Refurbish a Dynaco St70

This is my 2nd attempt at getting this thread started. 1st attempt never posted. Please forgive should a duplicate arise.

I have a Dynaco ST-70 that has been in storage for several years and has few hours on it. I would like to have it refurbished and updated for electrical components, inputs, and outputs. After having searched this forum I thought it might serve me best to initiate a new thread. Looking for contacts for having this done. Thanks in advance for your suggestions. Mike
Do not waste your time, and your money on a re build. It would not be cost effective. Even if you have the original circuit diagram, and original parts list, I doubt you could find current components that will work. Move on to other tube products, mostly from China that are a good value. Check out the Cayin line, great sound, great value. Better yet look for a Sound Values, M60 tube amplifier. The come up on ebay often. I have a pair and they sound great. They were a knock off of the St60, from the firm that brought up all the parts inventory of Dynaco went out of business.
Talk to these guys
I meant look at this. There are a number of places the redo these, although it may be cheaper to buy a renovated model used.
Thanks for the responses. I will check into enjoythemusic. I have considered that the cost might not be worth it on a strictly sonic basis. Such an historic amp and in such great shape thought it worth checking into. If is decide not to do anything with it I hope to find someone interested.
To rebuild a Dyna ST-70 is not that nuts! We do it all the time. The filter caps are readily available as the amp is so popular and the tubes are easy to find too, although the 7199s have been going up in price recently.

A properly-working ST-70 is actually a respectable amplifier. They have distortion figures that gave their competition conniptions back in the day (Hafler was an excellent designer) and so rival much more revered amps like the Marantz 7B.

There is an excellent thread about the ST-70 on audiokarma: [ur][/url]
Depending on how in-depth you'd like to go; Joe Curcio may be the best person to contact. He has decades of experience, in rebuilding/modding the ST-70. These amps have some excellent transformers, which are(arguably) the most important components, of a tube amp. The only major problem, with owning a completely stock ST-70, is finding a source of NOS or high grade 7199 tubes(my favorite was the Sylvania). They're almost extinct(Unobtainium) and there is no direct/plug-in replacement. It's not too hard to rewire the tube socket, to receive the pin configuration of the 6GH8A tube and adapters are also available. NOTE: When I say, "completely stock," I'm not saying one with the original caps and resistors, which absolutely should be replaced, to obtain the best sound and any kind of reliability. The only part that can be hard to find(for a stock rebuild), is the quad-cap. There's a seller on eBay( and Curcio Audio also offers a kit/upgrade for it: ( I used to buy mine from Antique Electronics Supply, in Tempe, AR. Replacing the inputs, outputs and power cord, without chopping up the chassis, is pretty much a cakewalk. Have fun!
Personally- if I were going to rebuild an ST-70 now days and retaining a MOSTLY stock(tube rectifier/greater filter cap capacity) power supply(like I used to), I'd use this board: (, stuffed with higher quality components. I just happen to love the Siemens CCa and this board would make it's use possible.
Thanks much! The responses are exactly in the form I was hoping for.

Atmosphere, am I correct in thinking that you (your company) rebuilds them for others that supply the amp?
Yes, we have done that many times. A typical rebuild (such that the chassis is reliable) is about $200.00. This is mostly just repairing the power supplies and making sure that the unit meets spec with good tubes, but does not include any tubes that might need replacement.

IMO its best to keep the amp as stock as possible; perhaps replace the coupling caps as an upgrade; the reason why is that the original design is quite competent except for the power rectifier system, which is handled by a single 5AR4. It should have been a pair of rectifiers. Some people replace the tube with solid state but you run into problems with too much voltage which can lead to other problems down the road- short filter cap life and possible damage to output transformers.

This issue is not addressed properly with most of the update paths I have seen- for example adding more tubes to the driver stage puts more load on the already stressed rectifier... I find that as long as the amp is not pushed hard it is very musical, so selection of the loudspeaker is important for best results. The harder you push the amp, the shorter the rectifier life will be, but it can last decades if the amp is loafing. So an efficient speaker is really helpful.
Thanks much Atmosphere! I will contact you regarding having your people do this. I believe it would be fun to play with and should I not find a place for it in a system and am sure I can find a AG member to enjoy it without much $$ loss.
Hello Ralph- "the original design is quite competent except for the power rectifier system, which is handled by a single 5AR4." Just curious as to whether you've ever added a second 5AR4(and heater transformer) to an ST-70.
No- not done that. There's not a lot of room for another rectifier, and unlike some I really don't like modifying the chassis of older units- it usually damages their resale. For something like that I figure if a person has a better idea, best to build it up from scratch.

