discuss and decide on his mod recommendations. Thanks.
Click to expand...

In general, point-to-point wired and use of discrete components sound better than printed circuit boards. In the case of the ST-70, the advantage was being able to use the VRD circuit in the Dynaco chassis. The VRDs are the best-sounding amps I've ever heard and the VRD circuit improves the ST-70's sound by a healthy margin. While the modded ST-70 doesn't sound like a VRD, it does sound much better than with the aftermarket board I had previously tried.
 
peterh said:
Unfortently he did not install individual bias . That should be the next mod
i think.

But shure it looks very nice and friendly!
Click to expand...

Craig actually did install individual tube bias controls. Might not be visible in the iPhone photos ;-)
 
Ultimate ST70 Mod ?

I have three (or maybe 4?) ST-70's but none in use at the moment. Many moons ago I did a conversion of one of them to triode mode input, based on Norman Koren's PSPICE work and modification. I knew Norman from the San Diego Audio Club when he worked at Kodak here. Very very bright and very nice guy! I think I was the first to do his mod ; pre-published (he eventually did publish in Vacuum Tube Valley (or was it Glass Audio) magazine. I made a few minor tweaks to his circuit but essentially the same. Takes an extra input gain stage via 12AU7 IIRC to get the 7199 in triode mode . The mod takes about a week working 2-3 hours per night to get it right-- teflon wired, etc etc.

The difference was astounding. For anyone that has a ST70 hanging around and really wants a great amp for a bi-amp setup (works best in mid-tweeter range, get something beefier for the bass) it is amazing

I did the Bill Johnson ARC mods 25 or 30 years ago to another one -- the cool thing about ST70 is its easy to work on and parts are easy to get --at least they used to be.

I will see if I kept the article and schematic if no one has it
 
That is really nice! Rebuilding like that is as much art as it is science. I completely rebuilt an ST-70 using Curcio boards and Joe's very detailed instructions. It turned out great (especially for me), but not nearly as well composed as this one.....nice job!
 
The best ST70 mod to my ears is the Welbourne Labs mu-follower circuit, but their customer service is unreliable and often horrendous.

I would like to know why the 12AU7 was selected for use with some other mods. I personally think it is a poor choice; the 6CG7/6FQ7 is more linear and better sounding, IMHO. Filament current draw is an issue with this tube because the Dyna doesn't give enough transformer reserve.
 
ricohman said:
But it would not look as clean as the OP's amp, which is one good looking ST70.
Possibly the cleanest example I've seen. Very nice!
Click to expand...
Myself is more used to listen to my amp then observing it's outer beauty.
And the suggestion to have the board connected via connectors AND
be mounted from above was a purley functional suggestion, done to
make it easier to remove/replace the board. All in interest of improving
sound and or function.

If main goal is to look nice, then by all means don't listen to my humble suggestion.

As we do have different goals we really should not have an argument here,
please do not see this post as critisism.

Regards
 
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