Dynaco ST-70

organ

I don't know what to do with my ST-70. I'm bringing it in for a complete rebuild and everyone is suggesting mods or a completely different circuit design. So the only things that won't change would be the output transformers.

I haven't used it in a couple years so I don't want to power it up again.

I want to try it on my Magnepan. Should I keep the stock design? Is there a different design worth looking into? The guy who will be working on it suggested the Audio Research mod.

I really like the stock design but I only heard it on super efficient horns. I'm afraid it may sound too slow and tubby on less efficient speakers.

Any suggestions?

Face

IIRC, I tried a Welborne modded ST-70 on a pair of MMG's and it wasn't a great combination.

organ

Do you know if it was the result of the mod or not enough juice from the amp?
I"ve tried mine on less eff speakers using the 4 ohm taps, but not as inefficient as my 12's.

Face

It wasn't the mod, it sounded fantastic on my Tannoy's.

organ

That's the thing I'm afraid of. It sounds sweet on my Klipsch speakers.

Timothy Smith

I have a stock Dynaco ST-70 which I tried on my Maggie MMG. It just didn't have enough power to play at a decent level. I tried a HK Citation 16 @ 150 watts/channel, still not enough, when I tried a Adcom GFA-555 @ 200 watts that did the trick.

Tim

dkg999

I'm looking at a Dynaco ST-70 build of some sort this winter. I bought a cardboard box full of baggies of parts and a ST-70 chasis at an estate sale. I've got stuff ordered from TubeforHiFi, Dynaco Dr, and Triode Electronics. I've got both KT-88 and EL-34 tubes, so I'm not sure what the original owner was planning. I'm working my way through all the information I can find and there are just so many options available for the ST-70's! Good luck and keep us posted on what you decide!

organ

Timothy,
My current Audio Zone amps are more than capable of driving them (200w@8ohms, 400w@4ohms). It's just that I have the Dynaco and want to try it out after some sort of mods.

dkg,
Will do. I"d also like to know what you end up going with. They're wonderful amps. Will you be using the original transformers?
I really like the stock sound. It sounds great if you're able to source good NOS 7199.

dkg999

I don't think I have anything original! I think my transformers are from some guy named Ed over at TubesforHiFi based on a receipt I found in the box. I'm having my chasis powder coated jade green.

schwarcw

Organ, if you are interested I can point you to a friend of mine that rebuilds ST-70s and sells them on Audiogon the the 'Bay. Great technical service. He is familiar with all the mods and will rebuild the ST-70 with the mod and the look that you want. He has over 1,300, 100% positive feedback. He has rebuilt some vintage Fisher and Scott units for me. PM me if you are interested in his 'Bay ID and email address.

Systems

You have to keep in mind what the ST-70 was designed to be driving back in the day when they were issued. Like Face said above, it is great driving his Tannoys. His Tannoys are a very efficient set of speakers which can be driven easily with a vintage amp of lower power. I tried a pair of MK IV's on my Dyn's and didn't get much of what I was hoping for. Connected to a pair of Tannoy Golds I got exactly what I was hoping for.

Timothy Smith

Organ / Polk Expert,

If you get the Dyna ST-70 to work well with the MMG's I would be interested in how you do it (what kind of preamp etc.)

I think part of my problem might have been relatively low gain in the different preamps I tried.

I love the ST-70 and the MMG's separately, and I am not really a loudness freak. I keep thinking they will be great together.

Thanks,

Tim

organ

dkg,
Sounds cool. I thought about doing that to mine as well but with grey or something.

Carl,
I'd love to have him do some work on mine but I already told my dealer I was bringing it in. He sold some of my equipments for me and I told him I wanted to use the money towards mods.
They're doing some heavy mods on my tube pre right now. I told them I'll bring in the ST-70 after. Their techie is very knowledgable and has restored/modded many ST-70's. Hopefully he can find something good for me. Thanks for the offer though.

187CAM,
Yeah, I'm aware of that. If things don't work out, I will be using it to power my Mirage and Klipsch speakers.

