Dyna ST-70 driver boards.

[mike stehr]

Ok, I just aquired a Dynaco ST-70.

I like the sound of this amp, but it needs some refinement or something.

It still use's the original ratty-assed PCB with the 7199's.

Why did they use pentode input with the Dynaco ST-70 circuit? I don't understand the idea behind this here.

It has a nice sound at lower levels, but it seems to come unglued pretty easy when you start pushing it.

The damping(bass)doesn't seem too special, either.

What are the favored driver boards for the ST-70 at minimal cost? Just the PCB is all I want.

What are the favored driver "circuits" for the ST-70?

I would take a guess and figure going all triode paraphase inverter would help quite a bit.

Like maybe the dynaco 6BQ5 PP paraphase inverter circuit shown on Uncle Ned's site. Except sub 6EU7 in place of 12AX7, because I think 7199 is a six volt heater.

Hell, in my situation it would probably be more economic to use a piece of aluminum to mount the chassis where the PCB went, mount a couple sockets and hardwire it.

Not because PTP "sounds" better, because it would be the cheapest route for me with most the stuff on hand.

Power supply improvements, anyone can give me a few ideas of what they have tried?

I guess I mean to ask how much capacitance can you use on the output cap in the PS before it's overkill?

What was that, the Dustpan Amp?

Pondering....

Mike.

[NOSValves]

Curcio boards are pretty nice. Also check these out www.tubes4hifi.com I have one here but haven't had time to try it yet. The thing you have to understand about all Dynaco amps is they were built to a very low price point and benefit in a huge way from upgrades if it wasn't for the excellent iron they wouldn't even be worth spit. The power supply in the ST70 is absolutely horrid in its stock form and this is what holds the amp back even more so then the front end board. If your going to do it do it right and put some serious filtering in the amp something in the neighborhood of 500uF total on the B+ will do nicely. You can do this with JJ telsa cans easily since they will lay flat under the chassis and there is plenty of room. You can use a standard twist lock to replace the one above chassis can if you want the stock look and then use the JJ's under the chassis.

Craig

[Thoriated_Tiger]

Mike:

Went through the same. The soft top and the flabby bottom are *not* how they sound. Soft top and flabby bottom is how they sound when the original PCB is dried out, the coupling caps are starting to leak, the filter caps are almost gone.

I restored mine with a Curcio board, but it's not a mod -- the board I use (PC3-R) is a duplicate of the original circuit. The one modification I did do was use an SDS Labs filter cap board to completely eliminate the 'can' cap atop the chassis -- I left it on for looks. ;o)

Result: Sparkling, smooth, detailed sound, with fairly good control (yeah, even the Fortes behave!).

The circuit may be odd, what with the 7199 driver stage, but it works, and works well -- as long as the parts are in tolerance. There's no way the original parts are in tolerance, Dyna used the cheapest stuff they could =o)

The one big weakness turns out to be the 7199. It doesnt sound bad at all, as long as you stick to RCA 7199 (Sylvania did 'em too.) But man.. stay away from the Sovtek 7199.. just stock up on RCAs in ebay, they do last forever.

What Dyna did (And it wasn't Hafler, it was 2 Dyna guys in a bar, while Hafler was away, or so the legend goes) was to use the triode half of the 7199 as the gain stage, and the pentode half has a phase splitter. People do the same job, but it takes 3 tubes instead of 2 (2 triodes and 1 double pentode.)

My suggestion? Build the Curcio PC3 R board, use either CUrcio's or SDS Lab's cap board (both wind up almost tripling the capacitance), both eliminate the selenium bias rectifier with something a little less prehistoric. Both also give the option to go with SS B+ rectification, I chose to keep the 5AR4 B+ rectifier.

If you find that a Stereo 70 built like that still sounds off, then by all means, go with one of the fancier boards, but it won't be a Dynaco anymore, it'll be a Curcio.

Just my two ¢...

[sfogg]

Mike,

Do you have NOS or Sovtek 7199s in it? On a stock St-70 I thought the Sovtek 7199s distorted very quickly compared to NOS tubes.

Try the mods Frank VanAlstine suggested in an old Audio Basics:

http://www.avahifi.com/root/equipment/amplifier/70_rebuild_plans.htm

I've done these basic mods to multiple Stereo 70s (and a MkIV) and it really improves them while still keeping that classic liquid Stereo-70 sound. Very inexpensive to do, I think you can do the mods and replace just about every part on the circuit board for under $50. The mods really give the St-70 decent bass and just cleans up the amplifier all over and lowers its noise floor.

Shawn

[CaptnBob]

If you really feel like sinking some money into this, in the middle 70s, Audio Research did complete rebuilds on Dyna 70s. They basically kept the iron and the chassis, and switched the tubes to 6L6s. The result, as I recall, (this is a thirty year old memory, so be guided accordingly) was most impressive - almost up to McIntosh standards. I've seen some repro boards on epay from time to time.

