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Is this Dynaco ST 70 FUBAR?

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solodolo47

New Member
Found this guy on CL for a decent price compared to what people are asking for these Dynaco units around me lately. However is this unit FUBAR? A lost cause? I have worked on some tube equipment but I have never removed a mod and reverted a unit back to stock.

It appears they have removed the rectifier tube and have installed a weird looking board as well as a multisection capacitor underneath. Guy says the unit does not work. What a surprise lol.

I'm wondering what everyone's opinion would be to restore this back to the original design and circuitry. Is it possible? Would it be a pain in the butt? Would it even be worth it? Is this ol girl a lost cause?
 

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^^ same, strip it down to the chassis and transformers, rebuild from there. I'm not even totally opposed to retaining the original driver board if its not trashed and the 7199 are still good. If thats not the case, there are many options for the board.

I think this one has been posted on here before, or at least someone had asked about an ST-70 with some big goofy board in the middle of it that had all sorts of unsupported high voltage crap sticking out of it.
 
gadget73 said:
^^ same, strip it down to the chassis and transformers, rebuild from there. I'm not even totally opposed to retaining the original driver board if its not trashed and the 7199 are still good. If thats not the case, there are many options for the board.

I think this one has been posted on here before, or at least someone had asked about an ST-70 with some big goofy board in the middle of it that had all sorts of unsupported high voltage crap sticking out of it.
Click to expand...
I appreciate the input! The board looks decent from what I can see in the pictures. Might be rougher in person though. The 7199s are mix matched. Half Zenith half another brand so the original might have been shot at one point. My biggest concern now would be if the transformers are still good.

As for the goofy board I can understand modding these as it's fairly popular but this looks like a big turd haha. I really love the simple design and layout Dynaco originaly created. Personally I'd never want to mess with that but to each their own.

I'm going to contemplate if I should get her or not. It's about half the price an unmolested unit in ok-ish condition is going for around me.
 
I would grab the basic of the many ST-70 schematics out there and just do a basic rebuild.

I would put in the original 5AR4 rectifier just because I like my tube amp to be all tube.

Just get an original 4 or 5 section can capacitor. No need for that other extra filtering crap.

Pull and test all tubes. They're most likely all fine.

Ohm out power and output transformers.

Looks like the driver board was already recapped so I would probably keep it and use it as is.

That's pretty much all you would need to do.

When you say it doesn't work, do you mean no power whatsoever or does it power up with tubes lit up and just no sound?
 
I would definitely plan on putting in the tube rectifier as I'd like it to be all tube as well. The tubes I can test no problem as I have a couple testers. All the caps look to be good quality Sprague caps which is nice.

As for the power situation he says it powers up, tubes glow, but there is no sound. So I imagine it has something to do with the work that was done to it. Bad solder joint, wiring, or etc. There are also extra terminals on the front of the face near the inputs? Not sure if these are speaker posts? The output trannys are still wired to the original speaker posts in the rear so not sure what's going on with that. The person selling it also may not know how to hook it up correctly to get sound. That is a possibility as well.
 
Before you rebuild it back to stock, you might want to try to figure out why it doesn't put out any sound. To me, this would be a much easier problem as no sound (both speakers) usually indicates a power supply high voltage (b+) problem and is usually fairly easy to fix.

I would first find the solid state rectifiers or diodes. If any one of these open, you get no plate current to the tubes. If one or more were shorted, then the main fuse would keep blowing.

Either/or, my bet is diodes. With all the mods, it had to have worked at some point.
 
solodolo47 said:
I would definitely plan on putting in the tube rectifier as I'd like it to be all tube as well. The tubes I can test no problem as I have a couple testers. All the caps look to be good quality Sprague caps which is nice.

