Thinking about buying this Dynaco ST70, is it worth it for a beginner?

[Fredark]

Hey guys, I've never been on this forum before but I was refered here and thought you might have some good info. I can get my hands on a ST70 for around $350, but only until tomorrow. I've been really interested in getting into speaker set ups and especially tube amps as a fledgling headphone audiophile, but am essentially a beginner for most of this stuff, especially when it comes to the DIY side of things. I know very little about vintage setups, but from what I can tell this seems like a really interesting way to get into it. Just to round off the background, I'm an engineer, not electrical but I have worked with electrical projects a decent bit at work, and I know my way around a soldering iron for simple stuff. I've never had to do anything extremely dangerous, and I only know what is essentially a components 101 class, but I at least know the basics.

As for my questions, I basically would just like any help I can get on the basics of this thing. The guy who is selling it hasn't personally tested it, so I am worried that it might not be working. From what you can tell, does it look like this thing needs to be completely redone? I'm totally into working on it, and have no problem spending more on replacements and upgrades, but I don't want to end up spending $1000 bucks just repairing it. Also, with my level of knowledge, how steep of a learning curve will there be for the repairs that I might need to do?

Also side question, and I REALLY hope this question isn't super dumb and nooby, but would it be possible to mod this thing to have a headphone out to run to some high impedance headphones? If it is, how difficult / expensive would it be? I'm currently using a pair of Senn 600's, but I know that I'm going to be upgrading sooner or later.

Here is a link to a picture of it. Any input whatsoever on if it is worth it is super appreciated!

https://i.imgur.com/N6kOTzy.jpg

Thanks guys!

 

[JDub]

I'm not a tube guy, so take my advice with a pinch of salt. However, if I was looking for a tube headphone amp I'd look at a new kit, such as a Bottlehead Crack, which comes with instructions and support, (at around the same price point), rather than try to mod a vintage unit that wasn't designed for the purpose. Also, in general, about 80 or 90% of the time "untested" is synonymous with "not working".

 

[HyKlas]

Really would need to look underneath with the cover off to see what's been done to it.

From that one photo I can see the can cap is missing (replaced with parts underneath probably), the input jacks have likely been moved to the back, the stereo/mono switch is gone, and the board has newer capacitors on it.

Need to fire it up and see what happens. Make sure speakers are hooked up first... If you can't test it, even at $100 it's a crap shoot. What if one or both of the output transformers are toast? Then we're getting up there in money for new parts...

I think I'm in to mine for about $700 and you can get brand new kits around the $750 mark from tube4hifi.com or triodeusa.com.

 

[captouch]

If you want it primarily for a speaker amp, that would be a fine price if the transformers can be verified to be working and it comes with tubes. Also, hopefully it’s in good/reasonable condition.

If it’s in bad condition, then I’d pass.

As far as modifying for headphone duty, it seems like it might be possible with dropping resistors, but I don’t know for sure since it’s a tube amp.

There’s a long thread about baseline testing on the ST-70 in the tube forum. I’d post your question there about mod for headphone use and someone will be along to tell you how possible it is.

 

[Bob]

the ST70 has had a minor update, the caps are not original and have been replaced
(the white caps). as @HyKlas indicated - the can cap is missing and hopefully
replaced by separate caps underneath.

if the rectifier tube is a GZ34 and the tubes are not Chinese and it works then 350 is a
good price. underneath, if the selenium rectifier used in the fixed bias circuit is replaced
then you're safe to go.

putting headphones on a tube amp is a delicate operation. solid state amps can handle
a no-load eg no heaphones or speakers plugged in. tube amps generally need a load
to prevent damage.

so, its easy to add a headphone circuit that series resistors to the headphone when plugged
in. problem is you need to have a load when they're unplugged intentionally or accidentally.
so one way is to add a 100R large wattage resistor across the output transformer's secondary.

and you'll need to learn to bias the tubes (first time when you get it, periodically, and after
tube replacement), download the manual, it has the instructions. and you'll need a VOM/DMM
any cheap one will do.

it will be a lot cheaper if you DIY. search the tube archives and look for tube amp schematics
that have headphone circuits and post the schematic here and ask for advice and adaptation
information.