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New to the Dynaco ST70 Community

RDW

RDW

Well-Known Member
I haven’t posted here in quite some time because life got in the way, but recently I was considering a tube amp to play with again (I’m 73 but haven’t had anything tube in decades). I was thinking about modding one of the cheap Chinese p2p amps but found working VT70 on marketplace for $600 and bought it so I got a “real” amp for not much more. I haven’t opened it up yet but assuming it’s mostly original I need to plan what to do and in what order. I’m sure if it still has the selenium rectifier that needs replacing first and then all electrolytics then setting the bias correctly. Since I heard it running I know the basics are there but I don’t know the condition yet. I will attach a few pictures if I can. I obviously would like to clean the chassis and pcb too.

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I also saw a “restoration” kit on that auction site for $95 that is probably higher than the parts separately but would save making a list and sourcing them myself. Has anyone had experience with these kits?

Also I found a Hafler DH101 preamp for $175 that’s had a lot the RCA connectors updated. Although SS is it a good enough match or should I look elsewhere? I don’t want to spend a fortune here.
 
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zackthedog said:
Do NOT purchase that "restoration" kit. That guy is a well known fraud. You've bought a nice-looking original unit. There's a whole thread here on the proper restoration of an ST-70. I csan't find it at the moment but I'm sure someone else can point you to it.
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Thanks I’ll look for it.
 
Congrats to a unmolested ST-70.

The Se rectifier may be left in place. They are no problem.
The can cap and the 2 electrolytes in the bias chain could be replaced. Do observe polarity of the bias caps.
EL-34 tubes is most likely subject to replace with a matched quad (or two matched pairs) The benefit purchasing a quad is that
you don't have to keep track of which ones that is a pair. The 7199 is most likely ok unless you have a hum
that wont disappear.

But first i would power it up connected to speakers and a source, adjust bias if possible and give it a good
listening session. This will give a baseline of what is working good and what might be replaced.
 
RDW said:
I haven’t posted here in quite some time because life got in the way, but recently I was considering a tube amp to play with again (I’m 73 but haven’t had anything tube in decades). I was thinking about modding one of the cheap Chinese p2p amps but found working VT70 on marketplace for $600 and bought it so I got a “real” amp for not much more. I haven’t opened it up yet but assuming it’s mostly original I need to plan what to do and in what order. I’m sure if it still has the selenium rectifier that needs replacing first and then all electrolytics then setting the bias correctly. Since I heard it running I know the basics are there but I don’t know the condition yet. I will attach a few pictures if I can. I obviously would like to clean the chassis and pcb too.

View attachment 3815911View attachment 3815912View attachment 3815913
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I'm also lately of the opinion that the selenium rectifier is fine to keep in for the short term while you get everything else in order. Its voltage drop is around 25 volts if I remember, so once you do swap it for silicon diode you need to check the negative bias value again and probably replace a resistor.

That's a great looking ST-70, you could make it shine like a mirror with some chrome polish.

I'd check all resistor values and replace anything way out of value, and then those black cat coupling caps likely have parallel resistance. You can use a multimeter to check for DC passing through them while the amp is in operation, to me the simplest way to check for leakage. It's worth it to hunt down sources of hum and eliminate them because it's pretty easy to get this amp dead silent. Usually the filter can or grounding issues from the original Dynakit builder.
 
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jr231 said:
About something like that that thing definitely humming and needing cleaned up. Dynakit or a macintosh Heathkit thing. I getting all kinda dumb whirrr noise agian use a wipe for jacks and still got noise i'm biased right too on the GX..just use to single coils and old mobile home I still can't gate enough or noodle through the hum and noise..I can but play loud through it..well thats about as bad solid state imitate tubes like that...some pics for ya still...combination of it all my speakers probaly making noise. I want plug blackie into it probaly rad and bad.

You need a few drinks and a black or something etc lol. I just think of it as a noisey reciever but yeah it looks right good tech. the ICs could be noisey that age they are known to be. and yeah jamicon could be suspect wiggle plugs maybe find it better. have good day ill tech with ya. should have good garage band.
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Good advice above, electrolytic caps are the low-hanging fruit. Can check the resistors on the board in circuit. If they read low, something is across it, but you can compare to the other channel. If it reads high, its bad. Those caps I think are supposed to be a di-film type that doesn't go leaky but no idea how well they really age. Change if you like, or not. the 7199 is a challenge to get, if yours are good, great, but if they are bad maybe consider 6U8 adapters, or a new driver board that just takes them natively.
 
RDW said:
I also saw a “restoration” kit on that auction site for $95 that is probably higher than the parts separately but would save making a list and sourcing them myself. Has anyone had experience with these kits?
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Don't waste your time on those kits, especially from the hifiaudio guy. Junk..