As for the power situation he says it powers up, tubes glow, but there is no sound. So I imagine it has something to do with the work that was done to it. Bad solder joint, wiring, or etc. There are also extra terminals on the front of the face near the inputs? Not sure if these are speaker posts? The output trannys are still wired to the original speaker posts in the rear so not sure what's going on with that. The person selling it also may not know how to hook it up correctly to get sound. That is a possibility as well.
Click to expand...
Well, it's usually easier to repair a working unit. Even if it's working poorly.

The owner has no idea why it is not working? It sure would save a lot of detective work. And/or major reconstructive surgery.

I hope it's cheap. You are basically buying it for the transformers and the chassis.

Good luck with the fix!
 
century tek said:
Before you rebuild it back to stock, you might want to try to figure out why it doesn't put out any sound. To me, this would be a much easier problem as no sound (both speakers) usually indicates a power supply high voltage (b+) problem and is usually fairly easy to fix.

I would first find the solid state rectifiers or diodes. If any one of these open, you get no plate current to the tubes. If one or more were shorted, then the main fuse would keep blowing.

Either/or, my bet is diodes. With all the mods, it had to have worked at some point.
Click to expand...
Beat me to the ENTER button.

:)
 
I aggree. Remove extra fluff and rebuild according to manual using a new can cap . The board might be saved
together with 7199 if components are of original values.
 
I can't quite tell the context, but if the OP is considering buying this amp, unless it is DIRT cheap I would suggest avoiding it and considering one of the thousands of ST-70s that is not in this state. If it's not working, it's not even possible to verify that the transformers are good.
 
Key is how cheap you can get it. Few different ways to go. Total strip down rebuild or strip off the add-ons and trace out everything as most is probably original. Then there's stripping the old chassis and buy a new kit parts without the transformers dump the the 7199's as they're already scarce and expensive. If your going with tube rectifier spend the extra and get a 5ar4 of euro or US origin. GE's are great 5AR4's. I think a total strip of the old chassis and new after market parts/board would be the way is head. Which ever way you go good luck.
 
solodolo47 said:
Found this guy on CL for a decent price compared to what people are asking for these Dynaco units around me lately. However is this unit FUBAR? A lost cause? I have worked on some tube equipment but I have never removed a mod and reverted a unit back to stock.

It appears they have removed the rectifier tube and have installed a weird looking board as well as a multisection capacitor underneath. Guy says the unit does not work. What a surprise lol.

I'm wondering what everyone's opinion would be to restore this back to the original design and circuitry. Is it possible? Would it be a pain in the butt? Would it even be worth it? Is this ol girl a lost cause?
Click to expand...

like the others, you should be able to get it back. Just a bit of work. May also want to consider a replacement PCB from CTech or one of the other on line suppliers.
their new boards are FR4 so much more robust than the originals and offer the same circuit topology.
 
century tek said:
Before you rebuild it back to stock, you might want to try to figure out why it doesn't put out any sound. To me, this would be a much easier problem as no sound (both speakers) usually indicates a power supply high voltage (b+) problem and is usually fairly easy to fix.

I would first find the solid state rectifiers or diodes. If any one of these open, you get no plate current to the tubes. If one or more were shorted, then the main fuse would keep blowing.

Either/or, my bet is diodes. With all the mods, it had to have worked at some point.
Click to expand...
That is great info thank you!
 
Selmerdave said:
I can't quite tell the context, but if the OP is considering buying this amp, unless it is DIRT cheap I would suggest avoiding it and considering one of the thousands of ST-70s that is not in this state. If it's not working, it's not even possible to verify that the transformers are good.
Click to expand...
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.
 
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.
 
^ true. tubedepot is selling used old Euro production EL34 for $110 each. A brand new quad of JJ EL34 is about the same as a single used old production at the same source.

personally the iron has slightly more interest, a new ST-70 transformer set at dynakitparts is $575 and currently out of stock. Thats also why they would have to be good for me to have any interest.

I have a set of JJ EL34 and some old Mullards. Might sound like heresy, but I don't hear that much difference between them.
 
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