[Deang]

"If you have the original OPTs - not the last version of the Japanese replacement - you have a great amplifier that can hold it's own with nearly any tube amp when you dress up the front-end and PSU."

That's a pretty stong endorsement. Cool.

[Thoriated_Tiger]

mdneen:

whoops.. been a long time since I built it or evem looked at the runes..

I like that tube. It's a shame no modern-day maker can do it right. At least NOS is still around, and at reasonable prices, considering the little tube's longevity in service.

FWIW, I found Jensen copperfoil pios to be quite effective as couplers in the 70. I initially did the board with orange drops, found it a bit held-back at the top and a bit 'dry' overall. The Jensens seem to let more of everything out.

The biggest jump in performance was the filter cap upgrade, and the effect was nearly all at the very bottom end. As in, it had none with the tired original. I suspect it was OK when new, but by the time it got to me, the filter can was almost gone. It wasn't humming, it was really just sagging bigtime whenever a good bass hit would happen. Now it's solid and seemingly bottomless.

[NOSValves]

You can forget getting a board from NED which is a shame he has not had them since Jan. of this year.

NOS 7199 are easily sourced but not cheap.

[mike stehr]

This was (Ken Johnson?'s) Dynaco ST-70.

Sheltie Dave picked it up from him, and now it made it to me.

[mike stehr]

The amplifier is a older version with the brown cage, and the US made cloth lead OPT's.

Not bad on the Cornwalls, the amp has potential...

[NOSValves]

Yup that amp already has a stock rebuild done on it so your most likely listening to a reasonable example of what the amp sounded like originally or just a bit better/worse since is has some modern parts installed. I suppose the original resistors on the board could be out of spec and the tubes could be weak ?

Craig

[bkrop]

Sorry to post off topic, but what is the deal with those circular 7 or 9 pin sockets on the front? I have seen other vintage amps with those and no speaker outputs??? What are their function?

[NOSValves]

The sockets are dual purpose on a ST70 and Mark series amps. The can power a dynaco preamp and also are used as bias test points.

[bkrop]

Thanks Craig, do they serve the same purpase on old McIntosh amps?

[mike stehr]

Ken posted this when he sold the ST-70:

The work was done by an electrical engineer and antique audio technician.

**The amp has a sticker on the back of the chassis. Classic Audio. Dallas, Texas. Dated august of 2003.

The work does look to be done very well. Though I'm not too thrilled with where the put that JJ cap right between the EL-34 sockets. The original can is just there.

I am currently enjoying the amp with my Cornwalls.

**And now I am.

The stock ST-70 had the following new parts:

4 EL34 Eiectro-Harmonix tubes by Sovtek

**These test on my B&K tube tester at around 3900, 4100 micromhos, whatever that means. They are about a quarter way into the green on the meter. I'm guessing around 80, 85%. Not wanked, but have seen some hours. A guess....

1 GZ-34 Rectifier

**This is a Sovtek 5AR4/GZ-34. This thing hammers my TT meter setup for 5AR4/GZ-34. When I setup for 5V4, the meter mellowed out at around three quarters into the green. ?!

Nonetheless, it's a strong rectifier.

4 Hovland Caps

2 .047uF/630V

**I don't think these are Hovland, but appear to be a nice film or poly or somethin'...

2 47uF/160V

**These are Hovland, but are 0.1 uF 400 volt. Which I think is the stock value.

1 JJ 20/20/20/40 Filter Cap (4 section electrolytic is mounted under the chassis, and the original has been left in place)

**Yup.

2 470 ohm 1/4W Res 1%

1 1N4007RT 5A diode

1 16GA H.D. Line cord

**Yup.

4 STD Rubber Feet

**I got better ones.

4 Gold RCA Jacks

Standard rat shack I think...

8 Gold Plated 4-40 screws

**I've yet to find those, maybe the screws on the speaker terminals, but they look like brass.

http://home.comcast.net/~kjohnsonhp/IM001901.JPG

I found the old Ebay link to your ST-70.

Was that amp strapped in triode?

Did you mount the JJ can on top with a clamp?

Did you use JJ's with 100 mic section's?

It looks like you changed out the choke.

I would guess by the looks of the resistors that the guy had swapped them out with newer ones using the old driver board. I recall kjohnsonhp commenting that the mica caps where replaced with the same old domino type that specced out, somthin' like that...

The amp is really quiet, though the PS trannie does need to be tightened up. And those 7199 sockets do the snap crackle and pop from time to time, mostly when cold.

I've been pumping a fair amount of metal and grindcore through the ST-70 because this is why I bought it.(I got my SET amp.)

With the right recording on the Cornwalls with the Fostex 15's and just a 1k stepped attenuator I have some boogie factor.

Bass on the Cornwalls is at the levels with the SVS subs. To the the point where I'll need a higher powered SS subwoofer amp, or just forget about it.(A cheaper alternative.)