This is what I would buy if I wanted to do a stock rebuild and didn't want to source the individual components

PC-3B ST-70 MF Resistors - Dynakit Parts (this one uses the 6gh8a tubes, but you can get their other version that uses the 7199's if you have good ones)
ST-70 BIAS CIRCUIT KIT - Dynakit Parts (Bias components kit)
MULTI-SECTION CAPACITOR - Dynakit Parts (I like the upgraded 80-40-30-20 can capacitor but they sell an original value one as well if you want to stick to lower capacitance, but it appears to be out of stock)
C-354 CHOKE - Dynakit Parts % (optional but I like to replace them if the wax is all melted out of the originals and stuck to the bottom cover - cheap enough)

There are a ton of other directions you could go with it too, of course
 
This~~! ^..^

Time to toss that old crusty driver board.

Replace it with a 6GH8 model, Replace filter cap and you good to go for another 25 years.
 
Congrats on finding a good ST70. Great amp with great potential....so much that can be done with them.

Tell us about your room and what's in the rest of your system.
 
Stereodiver

Thread 'Dynakit ST 70. A Restoration Journey'

I’ve always wanted to try a tube amp. I just haven’t found one I was willing to afford. Until now.
A few weeks ago a friend of mine turned me onto a Craig’s list listing for this amp. It happened to be at a local used record store. I used my iPod and a pair of medium sized Kef speakers that the store had to test it before purchasing. With the limited volume the iPod was capable of supplying it passed a signal with seemingly equal loudness from both channels. I couldn’t detect any hum. It seemed to be very quiet. A deal was struck, not a score, a fair price, and I brought it home...
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If you have time you can read through my journey with my ST70. I love it!

Good luck!
 
jr231 said:
I want to add it its like doing bias work...kinda like doing the threshold/depth mod/contour with a pot to the feedback circuit kinda like a Soldano you can get in there with a 500-1meg pot and some small couplings to ground kinda works with the jack coupling idea...no its not lars mod thats parlleling with the output couplings or whatever else put in there maybe some ceramics.
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Can you repeat this in english ?
 
Looks pretty clean. Nice find. Have owned five ST70s over the years. They are normally pretty trouble free once gone through.
If you don't mind the suggestion you may want to consider the following. I am sure there are tons of folks here who will have excellent input on this model as well.
1. replace the main filter cap with either an authentic cap or the CE version.
2. replace the bias capacitors below deck.
3. replace those .1uf black gate caps which usually are leaky.
4. measure all the resistors and see if any are out of tolerance.

those 4 steps would be a great starting point where you can at least get to the point where you can power up.
At some point you may want to replace the bias rectifier with a diode.
 
pac1085 said:
Don't waste your time on those kits, especially from the hifiaudio guy. Junk..

This is what I would buy if I wanted to do a stock rebuild and didn't want to source the individual components

PC-3B ST-70 MF Resistors - Dynakit Parts (this one uses the 6gh8a tubes, but you can get their other version that uses the 7199's if you have good ones)
ST-70 BIAS CIRCUIT KIT - Dynakit Parts (Bias components kit)
MULTI-SECTION CAPACITOR - Dynakit Parts (I like the upgraded 80-40-30-20 can capacitor but they sell an original value one as well if you want to stick to lower capacitance, but it appears to be out of stock)
C-354 CHOKE - Dynakit Parts % (optional but I like to replace them if the wax is all melted out of the originals and stuck to the bottom cover - cheap enough)

There are a ton of other directions you could go with it too, of course
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^^This^^
You should be able to find everything you need to restore your ST-70 from Dynakit including any add ons you want like an added bias kit. Buying one if Dynakit's replacement curcuit boards is a good idea. They a deffenet improvement over the original phenolic boards with the carbon comp resistors.
There's lots of mods for these amps since they're the small block Chevy V8 of amps.
One added mod thats a good idea is the
"Yellow Sheet" mod that adds a couple diodes to the rectifier socket. It adds some protection incase the Rectifier tube fails.
 

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knotscott said:
Congrats on finding a good ST70. Great amp with great potential....so much that can be done with them.

Tell us about your room and what's in the rest of your system.
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This is just to play with for now, so I don't really have a place or a system for it. My main system is a Marantz 2220B receiver, Marantz 6100 TT, Pioneer CS77 speakers, and various cd and cassette decks, but I've been wanting to get back into tube equipment for a while now. I expect after I get this going I will put it in my office with Polk T50 speakers and a Yamaha CD player for now; not sure what to do for a preamp but I don't want to spend a lot right now so maybe one of those Chinese tube preamps with phono and aux input?
 
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Thanks so much for all the great replies; somehow I didn't get notified about them. I found a "Dynaco ST-70 full recap" post at Cathode Bias that gave a recommended parts list and what they replaced and I got them all ordered from Amplified Parts. Of course when I get into it I will also check the resistors and replace any that look bad, then set the bias as recommended. All of the tubes seem to function but I don't know how well and as stated above the 7199s are hard to come by so I added the socket adapters and 6GH8A tubes (I think that's correct). I don't know yet what I'll do for a preamp; maybe get one of those cheap Chinese tube preamps such as Douk just to get started. There's a Hafler DH101 SS preamp close to me for $175 but I don't know if it would be a good match or what condition it is in beside having most of the RCA jacks replaced.
 
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6GH8 or 6U8 are the usual subs. Neither are precisely identical, or identical to the 7199 but "close enough" in most cases. Same pinout though so the same adapter works for both tubes.
 
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