ST-70 acquisition, now what?

[Dandy]

Good for you for getting this barn fresh ST70! It's a great way of getting to know how to work with tube amps.

After sorting out the mismatched power tubes, replacing the caps (electrolytics and film), and doing the selenium rectifier bias replacement, it should work fine. Then you too can have a personal opinion on this famous and likeable amp.

 

[ferninando]

IMO the driver board does not NEED replacement. It is not scorched.
A good cleaning and new parts will make it like new.. All the Rs and Cs are available.
Then just give it 6-12 mos of listening and then decide if it is really necessary to
get a new board. I think you will decide no.
But by all means , do just you want to do.
It also has the smaller power transformer that some folks hate so that may be another $$ item to think about.
Do the necessary stuff first and see how it goes before major replacements are made.
I am perfectly happy with mine in its original state. I have had NO problems with the power transformer.
The amp sounds great IMO.

 

[nj pheonix]

I agree with Gadget and Fernando.
There is an excellent break down and evaluation thread in the tube audio forum by Dcgillespie.
If you don't see it let me know. It's the perfect education for the new st 70 owner (or someone thinking of getting one). You can decide afterwards what you want to address.
Good luck and enjoy!

 

[Bob]

great news for you. you're not going to keep it mint, numbers matching, holy-grail the caps,
and replate nickel then chrome.

get the tubes, the Mullards command a premium just short of black gates, WE 300B, etc.

sell the 7199 and rewire for 6GH8/6U8s - don't forget the GZ34 if its British.

get cheapy el34-equivalents - Russian 6p3S (and others) cost $5/each, 6GH8s are about same.

you could bring the cost down by half. I'd recommend you plan B the polishing and
cleaning. focus on reconditioning/revitalizing/blueprinting the circuit, the
components, the solder joints, connections (fuse, speakers, inputs, power cord)
then chrome polish everything.

this ST70 is about 50 years old.

later when things are reliable (ask me how many I fixed and sold) you can
silver wire, Vcap, PSvane tube it, Cardas silver speaker connectors, vampire RCAs,
etc.

 

[primosounds]

So starting to make a parts list I need some feet for this thing yes? Also it would appear as both the s/m switch and power switch were never there and don't know if these pose any challenges in installing proper switches.

Sorry for the quickie post here but I have to go to work and time is short.

There is really no need to have a stereo/mono switch. The intention there was to allow one to get 2 st70 and turn them into mono blocks for doubling the power. Also a switch on the input signal does degrade it to some extant. Corrosion develops on the contact surfaces and that will affect signal integrity. The original power switch, being of the slide type is not the best type for a power switch. Replacing it with a more sturdy toggle switch with sealed contacts would be a better alternative.
There is so much info on the Dynaco st70 that there is no doubt we are all rehashing what has been mentioned many times before. But one thing that bears repeating is you should be very careful of the original phenolic circuit boards and the foil traces are very fragile and not very heat tolerant. So, a rookie mistake is to spend too much time on the solder joints and cause the foil traces to lift and/or break from the board. This is sometimes hard to detect and can cause you to go on a wild goose chase. It is probably the best reason for an inexperienced solder slinger to opt for all new boards. There is a way to use the original boards while changing out all the components but best to leave that for later. cheers and good luck with your project.

 

[stupidhead]

Hi folks, I appreciate all the input here. I feel as though answering or responding to some of the entries may help frame where my head is at on this piece.

So I love this remark

You have a Don Sachs Citation and your going to waste your time with that crusty ST70?

Unless its about 50 bucks, I would go back into the listening room and crank that Citation.

And I understand the sentiment. In fact Deucezilla will hold a prominent place in what will be the end result of our remodel project here. I have a couple sets of AR 3s (and one set of 3a) and will drive as et of these with the Citation. We will have three distinct listening spaces and I envision the Stereo 70 for the fireplace sitting area. A relatively small space with AR 4x bookshelf set of speakers I think. I have really liked the 4x through the HK tubes and hope it to be real nice with the ST-70.

I believe that you will find many threads on 'what' to do and 'what others' recommend. Google and research are your friends. Opinions vary all over the place about 'what' to do with an ST-70,...

One thing that I have found very helpful is to get the schematics and operating manual. The original schematic is large in print and easy to read/follow.

Also, several purists here recommend to get the ST-70 back to it's original state of operation (stock), then try/swap out parts and kits. I would recommend this option,... If you can, get it to stock operation first.