But if you did have dual rectifiers then the amp would be more reliable, as the 5AR4 is the most likely tube to fail, but that is only if you consistently push the amp hard.
"but that is only if you consistently push the amp hard." I had the same Mullard GZ34(Blackburn), for about 15 years, but always had the system bi-amped, below 200Hz(ergo: wasn't pushed). "I really don't like modifying the chassis of older units- it usually damages their resale." That's what stopped me from making a permanent, on chassis mod. I was thinking about punching out the quad-cap hole, to accept an octal socket, then installing four filter caps and a heater transformer, internally. Two GZ34s, each used for half the full wave(plates tied together, per the Radiotron Designer's Handbook) did sound nice though. Then I got power-hungry, gave the ST-70 to my son and moved on.
Atmasphere is spot-on with his recommendations - these are wonderful little amps; well worth rebuilding, and best left in stock form. I've rebuilt a pile of them over more than a couple of decades, including some that were brought to me with various mod kits . . . and virtually every well-known mod actually decreased the stability margins to which the amp was originally designed. David Hafler certainly knew what he was doing.

Of course, one has to bring them back to original operating condition, and replacement of electrolytics and coupling caps are obviously mandatory (use the original values for the latter!). Don't forget that these were frequently assembled on somebody's kitchen table, so inspect very carefully for poor solder connections, and very carefully re-form any stretched tube-socket pins.

Back in the '80s you didn't have to pay too close attention to the resistors, but nowadays ALL vintage carbon composition types should be carefully measured for drift before assuming they're okay (watch for voltage ratings if you sub modern metal films). Don't forget the "Biaset" resistor on the output tubes' cathodes, if this drifts, your bias settings will read incorrectly. Replace those silly 10-ohm input grounding resistors with wire links.

The power supply is indeed the weakest link, and some simple revisions can improve performance and reliability. Increasing the capacitance on the plate B+ supply helps dramatically, but don't add any before the choke, as it will then develop an appetite for rectifier tubes. There's some logic to adding a second 5AR4, but to avoid more filament load on the transformer you'll then need to use a separate 5v transformer for the rectifiers, and put this and all the electrolytics inside the chassis . . . which then gets a bit crowded . . . to me, this can easily be the first step down the road leading away from the elegant simplicity of the original.

I personally have had great results (in both sound and reliability) with a solid-state rectifier in ST-70s. Standard fare is a pair of 1N4007 diodes, and it's easy to wire them onto an octal plug so you can leave all the wiring original, and pop a 5AR4 back in if you like. A well-chosen snubber network in parallel with each diode eliminates their noise contribution (10nF in series with 47 ohms is a good one-size-fits-all snubber solution). You can use the same to replace the selenium bias rectifier if you like; simply alter the values of the bias supply filter resistors to get the bias pots back within a comfortable range. The solid-state diodes do increase the B+ a bit, but that's no problem as new replacement snap-lock electrolytics are available with slightly higher voltage ratings than the original.

A couple of final touches are an inrush-current limiting thermistor in series with the AC cord, and replacement connectors. For the input RCAs, it's just a hardware game, or you can make some RCA-to-octal-plug cables and use those front panel "preamp" sockets (probably for the first time in the amp's life!). For the speakers, I used to use replacements for a McIntosh 8-channel amp (MC7108 I think) as a perfect drop-in upgrade replacement; maybe Mac still has these available.

In any case, the ST-70 is just as fun today as it ever was, so enjoy it!
Thank you Kirkus for such a complete response. I am going to contact Ralph regarding getting this done and go with his suggestions.

The 1st system I heard that got me into this hobby was: AR XA turntable, Dynaco PAS 3 & ST-70, AR3a speakers. Couldn’t believe the sound. It was spring of 1969, I was 19.
Mesch- You were one lucky 19 y.o. I got my AR XA in 1970 or 71 (if you remember the 70s you weren't there!) but I had to wait another 20 years to discover the joys of tubes.
Swamp, yes I was and consider myself that way over my life. Hope to continue in same for some time. Took me some time after hearing that system to get to tubes, 2010 actually, now use a tube pre into SS amp. You don’t happen to live in south Florida, 'a walkin the swamps’, :-).
Mesch- I'm a new Englander thru and thru but must admit that w wind chills of -20 / -30 overnight it might be time to be walkin' some a them mangrove swamps y'all got down there.
Swamp, actually I am from southern Illinois. We do have a cyprus swamp, one of the furthest north in US. Seems you have been walkin' through allot of snow given the many major snow storms in the last couple years.

