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Purchasing a Dynaco ST70 that I can't Test

W

widows97

New Member
I recently got the opportunity to purchase a Dynakit ST70 from a friend. These belonged to his father who has now passed away. I don't have a way to test the amp and have limited knowledge about tube amps. From the photos he sent it seems it is missing the fuse in the rear side. Could this be an indicator of some kind of issue? I think I'll go and check it out this weekend, is there anything in particular I should be looking out for?

Thank you for any expertise :)


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widows97 said:
..it seems it is missing the fuse in the rear side. Could this be an indicator of some kind of issue? I think I'll go and check it out this weekend, is there anything in particular I should be looking out for?
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It is missing the fuse and could be as simple as the fuse blew at some point and was never replaced. Why it blew is the question it could be something simple all the way to pointing to an underlying condition.

Based the pics the unit looks to be in very nice cosmetic condition which for me is an indicator the previous owner took care of it which is a plus.

If you do go and look at it, check the tubes hopefully they are all original which again is a plus and also check to see if there are any that exhibit white tops, which means they've lost their vacuum

Example (random pic from the web)

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widows97 said:
.. I don't have a way to test the amp and have limited knowledge about tube amps.
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These are great amps with a lot of love here and elsewhere, if you do get it do so knowing that you will definitely want to have an experienced tech go over the unit and do a full assessment. It'll cost but worth it if you plan on using it
 
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The best you can hope for is an original owner unit that's just been sitting for 30 years. They were generally highly valued and cared for by the folks that built them. If it was your friend's dad's from new, that's excellent pedigree.

Not pitted, no heavy corrosion, cage is clean...

Looks like a winner.
 
It does look to be in nice shape and all original from what I can see through the cage. These are easy to restore and sound great. Even if a tube or two were bad, they can be replaced. Mine has popped a fuse at start up before. In the last 12 years or more that's been the only one. I put a fast blow fuse in it the first time anyway. It happens.
But it would be a good idea to have it checked out first to make sure something in the amp didn't caused the fuse to blow. Maybe a short in the cap can? Still, a minor thing to replace.
 
more likely the fuse holder got opened by accident or simply lost than any specific problem indication.

Almost nothing unfixable with an ST-70. The transformers are really the only thing to be at all concerned about, and they are not usually a problem. They can be had, but they aren't cheap and they aren't always in stock. Beyond that you can buy any part you can imagine for one of these. Fuse holders are cheap and easy to swap, possibly you can get just the cap, not sure who specifically made them but each mfg makes them just a wee bit different so the caps aren't quite universal.

from what i can see through the cage it looks complete and original. This is usually desirable because it means you won't be trying to figure out what someone did to it in order to figure out what you need to do in order to fix the thing. Its also not a rusty lump so it hasn't been used to prop up the sump pump in the basement or something like that either. The ones to hope for are ones that were taken out of service at some point and just left to sit in a closet for decades.

don't just plug the thing in and stick a fuse in it though. Thats usually a safe way to let the smoke out of something.
 
Great amps! And that one looks pristine—I would pick it up in a heartbeat. As others have said, they are pretty straightforward to work on, and there is a plethora of restoration documentation, both here on Audiokarma and across the web. Good luck!
 
widows97 said:
is there anything in particular I should be looking out for?
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If you have a multimeter I would recommend looking at the schematic and figuring out how to check each transformer coil for continuity. If the transformers are intact you have a nice amp there. As @gadget73 indicated, even a blown transformer could be replaced, but availability and cost are concerns in that case.
 
widows97 said:
@gadget73 Why not plug in with a fuse replacement?
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Anything this old, and tube, should be brought up on a variac or at least a dbt.

Hitting a 50+ year old 115v appliance with 125v and no current control is also an excellent way to make the magic smoke appear or grenade an old capacitor. Even my newly rebuilt amp gets 117v via variac.FYI. it's what the transformers were designed to see.
 
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You should be able to find a local tube tech in your area and as others have mentioned don't plug it in and turn on even with a new fuse. Some very salient points made in post #12 as to why.

If you need assistance in finding a local tech and your comfortable posting geographic location I'm quite sure we can point you in the right direction.
 
loudnoises said:
Anything this old, and tube, should be brought up on a variac or at least a dbt.

Hitting a 50+ year old 115v appliance with 125v and no current control is also an excellent way to make the magic smoke appear or grenade an old capacitor. Even my newly rebuilt amp gets 117v via variac.FYI. it's what the transformers were designed to see.
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This. Old caps don't love being shocked back to life.
 
Dswankey said:
You should be able to find a local tube tech in your area and as others have mentioned don't plug it in and turn on even with a new fuse. Some very salient points made in post #12 as to why.

If you need assistance in finding a local tech and your comfortable posting geographic location I'm quite sure we can point you in the right direction.
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^^ this, It's worth the effort.
There is plenty if support for this amp and you can download the complete assembly instructions along with the schematic too.
 
gadget73 said:
Almost nothing unfixable with an ST-70. The transformers are really the only thing to be at all concerned about, and they are not usually a problem.
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When I visit anywhere to check out an amp in such a situation, I carry my multimeter and check the OPT primaries for their DCR. That and visual inspection only. No powering up or anything like that.

Regards,
 
Thats a solid way to evaluate them. Not 100% but chances are very good if the ohm readings come out right and it doesn't smell like something has roasted its quite likely fine.
 
Don't plan on running it for any length of time before replacing a few bits. All the electrolytic capacitors and the selenium rectifier is, IMO the bare minimum. It could need new tubes too.

Douglas
 
PakProtector said:
Don't plan on running it for any length of time before replacing a few bits. All the electrolytic capacitors and the selenium rectifier is, IMO the bare minimum. It could need new tubes too.

Douglas
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We've kinda glossed over the missing fuse. But it could mean something made him shelve it...

I might replace those power and safety caps and the selenium rectifier first anyways. Very minimal cost and it's only a few components to update for a lot of extra insurance. Any tube is many times the cost of these parts.
 
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