You are spot on here. I recognized years ago that the ST-70 has a cult following of sorts, many examples in the field, all sorts of mods from folks with varying end results. All the while being an easy to work on unit, whether it be refurb or mod. While the Citation has been modded/upgraded by Don, that was a once in a lifetime opportunity as it were, and I had to do it. My intention with the ST-70 is in fact to bring it to stock condition and this example seems to be one that I will not have to undo someone else's work.

hope you didnt give more than 250 for it with all thats wrong with it.
Having said that, it is one of the simplest amps to rreferb/restore.
if the board is not fried you dont need a new one. Just replace all the Rs and Cs with new .
Get a quad of new output tubes , EL34 or KT77s. If 7199 are in the drivers are good keep them for now.
they are nearly unobtanium.
Check all wireing underneath. A 1N4007 diode replace the selenium rect. All New caps underneath.
A new can cap. Address the fuse and S/M switch as needed
Prepare the transformers for black semigloss paint. Polish/derust the chassis.
You will have something to be proud of in no time and it will sound very very good.
All this is what I would do. You may want to do it deff. whatever you do, enjoy doing it.
The amp is worth the effort IMO. I have one.

This response actually sounds like the approach I will take. My typical mentality with speakers at least is to restore as best I can with original parts as needed, and replace only parts that are either known problematic or worn out from time. I am not anal to the point of stuffing caps but I believe these things are so highly regarded 50 years later because the original engineer had a good design and components.

Market value and true value are different things. Dont worry if you overpaid. You now have a big project to keep you busy this fall and that can be fun.

You,ll be spending plenty more money too.... new driver board, components, other parts, tubes etc etc. When your all said and done unfortunetly you,ll have an amp that will be a huge step down from your custom Citation.

In addition, you will have likely spent more than what the VTA ST70 kit costs. The VTA is a nice upgrade in everyway to the original.... sure you will spend on lots of the same parts to put in your original dynaco but you,ll still have the original transformers and not the beefy ones provided by VTA.

On the upside, you,ll certainly gain some electronics knowledge too because it will be signficantlly harder to troubleshoot and modify the original amp than simply building the VTA with very clear instructions.

So again, dont worry about being way underwater on this purchase.... wearing a snorkle is par for the course, i have worn one frequently with audio purchases!

I don't believe this will be a huge expense to get it in good working order and I am far above water on many other things so I am far from unhappy being a little underwater with this project. Besides, no matter what you do to it the VTA will never be vintage like this one.

great news for you. you're not going to keep it mint, numbers matching, holy-grail the caps,
and replate nickel then chrome.

get the tubes, the Mullards command a premium just short of black gates, WE 300B, etc.

sell the 7199 and rewire for 6GH8/6U8s - don't forget the GZ34 if its British.

get cheapy el34-equivalents - Russian 6p3S (and others) cost $5/each, 6GH8s are about same.

you could bring the cost down by half. I'd recommend you plan B the polishing and
cleaning. focus on reconditioning/revitalizing/blueprinting the circuit, the
components, the solder joints, connections (fuse, speakers, inputs, power cord)
then chrome polish everything.

this ST70 is about 50 years old.

later when things are reliable (ask me how many I fixed and sold) you can
silver wire, Vcap, PSvane tube it, Cardas silver speaker connectors, vampire RCAs,
etc.

I figured mid 60's or thereabouts. Not sure about the rest of your post so I will just leave it alone.

Good for you for getting this barn fresh ST70! It's a great way of getting to know how to work with tube amps.

After sorting out the mismatched power tubes, replacing the caps (electrolytics and film), and doing the selenium rectifier bias replacement, it should work fine. Then you too can have a personal opinion on this famous and likeable amp.

Exactly!

Again, thanx for all the great input here folks. I suppose I still will have more questions on this but for now......

 

[stupidhead]

So a couple weeks later and the journey has taken some shape. I have entirely disassembled and evaluated. I am encouraged for sure, but still not sure what direction I am going to take. I am leaning towards the octal board from tubes4.

