So I'v crossed over to the dark side... Dynaco ST-70

Salbreth1

Salbreth1

Active Member
Hey guys,

So someone offered this to me 2 days ago unsure if it worked or not. I've never owned a tube amp or ever really care to but that all changed after I powered it up slow, biased the tubes and gave it a listen. Now I'v been up for 2 days straight fighting the amps mind control powers to impulse buy everything I see for it!:yikes:

Seriously though. I'm really enjoying the sound. Apparently this amp has bee a beaten subject for decades but i'm happy to contribute. I don't know much about the tubes or what tubes are good, only what i'v researched so far. I do know that this is not a factory wired version but am not sure how early it is. The cloth covered wires hints early. The output tubes are Phillips 6ca7, driver tubes are Dynaco 7199, and the rectifier tube is a Phillips ECG, looks newer.

After reading some threads I decided to pull the selenium rectifier and replace the 50uf filter caps. I don't have caps in the 500v range to replace whats in the can yet. I pulled the 10k carbon resistors and replaced them with two 10k metal films to make up for the new diode. I also re soldered every connection on the PCB. When I pulled the tubes to clean the contacts I had a significant voltage change from the left to right channel but as soon as i re flowed the board both sides were symmetrical and within several volts of the service manual with an input voltage of 120v. I was actually pretty amazed.

So here are some pics, I'm curiously to hear comments about this amp or if something looks funny. I don't have a tube checker, but visually there is some wear on the top of two tubes and also there is a vibration that iv isolated to one of the output tubes. its not audible and I didn't notice it until I tightened the tube sockets.

Sal
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rudedogg said:
Very nice amp Sal. Now, if your anything like me, you will be grabbing alot more tube amps!!! I see you like Marantz huh? Well.... I sold my 5 Marantz pieces to finance my growing tube addiction....lol
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lol, I hope this isn't the case. Transistors are cheap, tubes not so much. I am going to listen to for a while before I start doing surgery, but I already have a cart filled with new parts for the driver board and muti cap. Damit I cant help myself!
 
Tube amps are easy to work on. That saves restoration costs. And when was that made?? And yet it works well. Try that with a 50-60 year old SS amp! Lol. And i have yet to find a tube amp that has parts " no longer availabe" ...there are tubes that are no longer made, but parts are still out there....you will collect more, i promise! Welcome to the rabbit hole.
 
The other thing about Dynaco gear specifically is that it was a kit, and the assembly manuals are freely available. Makes it somewhat easier to troubleshoot as long as its stock. Other than the 7199 tubes specifically, nothing on that amplifier is not in current production, and there are fairly easy ways to work around the 7199.
 
One other clue as to age is the color of the hookup wire. The earlier kits were supplied with black wire, while the later ones were supplied with red hookup wire. Nice ST-70! Welcome to the tube club........

Dave
 
Welcome to the Dark Side! Please help yourself to some cookies.......... :D

Nice find! Looks like a nice early example; there should be an EIA code on the backs of the bias pots, at the very least, so it shouldn't be too hard to get an idea of when it was made. Replace those Black Cat molded-paper caps on the driver board with the film or PIO caps of your choice, and you should be all set. As for stuff to go with it, it shouldn't be too hard to find a nice example of the Dynaco PAS-2 or 3 preamp, and there's loads of info and upgrades available for them as well. Happy listening! :music:
-Adam
 
dcgillespie said:
One other clue as to age is the color of the hookup wire. The earlier kits were supplied with black wire, while the later ones were supplied with red hookup wire. Nice ST-70! Welcome to the tube club........

Dave
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I thought the legend said that the red wires were the early kits and then the black wires took over
 
Salbreth1 said:
lol, I hope is this isn't the case. Transistors are cheap, tubes not so much. I am going to listen to for a while before I start doing surgery, but I already have a cart filled with new parts for the driver board and muti cap. Damit I cant help myself!
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There Is always plenty of room for both. I still have stacks of Marantz SS seperates. However in my main system now sits a ST-70. My model 16 mono blocks and 33 preamp are now pulling duty at my bench.
 
Much nicer than the one that followed me home last week. Really a sweet amp. Parts trickling in this week. Going to go stock design with Dave's tweaks. Hopefully looks as clean underneath when i'm done as that one:thumbsup:
 
Marantz and tubes are a wonderful combination to fill a rack with. My two FAVORITE pieces are the Marantz 250M and the Latino Dynaco ST-70. Here is a not so great picture of my rack before I built a better one. The 70 is a wonderful piece and a joy to work on. Unlike my Sherwood that I love but you need sedatives to just look at under the hood.

Enjoy listening to your 70 in stock form before you start ripping into it. DO a lot of research before choosing boards. Also DON'T go nuts with crazy upgrades that overly complicate it and make it a nightmare to repair. Remember part of the charm of this amp IS the simplicity and almost bullet proof construction.
http://www.tubes4hifi.com/ST70.htm

Here is a site with some good stuff. I built the 70 from this site and use KT88 output tubes and 12BH7 drivers.
Enjoy and welcome.

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Well, it's not a super early one based on the lettering on the front. The earliest ones didn't use the "PAM-1 Power" labeling on the power takeoff sockets. Rather, they said "Preamp Power". Also, I've never seen the earliest ones use the "black cat" coupling caps. Nevertheless, this is a nice clean ST-70 - I'm going to guess mid-60s vintage. And...I can't believe no one has mentioned this yet... those certainly look like Mullard output tubes. Based only visuals, they look like they could still have some life in them. If that's the case, they are worth roughly $300-$400 alone! I see the rectifier tube also has a hole in the guide pin which may mean it's a Mullard as well.

If this was a freebie, that's a nice score!

P.S. The transformers will sometimes have a white two-digit year code stamped on them somewhere.
 
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Thanks for the replies, looks like I have a nice amp here. As Joeriz pointed out, the outputs are Phillips branded Mullard tubes. Nice! Wish I could test them. I ended up buying a new blank board along with ceramic sockets for the 7199s. Also new metal film resistors and solen metallized poly films. They were the closest to the original size. New silver micas as well.

I decided to bench test it the other day and with the bias set at 1.4v I got 32 watts before clipping both channels driven at 8 ohms. That was at the rated input sensitivity using a 1kHz signal.

I want to change the multi cap but there arent many caps available in a usable size over 500v. Looks like I either buy the SDA kit. (I thinks that's the name) or buy the new can from dynaco parts.

I really don't want to run it anymore until replace it.
 
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