Timothy,
I will definitely let you know. However, I think my MG 12 may be a little more efficient than your MMG's. Not too sure, I'll have to look into that.
When I was running my ST-70, it did appreciate a little current from the pre. But even that will only take you so far.
The best way to get it to work on lower eff speakers like panels would probably be a change in feedback, much higher PS caps, modify it to accept KT88 output tubes and maybe going to SS rectification.

mhardy6647

ST-70 doesn't really have the oompf for Magneplanars. Gary Kaufman has noted that even Mac MC-60s weren't really up to the task.

Slimmjay

This topic seems to have been answered quite thoroughly, Maggie's love power they're fairly inefficient and as people have said, need more than 200watts to get anything decent out of em. As far as the st-70, i love that amp! i highly regret getting rid of mine, although the upgrade to a rebuilt Baldwin was well worth it.

Face

Slimmjay wrote: »

This topic seems to have been answered quite thoroughly, Maggie's love power they're fairly inefficient and as people have said, need more than 200watts to get anything decent out of em. As far as the st-70, i love that amp! i highly regret getting rid of mine, although the upgrade to a rebuilt Baldwin was well worth it.

The best example I've heard of MMG were hooked up to a pair of Pass F4's, and they're far from 200wpc.

gidrah

There is a guy that's local to me and is pretty much a tube nut and specializes in ST-70 mods and rebuilds. It would have to be shipped, but if there's anything an ST-70 can do, this guy will bring it out. I don't know if he can make your Maggies sound to their best, but if anybody can,.......

kawizx9r

Here ya go - http://www.welbornelabs.com/st70.htm

Btw, +1 to what Mike said. Maggies may be power hungry, but you don't need alot of watts to drive them....it's nice clean current they strive for. My amp definitely won't go anywhere unless I spend a good +2k on a different one.

organ

Kawiz,
I need to look into the Welbornelabs first. Face mentioned above that it wasn't a great combo with his MMG.
So far I'm thinking about the ARC mod because it changes your output tubes to 6550/KT88 which will have higher output. Maybe an entirely new amp with 300B's in push/pull configuration. Or I may go for a stock rebuild.





I decided to dig deeper and went to audioasylum's planar board. It seems to be a hit and miss type thing. Some people are happy running tube amps on them while other's aren't. Under Magnepan's FAQ section they mentioned that some people run as little as 25w/ch tube on them. There was a guy on the planar board who was running 18w/ch SET. Weird.

Don't know if tubes will work for me. If not, I'll use the amp on my Klipsch Cornwall.

kawizx9r

In my experience listening to tube amps (notice I didn't say own any) they tend to clip much quicker than SS amps.

I can believe the 25watt thing on Maggie's FAQ, it's all about how loud you listen to things. Oh and of course if the amp driving them has the current to supply them with. I drive my maggies pretty loud, to where my wife says she can hear the music outside downstairs in the parking lot.

I only posted the mod so you can take a look at it, and since I've only heard good things about it (nothing specifically about driving planars) and the company itself. Hope everything turns out the way you want!

Big Dawg

I'm thinking of buying and rebuilding/modding an old ST-70. How well would it drive SDA 2Bs? And, has anyone ever compared that combination to same speakers driven by a Carver (like mine, an M4.0t)?

Timothy Smith

I have used my Dynaco Stereo 70 with Polk monitor 10's, RTA-12's, SDA-2's, SDA CRS+'s, and SDA 3.1TL's.

It work OK, loud enough, but the bass always seemed sloppy, and the highs limited, and I went back to my Adcom GFA555, usually with a Grant tube buffer to give some "tubey" sound.

For some reason, the Dyna ST-70 sounds great with my ADS-L810's though.

Synergy is a weird thing, and hard (for me at least) to predict.

Tim

Face

I agree with Tim, OK is the best way to put it.

But on my Tannoy's, completely different story.

Big Dawg

Thanks for the input.

What about something a "little" newer, like a modded Golden Tube SE-40? Would that yield similar results?

Are there any tube amps that are available in the sub-$600 range that will improve on my Carver to drive the 2Bs?

I may get the Dynaco anyway, and use it with different speakers.

I'm guessing Maggies or ALIIIs would be an even worse match.