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Is this Dynaco ST 70 FUBAR?

solodolo47 said:
I wouldn't say it's dirt cheap. He wants 400 "firm". All the other ST 70s around me are 700 and up in ok shape. I'd say most of them are in the same cosmetic condition as this unit but without the crappy mods. I figured for 400 I could spend less than 100 on caps and resistors. I might have to purchase a rectifier tube. I'd have to check my stash to see if I have one. I do have 12AX7s but I do not have spare 7199s.

I don't mind doing the work as I enjoy it and I would like to learn more about the circuitry of this amp in the process. However, I do not want to pour a lot of money into it as at that point I might as well buy a fully functional stock unit.
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I'm not sure what your level of knowledge or experience is with tube circuits, it's certainly possible that you could revive it for not a lot of money but it's also possible that it could need close to a 100% rebuild. Time and success being the issues there, not really cost.

I wouldn't buy a DOA amp under most conditions unless it were nearly free, I would basically assume that the transformers are no good and evaluate the price on that assumption. While $700 seems typical for a ST-70 there many options for working amps in the neighborhood if price you're talking.

Obviously just my 2c and do as you wish, if you go for it hope it comes out well and you learn a bunch in the process!
 
I have restored and put together many of these. If the transformers, all 3, are good and the chassis is not rusted, that is normally the only items I keep and then rebuild from there. Of course some of the tubes might be good as well as that board, that should have all the components tested. I see those orange drop caps on the board were installed with the leads cut from the original caps, with solder blobs. Look at the stack height of the power transformer. Is it short or tall? The taller ones are more desirable, for one they dont overheat as easily. And a couple white wires coming out of the power transformer? Unless those are way faded, there are no white wires from the stock transformer. Dont assume those power tube sockets are any good, see if they wobble around with the tubes in them. Mine get replaced. With so many unknowns, the asking price seems high to me. You might want to have a closer look at what else is available for a bit more, it might be worth it.
 
Given that it doesn't work and had been modified by someone of unknown qualifications, I'd definitely be trying to buy it as cheaply as possible. You really don't know what's good and what's not, so you need to either check it out extensively prior to the sale, or offer them a low enough number that it's not going to be relevant...
 
Some pretty nice looking tubes there, but always the risk that one or more OPTs are shot. But I don't see any catastrophic damage underneath. Definitely looking at a complete strip-down and restore though. I wouldn't trust that mod at all.
 
A new VTA ST 70 kit is about a grand.
You'll end up with great sounding amp that you can use for decades. Why throw $400 at a pig in a poke??
 
gadget73 said:
400 *might* be OK but that would require the tubes and transformers being good to spark my interest. Also that would only apply if they are old production EL34 in very good condition. Modern EL34 are fine but they can be had reasonably cheap, and often quite a lot cheaper for lightly used ones.
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I just looked at the photos more closely and two of the EL34s are indeed Mullards. The other two are Zenith as well as the 7199s.
 
Zenith never made a tube, they bought them from manufacturers and paid to have their logo printed on. Those look like some flavor of Euro production EL34 to me. Might be Mullard, but just as likely to be from another Phillips factory, or maybe even Matsushita. I'd be fine with any of those options, so long as the tubes are good. There are codes on the side that can be translated if you care to.

The 7199 would also be re-branded, guessing RCA or Sylvania may have made them. Usually will be an EIA code that will tell you who and when. Same general idea as the Phillips codes, but a completely different format.


honestly it matters less who made it than the condition.
 
FB night said:
I have restored and put together many of these. If the transformers, all 3, are good and the chassis is not rusted, that is normally the only items I keep and then rebuild from there. Of course some of the tubes might be good as well as that board, that should have all the components tested. I see those orange drop caps on the board were installed with the leads cut from the original caps, with solder blobs. Look at the stack height of the power transformer. Is it short or tall? The taller ones are more desirable, for one they dont overheat as easily. And a couple white wires coming out of the power transformer? Unless those are way faded, there are no white wires from the stock transformer. Dont assume those power tube sockets are any good, see if they wobble around with the tubes in them. Mine get replaced. With so many unknowns, the asking price seems high to me. You might want to have a closer look at what else is available for a bit more, it might be worth it.
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I love your avatar! :thumbsup:
 
solodolo47 said:
I just looked at the photos more closely and two of the EL34s are indeed Mullards. The other two are Zenith as well as the 7199s.
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Can you take a photo of the the small etched alphanumeric printing, at the bottom of the tubes?

It's been a long time since I've heard an amp with healthy Mullard (Blackburn) EL34 xf1 or xf2 tubes. I remember that I was not disappointed.
 
Wow, the Frankenstein monster who worked on that must have got some memo that you can't have too much power supply capacitance.

Per replies above, $400 seems ok if all the transformers are in order. (And bonus points if there is a cage and bottom cover, i.e. helps with resale value, if that's important.)
 
This may or may not help you decide but I paid 380 for mine.

Dynakit ST 70. A Restoration Journey

…..say Selenium Rectifier. I also dont want to loose the “Soft Start” aspect. I know there are many opinions about it. I’ve read both good and bad things about them. Right now I see no reason to change it. If it blows,,, I’ll cross that bridge when I get there. “If it ain’t broke, don’t...
audiokarma.org

Good luck!
 
My ST 70, along with an Eico HF-81 was pulled from an old shed that had been flooded. I stripped the chassis completely down, reassembled using the original Dynaco instructions. It was fun to build the amp as someone would have years ago.

I took an entire weekend to clean all the grime off the chassis and make it shine again. The transformers also took almost another weekend to clean and paint.

I bought a bare board and stuffed it myself.

The EL34s along with the 5AR4 were missing. 7199s were in the old board and test great.
 

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Jalousie said:
Wow, the Frankenstein monster who worked on that must have got some memo that you can't have too much power supply capacitance.

Per replies above, $400 seems ok if all the transformers are in order. (And bonus points if there is a cage and bottom cover, i.e. helps with resale value, if that's important.)
Click to expand...
If all the iron is good, and along with a chassis, its well worth $400.
Go price all that stuff new at Dynakit.
 
It's worth $400 only if :

A) All three transformers are verified good.
B) The chassis has minimal rust and won't need to be replaced or re-plated for decent cosmetics.
C) It doesn't have added mounting hardware that will leave numerous ugly holes when reverted to stock.

Of course, buying in this condition means you'll need to have the necessary skills and equipment to perform all the electrical restoration work yourself. Also, I disagree with previous comments regarding the tubes. Unless the outputs can be tested at full power and the testing indicates they are reasonably well matched and have not had excessive use, they're worth zip. The question is how to test the transformers if the unit is totally inoperative. Check pricing for replacement transformers to fully understand the risk, and bear in mind that output transformers are best replaced in pairs.

I hate to say this, but eBay is a much safer place to buy an amplifier than CL. Anything claimed to be in working condition (or at least guaranteed to have good transformers) can be returned for a refund if it's broken. Just be sure to pay with a credit card.

Jack
 
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