Triode strapping would be a easy thing to try.

I probably should pay Dave before I rip this amp to pieces....

[kjohnsonhp]

Here are some comments:

1) The amp was purchased stock off ebay--seemed vintage stock

2) An EE friend and antique radio restoration enthusiast did most of the work. The parts list posted above was from my bill.

3) The amp was then taken to Classic Audio. Dallas, Texas for inspection. He found that a resistor was out of spec causing one channel to be louder than another. He checked it out and felt it was a good example of a stock Dynaco from the era with some newer parts.

NOTE: The person behind classic audio was a tech at Hillcrest Audio back in the glory days of the kit Dynas. All the kits sold were brought back in and he checked out or repaired the build jobs done by the owners. He also did repair work on McIntosh and all the other popular amps of the day. He currently is a vintage gear repair shop with a specialty in record changers. He is well respected in Dallas and was recommended by someone on this board.

NOTE 2: Classic Audio did a repair and check on my amp...he did not do the restoration and parts selection.

3) I sold this amp and bought the NOSValves restored ST-70 with curcio board and other work by Craig that he can describe. I felt this was a notable improvement and would highly recommend upgrading from stock...

4) I can ask my friend who did the original work questions if there needs to be clarification.

5) I sold all the tube stuff and went modern multi-purpose and have on order an ONIX Emotiva pre/pro and 7-channel amp which I hope will compete with products like HALO and Krell Showcase...another breed compared to vintage tubes....it will be interesting to hear what this provides other than a bigger expense and more switching and features.

[Deang]

Looks like you have some fun coming your way. Any way to describe what you're hearing compared to your SET amp?

[Marvel]

I have one of the middle era ST-70 models. I bought it from a friend in the early '70s. The tranny leads aren't clothe covered, but plastic/vinyl. The driver board is FRP and not phenolic like the first series. It has all original tubes and other parts, but sits on the shelf until I finally get around to rebuilding it. I did do a couple of Frank's sugestions, removing the sitch on the front that ties the two inputs together for mono, etc.

When I got it,the sound was superb. Epsecially for the money. Since it was a friend, I paid him $50 for it.

mark,

It was mentioned rewiring for a 6L6. Wouldn't this give a lower output? Maybe only 25 watts max compared to the EL34? I was thinking it might help out the power supply as the heater current on the 6L6 is lower than the EL34.

Marvel

[tillmbil]

I had Craig rebuild my ST-70 with a new driver board. Purchased some Bugle boy 6DJ8's and it sounds way better than before. I am using it with my Heresys. Incredible garage sound. WHen I get time I will bring it inside and try it with the Cornwalls.

[mike stehr]

"NOTE 2: Classic Audio did a repair and check on my amp...he did not do the restoration and parts selection."

Thanks for the reply and pointing this out.

"4) I can ask my friend who did the original work questions if there needs to be clarification."

I would think you had pretty much clarified everthing already.

"5) I sold all the tube stuff and went modern multi-purpose and have on order an ONIX Emotiva pre/pro and 7-channel amp which I hope will compete with products like HALO and Krell Showcase...another breed compared to vintage tubes....it will be interesting to hear what this provides other than a bigger expense and more switching and features."

Sounds like some serious 7 channels...

__________________________________________________________

"Any way to describe what you're hearing compared to your SET amp?"

Ok Dean, since your question bugged me all day yesterday.

When I got home I hooked up the Triode amp, I had been listening to the Dynaco most the week.

Immediate notice in more midrange and top end presence, a wider and more fuller sound. Way better imaging, or stereo separation, whatever imaging means.

But you gotta crank it up a bit to get the bass levels up. The bass is fine, you just gotta crank it a bit.

OTOH, the Dynaco has more bass presence at low levels, but you gotta crank it up a bit to get the midrange and treble levels up. Midrange and treble presence are pretty mediocre, kinda laid back.

Stereo separation is barely there, and the width of the sound is kinda smashed-in compared to the triode amp.

The amp has way more noticed power, bass levels aren't bad at all, but could use some more control. A little tightening is in order.

I do like the tone of the EL-34, whatever that means....

Now I blame the driver circuit, and the old rotten phenolic PCB for the Dynaco's shortcomings in midrange, treble and lack of stereo separation as opposed to the triode amp.

With a good linear all triode driver circuit, and a critically damped power supply, the Dynaco should have tighter bass, and a lot more open on the midrange and treble, with improved separation. IMO....

It may not equal the width and openess of the triode amp in that area, but should be real damn close.

And my small room, I don't really think I need the UL connection. 30 watts or so a side in this room probably overloads it. The amp seems to be coming unglued at higher levels, dunno if it's the amp or the room.

I figure even triode strapped at 12 watts a side or so still would be plenty of power, and would address all the shortcomings described above.

It's cheap to ponder.

The ST-70 may get a bench test next weekend........