New to the Dynaco ST70 Community

[pac1085]

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.

I looked at that website and noticed he didn't mention any of the carbon comp resistors. Be sure to check the 1.5m screen grid resistors on the driver board since those seem to drift very high. Worth checking the others as well. performance could be atrocious if any of those are too far out of spec.

I've worked on a bunch of ST-70s where they were drifted very badly, and others where they were fine... may have to do with the environment they were stored in over the years.

 

[RDW]

I looked at that website and noticed he didn't mention any of the carbon comp resistors. Be sure to check the 1.5m screen grid resistors on the driver board since those seem to drift very high. Worth checking the others as well. performance could be atrocious if any of those are too far out of spec.

I've worked on a bunch of ST-70s where they were drifted very badly, and others where they were fine... may have to do with the environment they were stored in over the years.

Thanks for the tip I’ll definitely do that!

 

[gadget73]

carbon resistors in general tend to drift, but those high value ones especially. Possible it wants to be a little smaller with a 6GH8, something in the 1-1.2M range may get the plate and cathode voltages closer to proper. The smaller the screen resistor, the higher the cathode and lower the plate voltage at the triode section will be.

 

[tonyp063]

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.

6BL8/ECF80 also work fine with those adapters.

 

[pac1085]

6BL8/ECF80 also work fine with those adapters.

They "work" but when I tried it on my ST70 I got horrible measured distortion performance. I have a large number of 6BL8's and including many new ones. I also tried Russian 6f1p (equivalent to 6bl8) with the same poor results. 6gh8a seemed the best/most consistent followed by 6u8.

 

[peterh]

They "work" but when I tried it on my ST70 I got horrible measured distortion performance. I have a large number of 6BL8's and including many new ones. I also tried Russian 6f1p (equivalent to 6bl8) with the same poor results. 6gh8a seemed the best/most consistent followed by 6u8.

What is different on your st-70 ?

 

[pac1085]

What is different on your st-70 ?

Nothing, it’s a well documented/tested stock rebuild. I actually have two of them like this and I will start a thread at some point with test results from the various tubes in that position.

here is a link to the latest video I did on the second ST-70 including comprehensive performance tests on the analyzer (Rohde & Schwarz UPL - towards the end). I could swap 6BL8's into that amp and record the poor performance, since I have a good baseline.

 

[peterh]

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.

You better run slow and careful. Your 7199 is likely ok, no need to fuzz with them. Most components also ( unledd measured faulty)
The EL-34 is probably in questinable condition.
What is needed is EL34( well matched ) and the can cap.

It's easy to spend a fortune in vain. Replace what's bad and enjoy the music.

 

[RDW]

You better run slow and careful. Your 7199 is likely ok, no need to fuzz with them. Most components also ( unledd measured faulty)
The EL-34 is probably in questinable condition.
What is needed is EL34( well matched ) and the can cap.

It's easy to spend a fortune in vain. Replace what's bad and enjoy the music.

Thanks you’re right but free shipping basically paid for the extra caps, and the tube adapters are for future use so I don’t have to pay shipping later when (not if) I need them.

 

[Stereodiver]

Your 7199’s are most likely pretty tired. You may find you like the sound better with the 6u8a or 6gh8’s better. I was lucky enough to find a nos pair of Tesla branded tubes that I like very much!

Plenty of nos 6u8a’s on the big auction site for small money.

Have Fun!

 

[RDW]

Finally got a chance to open it up today. Everything looks pretty original to me (but I could very well be wrong as this is my first ST70) and mostly good except for the extreme amount of gunk all over the choke.



My parts order gets here next week and I’ll get started slowly changing out parts. I still don’t have a preamp or source to use with it in the shop and don’t know what I want to do yet. Until then I can’t really test any changes that I make except visually. I do have a cassette deck (TASCAM 112) with an output control so I could bring that down if I need to.

 

[gadget73]

If its not all original, it sure hasn't been messed with much. No obvious mods, and it appears fairly well built. Slightly later one, PVC insulation on the output transformers rather than cloth but thats about all I got. If you look at the numbers on the bias pots that will give a better clue about when it was made.

 

[RDW]

I have a question. I currently have a Marantz 2220B in my main system but it doesn’t have preamp outputs. I have thought about upgrading to an amp or receiver that does have preamp outputs and using the new amp’s preamp output to drive both the main system and the ST70 via y cables on the preamp out/main in terminals. My question is how will this affect the impedance and is it safe? The advantage is that I could then use all of my input sources hooked to the main system and control both amps with the controls on the main system. Would I need some kind of buffer to address any impedance mismatch? Could it be done this way?

