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Free New Big Score: Dynaco ST-70 Dynakit & Preamplifier

realjimjim

realjimjim

Well-Known Member
DSCN3028.JPG We were on our way to Long Island from Virginia to visit our daughter and stopped into Philadelphia to visit my long time old buddy. As we jabbered away to catch up, the subject of guitars, amps, music came up which led to stereos. He tells me he's downsizing and going all MP3 thus he is getting rid of his stereo system.

I asked what he had and he said "An old tube system, a Dynaco ST-70 & preamp . . . " I tell him I've never had a tube system and knew nothing about them. He says "It's in my trunk now, I was going to take it to the pawn shop to sell. You can have it if you want it. I love free stuff and especially electronic free stuff so we went out and put it in my car.

He told me he'd bought it many years ago from a guy who actually built them at Dynaco in Philly. After the company closed he worked out of his garage and there was about 100 tons of brand new Dynaco parts in there. That's where this ST-70 came from.

Funny thing is . . . we talk on the phone all the time and it never came up that he was downsizing. He tells me "I'm sure sorry I didn't think about it before because I've already given away the tuner, speakers and turntable."

Some pics are below.

Well there's the story. I don't know enough to ask many questions but here's few for starters. Why are the knobs missing on the preamp? It appears to me it was built that way. Also, my pal told me a tube went out so he pulled the other (see pic) and has run it in Mono for about five years. Can anyone ID the tube and let me know a good place to get another so as to switch this back to stereo?

I've also attached his hurriedly scratched instructions . . . what about these "7199" tubes he's talking about. I think he told me they were rare or something.

Anyway, I'd appreciate any information at all to help me get to know this system and get it running. I have not plugged it in yet because I'm unfamiliar with it. It does work fine on Mono he told me, and it does need cleaning because it has been sitting unused for a long time and is really dusty.

Thanks for any help!
 

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Its an EL34, one of the output tubes. You can use an EL34, KT77 or 6CA7 in that spot. Available all over the place, new, old, depends what you want to spend. I'd be inclined to pick up a new matched set of 4 and go with it.

Not wise to run one of these "mono" though. The power supply was designed to have the load of all 4 output tubes on it. With only half installed, the voltages will be all wrong. It looks like someone has gone through this at some point. The can cap and the orange caps on the board are not original.

The 7199 tubes are the small ones on the front board. Not the most common things anymore, but adapters exist to sub them with more commonly available tubes.

The preamp was probably built "minimal", with the non-essential controls bypassed. Its not particularly uncommon for people to remove them. You can even get faceplates without the extra holes in them to go with that sort of setup.
 
20161004_092041.jpg Dynakit has a few ready to buy custom face plates for the PAS. I bought one from FM3 Zone and had one made just how I wanted it. Silver or black.
 
Looks like the ST-70s driver board has already been partially redone...the big Orange Drops replaced original caps. Possibly they were upped to 1uF instead of the stock .1uFs...a mod favored by many including Frank VanAlstine. There's four more caps on that board that need to go...the two black ones and the two smaller brown caps. Possibly some of those old carbon resistors have drifted in value as well. Main power supply multicap looks to have been replaced as well. What's under the chassis? Has the bias supply been redone? New caps and a 1N4007 diode to replace the selenium rectifier are inexpensive. All in all, a neat gift from a buddy!
 
Last edited:
realjimjim said:
View attachment 847511 We were on our way to Long Island from Virginia to visit our daughter and stopped into Philadelphia to visit my long time old buddy. As we jabbered away to catch up, the subject of guitars, amps, music came up which led to stereos. He tells me he's downsizing and going all MP3 thus he is getting rid of his stereo system.

I asked what he had and he said "An old tube system, a Dynaco ST-70 & preamp . . . " I tell him I've never had a tube system and knew nothing about them. He says "It's in my trunk now, I was going to take it to the pawn shop to sell. You can have it if you want it. I love free stuff and especially electronic free stuff so we went out and put it in my car.

He told me he'd bought it many years ago from a guy who actually built them at Dynaco in Philly. After the company closed he worked out of his garage and there was about 100 tons of brand new Dynaco parts in there. That's where this ST-70 came from.

