Newbie ST-70 Project: What Modifications have been done? Pics Included!

I’m new to the forum, and would appreciate any advice about my new project. I’d be especially grateful for anyone who could give me a rundown of the modifications that have been made and why they were done. Nothing is too obvious!  

I’ve long been a fan of Dynaco stuff and have owned a couple of tuners, a PAS-3 and a PAT-4. Recently a friend gave me his old ST-70, an amp I’ve always wanted but never been able to afford. He was given it by another friend and never did anything with it.

I’d like to start the restoration but am a bit lost right now. I have basic electronics skills: I solder well, can read a schematic and have done a bunch of basic audio repairs in the past. But I’ve never worked on tube gear (Danger! Danger!!) and am out of my depth in terms of diagnosing whatever the previous owner did. Hopefully I can use this project to further my knowledge.

I know I’ll do the following:

• Replace or restore the driver board: I’m inclined to replace it with the Tubes4hifi ‘Classic’ board. I’ll buy the kit and solder it myself. It may take me a while to raise the $$ but I’ve heard it’s a great mod.

• Replace the missing multi-section cap. I know that the PO must have found another way to do the circuit, but I’d prefer to restore the wiring to the original. All the caps need doing anyway I guess.

• Reinstate the rectifier tube. Again, I guess that the PO found a solid-state solution, but I’d like to bring it further back to stock unless you guys advise me otherwise.

• The filter choke looks like it’s leaked most of its wax out, so I guess that’ll be on the shopping list.

• Should completely replace the bias circuit components?

• Replace the spring-loaded speaker terminals. I really hate the ones the PO added!!  

It’s going to be a massive learning curve: technically it’s out of range of my skills, but if I can find a couple of patient people on the forum I’m sure I can get through it. I downloaded the original build manual and have been looking at as many photos of the original wiring as I can find.

Any advice is appreciated: I’d love to get this going and then pair it with my PAT-4, which is currently in a long process of modification. But that’s another story…………..

Thanks in Advance!!

Sell it on ebay and buy an unmolested one. Or buy a vta-70 from tubes4hifi.

The mess shown will take your time and cost more money then starting fresh. Selling it
will reimburse you some money that is better spent with fresh stuff.

The chassis, the transformers and hopefully the tubes look to be the only salvageable parts. Those, however, are the most expensive parts! If it were me I would keep it. I would highly recommend the VTA board. You will find that Bob's instructions are easy to follow, and well within your skill set. Plus you'll end up with a fabulous sounding amp.

For the quad cap and other parts you'll need, check out Dynakitparts.com, Antique Electronics Supply or Triode USA. Kevin at Dynakit is particularly helpful. I'm sure there are more but these are the ones I've used.

Keep us posted!

Kaner

yes, I would keep it too.
Pretty much a complete tear down, clean the chassis, recover parts such as the tube sockets, transformers and whatever else is salvageable.
VTA noval driver board, new quad cap, new choke and other parts you wish to upgrade, and start over.
It will be a good learning exercise for you, and you'll be proud to say that you built this amp when you're listening to tube music!

I agree with what the others suggest with one exception. Sell that power transformer on ebay and pick up one of the newer and beefier PTs from dynakitparts or Triode USA. It's just the smart thing to do.

I think either restoring back to the original circuit or going with the new VTA driver are equally good choices however the VTA may get you better sound in the long run.

Whichever way you choose these are great amps to play with and learn from. Mind the high voltage and stay safe!

Hi, Doc; Welcome to our forum.  Keep it! Take it all down, clean the chassis and paint the xformers
(Bob Latino here recommends the Krylon FUSION satin black, that's what I used). Mount the formers back on the clean chassis w/new feet and you'll be inspired! Part the way there already.  And do make this a build thread for us.   Dale

Thanks guys!! Nice to hear the encouragement: I'll make a build thread and start with the tear-down.

I've taken a bunch of photos, so next it'll be dismantling and cleaning of the chassis/transformers

Watch this space!!

I concur to gut it and start over. I did just that in 2011. Why would you keep 50 year old wiring, sockets, capacitors and resistors!!??? That's like keeping 50 year old gas line when restoring an old car. Asking for nothing but trouble. I rebuilt mine and got all my parts from Bob. I ended up replacing the power transformer too as it was toast. So all I have original is the chassis, cage, and output transformers. I do think I ordered a different set of output posts (I have two different sets, only used one). I have the VTA classic board. I'd probably go octal today, but I have no complaints about the sound quality. I did install the volume control (attentuator) in place of the mono switch and like it a lot. I run it with NO preamp.

My recollection is that with the power tranny replacement, I ended up saving nothing on my rebuild. But I left mine with an non restored cage, and a carefully cleaned chassis, I have an AMP that looks 50 years old but performs better than new.

Jason

Somebody has done a modification on the driver board anyway, note the new resistors running willy-nilly on the board. Most such inexpensive modification were by crackpots; so it's best if you toss that and start over with a new one.

I concur with the advise everyone else has given. Keep the chassis and transformers, throw the rest away.
I would also agree to replace the old original power transformer with a new one, as the new ones have better current output
and run alot cooler. With our VTA driver board, you won't need any new bias components.
Do buy new tube sockets, new choke, new quad cap, new input RCAs and new speaker terminals.
Most of that you can buy from my website or from TubesandMore.com
If you don't have a manual you can download one for free here: www.tubes4hifi.com/downloads.htm

GP49 wrote:Somebody has done a modification on the driver board anyway, note the new resistors running willy-nilly on the board. Most such inexpensive modification were by crackpots; so it's best if you toss that and start over with a new one.  

Nicely put, thanks! Yes, the driver board is going in my parts bin and a VTA board is on the shopping list. Tubes4hifi, many thanks for your post: quad cap and choke are on the list!