 

[Oerets]

I will echo others in getting your needs filled by Dynaco replacement parts & vacuum tube audio amplifier kits from Dynakit.

Redid mine with all new board caps diode and resistors. Also replaced the choke due to it looking a bit toasted. Figured in for a penny....

Figuring planning on keeping it around for some time, so did a total overhaul.
One and done.

 

[peterh]

It looks like the stuff on the choke is added silicon adhesive, possibly to reduce mechanical hum ?
If it is silicon then the coke is OK.

 

[albusark]

Finally got a chance to open it up today. Everything looks pretty original to me (but I could very well be wrong as this is my first ST70) and mostly good except for the extreme amount of gunk all over the choke.

View attachment 3817338View attachment 3817339View attachment 3817340

My parts order gets here next week and I’ll get started slowly changing out parts. I still don’t have a preamp or source to use with it in the shop and don’t know what I want to do yet. Until then I can’t really test any changes that I make except visually. I do have a cassette deck (TASCAM 112) with an output control so I could bring that down if I need to.

This is expected for the original chokes, gravity and time. If I remember correctly it should measure 1.5 henries. They typically get replaced for cosmetic reasons rather than failure.

 

[Quadaptor!]

A great and classic amp and the "gateway drug" for many getting into hifi back in the day and for getting into "tubes" here now in our day. My first ST 70 was in in the mid 80s and, to me, it was the working man's McIntosh. I got mine at the Sound Well in Berkeley for about, I think, a hundred-bucks. That same shop had McIntosh amps they were selling for about 800-900 bucks around 1985. I also got a PAS 3X a little later from the same shop for $125.00. Long story short, I think the ST 70 still stands on strong legs today however I'm not so sure about the PAS preamps. When you get around to a preamp, I would give some consideration to more modern tube units to make the ST 70 shine. And I don't consider Dynaco amps and preamps modified with "drop in kits" from the likes of outfits like Tubes 4 Hifi to be Dynacos anymore as they basically use the ST 70s hardware to make a completely new circuit. Have fun with your amp. As a "gateway drug", with this one you're allowed to "get hi(fi) on the supply"!

 

[TV MAN]

I'll jump in here with a few cents worth. I too found some Dynaco stuff in the 80's and 90's. I hadn't used my 70 in years, many years, and brought it up slowly. Works great! I did the same with my Pas3. Works perfect, sounds super. I even recently found a PAS 3 on the auction site, and it too works good. I've tried a PAT, they are ok, but not a Pas either. I also tried a Douk. NOT impressed. Mine is a small black box with bass, treble, volume and a couple small tubes sticking out of the top. I'd say don't waste your money. I'd used mine on a couple amps I built, and felt they were a little lame., but when I connected my Pas3, they came to life! I think you'll like that st70. yea, some quirks, but mine (both) are factory original, and to me, sound great. I'd check it with the 7199s. May be fine. Looks like you may have the German EL34s. good ones. I'd use 'em. So, just my 2 cents.

 

[Quadaptor!]

I'll jump in here with a few cents worth. I too found some Dynaco stuff in the 80's and 90's. I hadn't used my 70 in years, many years, and brought it up slowly. Works great! I did the same with my Pas3. Works perfect, sounds super. I even recently found a PAS 3 on the auction site, and it too works good. I've tried a PAT, they are ok, but not a Pas either. I also tried a Douk. NOT impressed. Mine is a small black box with bass, treble, volume and a couple small tubes sticking out of the top. I'd say don't waste your money. I'd used mine on a couple amps I built, and felt they were a little lame., but when I connected my Pas3, they came to life! I think you'll like that st70. yea, some quirks, but mine (both) are factory original, and to me, sound great. I'd check it with the 7199s. May be fine. Looks like you may have the German EL34s. good ones. I'd use 'em. So, just my 2 cents.

And I do have a lot of respect for running both units as a solid two channel stereo today.

 

[Dandy]

Another ST70 owner here. Congrats on getting a nice, clean, original unit.

I use mine with a passive pot in a box volume control and a separate modern DIY tube phono-stage. Dynaco preamps are very scarce in the UK, so I can't speak from personal experience but, if I were living in the US, I would be looking for a PAS-2 or PAS-3 to play with. Of course, tube preamps from that era are usually not as good as modern ones and will need much more restoration than your power amp.