Funny thing is . . . we talk on the phone all the time and it never came up that he was downsizing. He tells me "I'm sure sorry I didn't think about it before because I've already given away the tuner, speakers and turntable."

Some pics are below.

Well there's the story. I don't know enough to ask many questions but here's few for starters. Why are the knobs missing on the preamp? It appears to me it was built that way. Also, my pal told me a tube went out so he pulled the other (see pic) and has run it in Mono for about five years. Can anyone ID the tube and let me know a good place to get another so as to switch this back to stereo?

I've also attached his hurriedly scratched instructions . . . what about these "7199" tubes he's talking about. I think he told me they were rare or something.

Anyway, I'd appreciate any information at all to help me get to know this system and get it running. I have not plugged it in yet because I'm unfamiliar with it. It does work fine on Mono he told me, and it does need cleaning because it has been sitting unused for a long time and is really dusty.

Thanks for any help!
Click to expand...

Nice score! Hope it works out for you! You will become addicted to the sound of a tube system.
 
Nice stuff.
A great intro to tubes, especially if you're the DIY type. An excellent platform.
Do some research before you plug either in, both should be checked out before use.

I was gifted an ST70 years ago and didn't heed my own advice. A very nice Mullard EL34 redplated and died as it took out a resistor because of my impatience. I was lucky the damage was limited.
Good luck and enjoy.
 
We (pretty much) all love Dynacos here. You've got the makings of a keeper system there.
 
I really tried to talk him out of going "All MP3" . . . I really did.
He had some killer LPs too.
Very sad to see them all gone off his shelves. . . maybe there's something wrong with him I'm not aware of?
Like he's taken leave of his senses? LOL!

Here, take a look, does he appear sane?IMG_6255.JPG
 
Here, take a look, does he appear sane?
Click to expand...

He is wearing a wedding band, so I can understand your concern.
Everything else about him seems normal. Everybody tune's their guitar's unplugged.
 
Don't plug it in.

That is awesome gift. I picked up an ST-70 and a PAS-2 preamp two years ago. Last summer i got the ST-70 going. I was new to tubes and a sophomore in terms of electronic repair. I'd guess that I spend 30-40 hours reading, re-reading, and studying to get a fair understanding of the circuitry and what basic modifications need to be made. Also learning high voltage safety basics -- that thing can kill you. Starting point was checking the transformers as value of the unit is really dependent on transformers. Cost of parts needed for minimal refurb was about $150.

I'd guess that I spent 15-20 hours studying before I got near the plug it in part.

It is not like you can say, "give me a checklist on the process to plug this thing in." The answer is more like: study how tubes work, read the manual on the unit, read threads on the unit, have a basic understanding of the unit; and, when you yourself understand enough that you know what to do before plugging it in, then you are ready to start. The reason that this complete understanding is needed is that everything in that unit is inter-related (and also since it is high voltage you better know what is what to stay safe).

For me, the process was check the transformers (per youtube video re: checking SCA-35 transformers), check all wiring (all good, not hacked up), replace quad cap, replace selenium rectifier, replace bias circuit (all little kids from dynakit parts), bring up slowly on variac checking voltages during power up, test tubes, install good tubes, turn on on with dim bulb tester. (If I had not planned on keeping the unit, I would have done it differently.)

You will find threads that seem to indicate that the process for testing a vintage tube amp is simpler and quicker. I'm not contradicting those threads or those processes. Now that I have some idea of what I'm doing, I can perform more of a quicker diagnosis/test/startup on an amp.

There are places that will refurb this for you. Not sure of cost but think it is several hundred dollars and up.

You can also just sell it as untested/not working on ebay for $200+

For me the ST-70 project was fantastic. I am hooked to the sound.
 
That almost looks like a Toshiba or Matsushita 5AR4 rectifier there (next to the power transformer). If so, and if it tests well, hang on to it as those were some very nice and reliable rectifiers (on par with Mullard 5AR4s).

-D
 
Thanks HH-Bham! he was using it all along until he decided to unload all his clutter, I'm sure it is okay, but I do intend to double check it anyway and do some reading about the ST-70s. Thanks for the reply!
 
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