Anyway, back to the post, I currently have an working AR XA turntable. Cartridge on it need replacing. Given that I have a start on a vintage 60s system, I might pursue it further.
Mesch, don't know if it is so but read that Cario, IL is closer to Jackson , MS than Chicago.
I'm in CT, where most years, the snow cover is more off and on, so we do get some field work done. We also do a fair amount of report writing in the winter. Then we go to Cancun and sip margaritas!
Schubert, i believe you are correct. I live ~60 miles north of Cario so more of a toss up.

Swamp, field work, report writing, Cancun & margaritas; been there and done that! Cheers.
"Pry- I meant with modifying Dynacos, not where he lived. :)" Actually: Mr Johnson began a company known as Electronic Industries, in Minneapolis, Mn, in 1951. He was building some amps back then. Another company assumed ownership, but- he stayed on as Research Director. He didn't start modding ST-70's until the late 60's. Started ARC, in 1970.
Rodman, thanks for the additional, and corrected info. I remember Pelpoe Industries as well. My point was that modding that amp has some serious cred behind it, and if memory serves, some of the earliest ARC offerings were based on Dyna mods.
YEP! ARC's first FM tuner was based on the Dynaco FM-3. The M-60 MkIII C3 amp was a redesign of the Dynaco MkIII. ARC offered kits, to remake the ST-70, using only the chassis and transformers. Imagine what we'd have had, back then, if Hafler and Johnson had formed a company. But, HEY- over 350,000 STOCK ST-70s were sold, making it the most popular tube amp ever. Hafler's design wasn't too shabby, on it's own merit(what an understatement)!
ARC was a spin-off of Electronic Industries. EI is still in business today, but has switched over to making circuit boards.
And before Electronic Industries, Bill Johnson's company was known as Peplo. I don't know why or the significance of the name.
As an update I have contacted Ralph (Atmosphere) and intend to ship him the amp for refurbishment. When I have this done and try out the amp I will provide my thoughts. Thanks to all!
My first home experience with tubes was a ST-70 and a PAS-3 that I bought at a garage sale for four dollars each. No joke, and it was at the end of the day too. Those two pieces sat on a table at that garage sale all day and no one bought them. Go figure.
Wow Papa, what a great deal! My ST-70 came to me free as it was going to be thrown out. Have not yet sent it to Ralph however the intent remains.
As an update, I was distracted from this however finally got around to boxing my ST-70 to be sent to Ralph. It will go out this week. I will keep all informed as this project progresses. May try to find a PAS-3 thereafter. Oregonpapa, maybe I need to go garage sale shopping. 

You got it out in under a a hair. :-)
Good luck with the renovation. I'm sure it'll be worth it!
OK, so I have received my ST-70 from Atmosphere, purchased new tubes as recommended by Ralph, biased the tubes, and hooked to a set of Clestion F15 speakers for a trial run. Amp works perfectly and sounds wonderful. Ralph was a treat to work with, had several conversations around his recommendations as what to do and pretty much followed his recommendation to a 'T'. Ended up keeping the amp as original as possible.  I am very pleased with my choice to refurbish. Thanks Ralph! Thanks to all for their input toward this end!  As I indicated on another thread, I am still looking for speakers that mate well with this amp.
mesch, congrats on your decision and being pleased with the results.  Your initial question didn't receive unanimous support but pretty close to it.

My first stereo was built around Dynakits back in the '60s, which included a ST-70.  There were musical characteristics in the sound of that system that still influence me today, all these decades later.

I'll also say good choice in having Ralph do your restoration.  I owned Atma-Sphere amps years ago (wonderful sonics with some music but not quite enough power for full orchestra or big band with those speakers) and will say he is among the best people in this industry to deal with.

Wondering if Ralph is still repairing St 70 amps?  I have one I'd like very much to get breathing again.  Any help would be very much welcomed 

Thank you

I have rebuilt 3 of the Dyna 70s and more than a dozen of the MK IIIs.

they are decent amps, and can sound good, but don't go crazy with after makret parts, or you will soon have way more cash in the amp than it is worth. or alternatively you can begin to ask what tube amp can you now buy on the used market for 1.5K dollars....quite a few.

would suggest looking at and  for starters.

as one of the other posters noted, the Cayin amps are really well built and can be bought used for a very reasonable price.

I'd say have fun with your rebuild but dont go crazy with aftermarket parts, NOS tubes and the like.

happy soldering.