 

[triode17]

I'll tell you what NOT to do, do not wire brush the transformer 'plates' (laminations). That can cause the insulated lams. to short together which could damage the transformers. I would lightly steel wool them just to get the rough rust off. Then , use some Rustoleum to paint them.
Another potential problem (that I saw in a friends 70 that I was rebuilding) is the bias circuit ground connection. Each pair of power tubes has a 15.6 Ohm to ground used as a bias level monitor. You will notice that they are grounded to the metal socket mount. This metal to metal connection was fine when it was built, about 50 years ago, but by now may have acquired some minor corrosion beneath them. This can cause the bias level to be unsteady. Add a piece of wire from each socket mount back to the minus terminal of the main capacitor can. This will insure a steady level for years to come.

 

[gadget73]

Grounds to screwed or riveted joints can definitely become a problem after a while. If you have the inclination, building yourself a dedicated copper ground buss that connects to the chassis only at the main can capacitor and has all the circuit grounds tied to it will very nicely take care of that. It also tends to make things quieter if you run it the right way.

 

[stupidhead]

I'll tell you what NOT to do, do not wire brush the transformer 'plates' (laminations). That can cause the insulated lams. to short together which could damage the transformers. I would lightly steel wool them just to get the rough rust off. Then , use some Rustoleum to paint them.
Another potential problem (that I saw in a friends 70 that I was rebuilding) is the bias circuit ground connection. Each pair of power tubes has a 15.6 Ohm to ground used as a bias level monitor. You will notice that they are grounded to the metal socket mount. This metal to metal connection was fine when it was built, about 50 years ago, but by now may have acquired some minor corrosion beneath them. This can cause the bias level to be unsteady. Add a piece of wire from each socket mount back to the minus terminal of the main capacitor can. This will insure a steady level for years to come.

thanx for the warning, but in fact there was very little rust/corrosion on the transformers once disassembled. I have surface cleaned them with a mild detergent and followed up with some wax and they look great. I will post pics soon of these and other components. I am inclined to not repaint for a number of reasons, but mainly because they are quite presentable now.

thanx also for the thoughts on grounding that you and gadget have shared. I like the idea of a dedicated ground and will consider the idea as the rebuild unfolds.

for the past couple weeks I have read a ton of threads, blogs etc. and have communicated with a variety of like minded folks in an effort to decide the course I am going to take. this community of people is a fabulous resource and I take none of it for granted. the number of opinions is basically equal to the number of people/resources you encounter, and that seems to be particularly true with this model.

What I have decided is I will restore this to as original as possible, including acquiring a replacement board with the original design which includes using the original 7199's.

I have seen a few resources for such a board but if folks will chime in on resource for this option I will make an order for things soon and proceed with the rebuild.

I am quite happy with the end result of cleaning and polishing the chassis, more pics to follow.

 

[peterh]

One supplier of the st70 board in the original design for 7199 is :
http://www.dynakitparts.com/dynakit-products/PC Boards/PC-3A-ST-70-Metal-Film-Resistors
( they also have a similar board adapted for 6gh8/6U8 instead of 7199 )

 

[crispycircuit]

Great project ! Nice work ! Classic ! This is the amp that got everyone hooked on tubes. Enjoy.....

 

[Omegaman]

I don't know if this has been covered

dating your ST-70

Yours has paper caps in the supply, that is an early indicator
Yours has the later PT slightly shorter than the original
Yours has black hookup wire. Some say Red was first and black came later, some say the other way around. I say Red = earliest
If they are original you can check the bias pots they might have the year in the code on them

The 4th and 5th digit are the year. One of mine says 1376512 making it a '65. My earliest one does not have the year code one them.
My earliest PT has 59 stamped on it

My earliest ST-70 driver board shares one hole for the two 270k resistors whereas later versions has two holes for each resistor
The two 270k brown resistors outboard port and starboard


Also it looks like the date is printed on the transformers

Tha's all I learned or made up on dating the ST-70

 

[gadget73]

If you're really trying to keep it stock but with a new board, honestly I'd go for a 6GH8/6U8 driver instead. its a very similar tube but good ones can be had. 7199 stock is pretty limited and the ones you can get may not be much good.

Elsewise honestly I'd just rebuild the stock board if you're really set on keeping a 7199 in there.

 

[nj pheonix]

For what it's worth I agree with Gadget.
I did mine within the last year with a new ,stock design. 7199 board. I had several 7199s in my parts box. I've come to find these are the last of the 7199s. One of my NOS was noisy.
I've since sprung for the gh8 adapters and 2 sleeves of gh8s (for future) (probably for the cost of less than 2 7199s of unknown 7199s.
I'm very pleased with the way it turned out but if I had to do it again, I think I would have used a 6GH8 board.
Just my 2c..
If I may I'd suggest you do as suggested in the article. Build the bucking transformer for it. Get a SS plug in rectifier and Add the B+ dropping resistor.
Its win win. Lowers operating costs and improves performance.

