ST-70 Rebuild Thread

A few days ago I posted a new thread about my ST-70 project: I had a lot of helpful replies which have encouraged me to make a start. Here's the first post of a (probably long) rebuild thread.

I'm starting with a couple of pics from the other post showing how the amp was when I got it: complete but dirty and unused for many years. Also substantially modified!





I'm sorting out my shopping list.    Right now, it's looking like this:

VTA Board
New multi-cap can
New bias circuit
New choke
New speaker terminals

As well as caps, resistors and wire. I'm planning on using solid-core 20AWG wire: I read somewhere that this is a good replacement for the 22AWG originally used.

Pics of the stripdown coming next!!

looks like all parts are there ..as for substantially modified..parts have been upgreaded an B+ is setup with SS diodes ......on the stock setup...lot of people here have ss diodes on the B+........an upgreaed cap on the B+ like your amp.....were is the substantiall mod..stock input board an tube that I like a lot myself......as for dust looks like my MK3s after a week on my shelf...hehe
Have you turn it on?.....lot of ST70 an MK2s...3s...4s...have been setup the way this amp is...an minny like the sound ....SO
Do you know if the amp works?....an the tranfourmers are ok.........do you like tube sound?.......why would you put $$ into any amp you don't know if it works?...........less you just have the cash to spend?...............I have owened a lot of stock Dynaco Amps an have MK3s now over 50 years old...with all stock parts an input tubes...............sound great.....work fine......good luck





thanks for site Bob......

Again, why not ebay this for big $$ and buy an all-new ST-70 kit from Bob?

You'll probably spend less and get more.....

audiobill wrote:Again, why not ebay this for big $$ and buy an all-new ST-70 kit from Bob?

You'll probably spend less and get more.....

I think Audio Bill is right. Plus with the new ST-70 in addition to a much upgraded driver board you'll have all new caps, better resistors and new trannie iron. Might be just the photo but it looks like that transformer has oozed a bit.

I second that. Sell the st-70, don't try to fix it ( it's munged/broken/destroyed ) and buy a vta-70 kit.

Doctor Hugocat words

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.

key words.............I’ve always wanted but never been able to afford!....did I miss ...you now has cash?


There are 10.000 of these ST70s out there all stock working fine just like my 3pr of MK3...I know people that own stock an upgreads like this ST70s with stock  50 year old tubes....sound better than some high $$ tube amps....
...the VTA70 well be over a $800-1000......less I missed some thing...
an as for saleing the amp.....that he dose not know if it work...for what $500...good luck with that
....you could be playing sweet tube sound today.........................................justsaying

advice...........spend $50. on a Varic....less this is the only tube amp you well be working on ever............I run all my tube amps an tube preamps on a Varic.....lower B+ just sound better......too me.
all this is just one Dynaco owners o-pine....



thanks for the site Bob.......

If you look at the background threads, everything except the chassis and output transformers has been recommended for replacement. And the old amp will bring more as is than parted out on ebay.

The Op will spend to upgrade the old amp anyway.

Thanks for all the input guys, keep it coming!

I'm sure you're right that in the long run selling this amp and putting the money towards a new kit would be the sensible choice, but I'd like to go ahead and restore this one. I've been involved with classic cars for a long time, and I frequently give people similar advice: better to start with a good car or buy a restored one, than to spend a fortune on rebuilding a wreck. So why am I gonna do it? Well, partly because I enjoy restoring something that's old and tired into something fun and shiny. Partly it's also the challenge and the process of learning. There will be problems to solve that I won't get if I buy a kit off the shelf. And lastly, this way I can spread the cost and have fun restoring some bits while I'm waiting to find more money. If I sell this amp, the $$ will go on something trivial like the rent and I'll probably never have the lump sum to buy a kit: but if I do the restoration, I'll buy a few parts whenever I have the cash.

It makes sense to me anyway......

[/quote]
Might be just the photo but it looks like that transformer has oozed a bit.[/quote]

I thought that at first but it's glue from a hot glue gun that the PO used to glue caps etc to the chassis.........The choke has oozed BIG time tho.

Doctor Hugocat wrote:f I sell this amp, the $$ will go on something trivial like the rent and I'll probably never have the lump sum to buy a kit: but if I do the restoration, I'll buy a few parts whenever I have the cash.

It makes sense to me anyway......

I used to restore old (1920s vintage) yachts so I know whereof you speak.
As for rent, never put rent ahead of good hi-fi. Landlords and banks are a dime a dozen; good sound is rare!

So why am I gonna do it? Well, partly because I enjoy restoring something that's old and tired into something fun and shiny.....................