 

[crispycircuit]

+1 for 7199 to 6gh8 adapters. I use them in my Sherwood. A sleeve of NOS 6gh8s $10 on fleabay. The lowest cost used 7199 was $25. Who knows what you'll get? Keep it in mind when your driver tubes go bad.

 

[stupidhead]

If you're really trying to keep it stock but with a new board, honestly I'd go for a 6GH8/6U8 driver instead. its a very similar tube but good ones can be had. 7199 stock is pretty limited and the ones you can get may not be much good.

Elsewise honestly I'd just rebuild the stock board if you're really set on keeping a 7199 in there.

I am a bit torn on this angle. I fully understand the equation of old tube 7199 becoming harder and harder to find. However it is far from unobtainium as some suggest, more that it commands a premium due to availability. The actual struggle I am really having is whether to replace the board at all? My soldering skills and equipment are of a novice level, and I am dubious of reworking the original board myself for fear of the traces lifting or otherwise mucking it up as I have read repeatedly. Seems replacing board (oem design) with a more recent production/materials/pre-populated is the easiest approach, but does veer a bit away from oem? I would love to find some like minded AKer local to me who has the skills needed to rework the original board, and as Ferninando said

IMO the driver board does not NEED replacement. It is not scorched.
A good cleaning and new parts will make it like new.. All the Rs and Cs are available.
Then just give it 6-12 mos of listening and then decide if it is really necessary to
get a new board. I think you will decide no.

So again, if anyone local to me (Southern NH) is of a mind to give me some assistance with this, I believe I would like to keep the oem board and rework with fresh components. I would be happy to drive to or host an afternoon of board rework.

As promised, a few more pics of the disassembled guts of this thing, first the transformers.



and again, all I have done is extract from chassis (desolder not cut wires) and of course I tagged and recorded all leads. A bit of mild cleaning and a coat of wax leaves these in very presentable condition and I am not even considering repaint. I much rather the romance (and appearance) of original, a little beat up as opposed to a perfect appearing repainted anything.

The choke shows no evidence of overheating and I have done virtually no cleaning prior to taking this picture.



and I have seen a 65 on this and all transformers as well. Safe bet it was produced in '65 yes?

Then this pile of pieces/parts is all in decent shape, depending on final decision of board approach will dictate just how much of this gets undone. I will get new (oem style) switches for power and m/s. The fuse holder is not oem I am told so will effort an oem style (or original) if available. I have not taken close ups but a question about the tube sockets. On one side it appears as there is some crusty remnants on the top surface of the sockets. Not a lot, but definitely there. The other side pair is clean to the original surface and no evidence of crustiness. In the below pic the bottom pair of sockets is the clean pair and the upper shows the crustiness. I will get some better pics of this later but thought I would mention now.



Then there is the cap can. Again I did not take close up but there is definite evidence of crusty leakage coming out of the bottom of this thing. Forget the romance, just replace with a new version unless someone has other ideas?
My real quandary is about the cord. I suspect I can clean this original one and reuse, but I am leaning towards replacing (and grounding) cord with something a bit more substantial. Where do I draw the line for keeping it original?




Folks, I greatly appreciate the input here.

Geoff

 

[dmag]

For simplicity sake, you might completely disassemble and clean the parts, buy a new can cap and wire and reassemble according to the original dynaco instructions. Dynaco's manual was truly idiotproof. Then you can mod or not.

 

[Lavane]

Personally, I would buy a new board kit. You will get better sockets and a much better made board. Those phenolic boards are like old dried out Graham crackers. Brittle and weak.

 

[gadget73]

Ditch the can cap. Replace the ones in the cardboard tubes too. Those old style components aren't really obtainable anymore, and frankly they do not age well so I wouldn't go seeking NOS ones either. On some level you're going to have to accept that this will not look how it did in 1965 or whenever it was made.

If you do want it to look like it did, you're probably going to have something that is really only suitable for shelf decoration. I wouldn't be inclined to run an all-original amp on a regular basis for fear of it having a meltdown.

The residue on the tube sockets is probably solder flux. If it chips off with a little bit of poking and is an amber sort of color, thats what it is.