An have some fun on the ride..........you can just run the amp with out the choke....look at the Mk2 schematic B+ setup with a res on the sites free dynaco schematic...lot of great tube amp are ran with out chokes.....an I don't use them... less ther over 5h ......I like the no choke sound..but that's me................
cars when I was a kid all had tube radios... an some with .Reverb ...wild sound even if it was only Am.... low volt tube are in some tube preamps today ...some runing on 12-24V  Battres sweet sound.......
cars are not my thing...but things are just better.... once the owner....puts there hands into any of the re-working....we don't ever.... wont to be to young to learn...hehe...... ......................
as for the tube4hifi.... all of the amps an drive boards.......
I will have a pr of each..............good luck




thanks for the site Bob.........................

Well, I've been putting some time in and have got it all in pieces! I've given the chassis a quick polish:



I've cleaned up most of the parts  I'm going to re-use (not many) and have made my first order with Dynakitparts:

New stereo/mono switch (the old one had one terminal snapped off)
New tube socket
New bias kit

I'm going to start building it all back up again using the original Dynaco assembly manual, as a member here kindly suggested.

Very nice!  Are you going to replace the choke?  Bob's new choke is rated for higher current so it won't overheat like the old one.

Also, I like the miniatures you have as well!

Tubes4ever wrote:Very nice!  Are you going to replace the choke?  Bob's new choke is rated for higher current so it won't overheat like the old one.

Also, I like the miniatures you have as well!

Thanks! Yes, a new choke is on the list: I'll be buying the multi-cap and the choke next when I have some more $$ to spare. Watch this space!

That chassis looks good, nice job. I've got a '64 Falcon hardtop that could use a little help....

So here I am again not much done since last time, mostly been assembling pics of finished units. I've got the tube sockets in, along with the new stereo/mono switch (the old one had a lug snapped off).



Next I'll take a pause while I assemble the money for the rest of the parts! I do though have the new bias circuit from Dynakit Parts, so I can be putting that together at least.

I don't know whether anyone's still following this thread, but just in case: Here's progress!!

I got the new input jacks fitted, and made new speaker binding posts with banana sockets and g/f board from my local electronics store. They're correctly spaced for twin banana plugs (I only want to use the 8ohm).

I've also made my other order from Dynakit Parts: once I've installed the choke, I can refit the transformers and start some wiring. Next thing is to order my VTA Board kit and then it's solering all the way.

Thanks Bob! In that case I'll take out the tube sockets etc. when I install the VTA board.

I put the bias pots in just to fill the holes in the chassis: the plan is to temporarily get some vintage-looking knobs for cosmetic purposes and in the longer term buy your triode/pentode kit which will fill the holes AND do something useful!!

The advice is much appreciated.

Not much progress over the last few weeks: but at least I ordered my VTA Board and Parts Kit today!!

I discovered a few weeks back that all the wires in my power transformer were shot: insulation all cracked and flaked off. That's really set everything back, but at least I can be building the VTA board while I think of what to do about the transformer.

If there is still enough insulation so you can identify colors, that's not so bad.  Carefully mark the wires (perhaps by tracing them from where they were soldered) to identify them, and then disassemble the end bells from the laminations.  You should be able to access the wires where they are soldered to the windings.  Some new, properly colored wire, judiciously placed insulating tape, reassembly of the end bells, and you're set to go.  I have not rebuilt a whole lot of them this way, but certainly four or five of them.

Another option is a new power transformer.  The one sold by Dynakit Parts has a higher current capability than the original Dynaco transformer, which is operated very close to the limit of its capabilities, and runs hot.  In the repair shop I had to replace two or three of the original transformers on Stereo 70s, including a dead one that was given to me...it now has the transformer from a Stromberg Carlson EL34 stereo amplifier that had been dropped; it's bigger enough that the Stereo 70's cover won't fit.  I have kept that amplifier.

Thanks GPP49: I think I'll end up going to Dynakit Parts as you suggest.

I thought I would repair the wires as you detailed, but when I tried to remove the bottom cover it all went wrong.......its been potted in some  sort of black epoxy.......not original? Getting to where the wires solder on would be impossible without a jackhammer! Very frustrating.......

Not much more to do, I guess. If it were an irreplaceable transformer I would probably try with careful splicing, and heat-shrink insulation of the wires. But it would probably be preferable in this case to go with a new DynakitParts transformer. You don't know how hot this one has been run, and for how long.

Dynaco used several subcontractors for power transformers. I may have been lucky not to have encountered a potted one. I actually have a Mark II-III power transformer, without its end bells, and a Mark III with a shorted transformer. Actually I've had them for a long time...never needed the amplifier! The wires on that good transformer would be easily replaced if I needed to do it.

Thanks GP49 - I found some pics of other transformers from ST-70s which had no epoxy, so maybe it was added after a rebuild.....either way, it's over half and inch thick and rock hard. I would have thought it would have implications for heat build-up, but I don't know enough to really tell.

I'm putting a Dynakit Parts transformer on the shopping list! And just when I thought I had everything, too......

Hi any progress?