Hi, I've been searching through here looking for a suitable project, but it's a difficult thing to search for, as I discovered. I've been reading random threads here that looked promising only to emerge hours later empty handed. But I've enjoyed what I've read so far, so it wasn't wasted time, and I'm in no hurry...
So I need your help and advice: I would like to build a tube amp that is pretty much everything an ST-70 is I suppose. Push pull, stereo, 30-50 WPC, commonly available (and reasonably priced) parts, well respected sound, lots of expertise and experience floating around, etc., etc. Definitely nothing exotic. And I'd build an actual ST-70 if only I could find a well-documented project that tells me which parts (and vendors) to avoid, what to watch out for, how to lay it out, yada yada. (Someone to hold my virtual hand).
I've been building SS amps for years and am beginning to understand their operation, so I want to learn the same way with at least one tube amp, and see how it goes from there. I'd like to build it from scratch, I'm not a kit buyer/builder -- no fun there. But I know very little about tube amp specific parts like the transformers, tubes, chassis, hole punches, etc., and must rely completely on a well documented project. Otherwise I'll screw it up by buying crappy parts for too much money, or laying it out so that it hums, burns and attracts mosquitoes.
Any advice, directions, referrals?
Alternatively, I could just "go for it" and ask a zillion questions here. Does that sound like a more reasonable approach?
..Todd
So I need your help and advice: I would like to build a tube amp that is pretty much everything an ST-70 is I suppose. Push pull, stereo, 30-50 WPC, commonly available (and reasonably priced) parts, well respected sound, lots of expertise and experience floating around, etc., etc. Definitely nothing exotic. And I'd build an actual ST-70 if only I could find a well-documented project that tells me which parts (and vendors) to avoid, what to watch out for, how to lay it out, yada yada. (Someone to hold my virtual hand).
I've been building SS amps for years and am beginning to understand their operation, so I want to learn the same way with at least one tube amp, and see how it goes from there. I'd like to build it from scratch, I'm not a kit buyer/builder -- no fun there. But I know very little about tube amp specific parts like the transformers, tubes, chassis, hole punches, etc., and must rely completely on a well documented project. Otherwise I'll screw it up by buying crappy parts for too much money, or laying it out so that it hums, burns and attracts mosquitoes.
Any advice, directions, referrals?
Alternatively, I could just "go for it" and ask a zillion questions here. Does that sound like a more reasonable approach?
..Todd
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For the last month i've been listening to Gregg-the-Geek's DynaMutt.
I gave him 2 ST70 donors and got DynaMutt back. A step up from what i had before.
Gregg has circiut boards
dave
I gave him 2 ST70 donors and got DynaMutt back. A step up from what i had before.
Gregg has circiut boards
dave
Yeah I saw that. Gregg has some nice stuff there. Looks well done. And a convenient drivable location to boot.
But it's easy to stuff and mount a board (famous last words). Gregg's done all the hard work there. It's the other stuff that has me more stymied; transformers in particular. What are (in anyone's opinion) the best bang for the buck in power and output transformer options? ...Kind of a good/better/best ranking. I only know of the Hammond and Edcor brands. How do they compare? What other brands should I know about for an ST-70?
er... where'd you find all the surplus ST-70's, Dave? Is there a pawn shop nearby that specializes in them? Got 2 more to give me? I'll give you something back in return. (glancing around... really old cell phone, box of obsolete computer parts, ornery spouse...) Well, I can always hope.
..Todd
But it's easy to stuff and mount a board (famous last words). Gregg's done all the hard work there. It's the other stuff that has me more stymied; transformers in particular. What are (in anyone's opinion) the best bang for the buck in power and output transformer options? ...Kind of a good/better/best ranking. I only know of the Hammond and Edcor brands. How do they compare? What other brands should I know about for an ST-70?
er... where'd you find all the surplus ST-70's, Dave? Is there a pawn shop nearby that specializes in them? Got 2 more to give me? I'll give you something back in return. (glancing around... really old cell phone, box of obsolete computer parts, ornery spouse...) Well, I can always hope.
..Todd
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Hi,
You can get iron equivalents from Triode Electronics or Dynakit Parts.
Edcor's closest combo is XPWR002 (the 6.3V windings would **just** make the grade) and 2 x CXPP60-MS-4.2K
If I were building an ST-70 from scratch, I would use the Triode Electronics power transformer and the Edcor CXPP60-MS-4.2K OPT's.
Well, I had to beg one from Dave to work on... they still go for $$$ even completely kaput
Cheers!
You can get iron equivalents from Triode Electronics or Dynakit Parts.
Edcor's closest combo is XPWR002 (the 6.3V windings would **just** make the grade) and 2 x CXPP60-MS-4.2K
If I were building an ST-70 from scratch, I would use the Triode Electronics power transformer and the Edcor CXPP60-MS-4.2K OPT's.
Well, I had to beg one from Dave to work on... they still go for $$$ even completely kaput
Cheers!
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I'm very curious to try the O-Netics myself. I've also been happy with the Dynaclone iron from Triode and ecstatic with transformers from James. The latter will take a bit of work to procure, but they're worth it- performance is excellent, pricing (including shipping and duty- DON'T use the air freight option!) is not outrageous, and they're beautiful.
There is loads of ST-70 info over here, including boards, kits, etc:
diytube.com :: View Forum - st70
diytube.com :: View Forum - st70
The Triode ones are easy to buy. That's a positive compared to the others.
The James transformers (and chassis) look interesting. I have family in Taipei, so I'll to check further into those. Maybe I'll go visit this year, with an ulterior motive.
I cannot find any official information about O'Netics. Just lots of kudos and whatnot. Does he do strictly OEM sales, or does he have dealers, or sell privately, or is he just a very vivid figment of everyone's imagination?
..Todd
The James transformers (and chassis) look interesting. I have family in Taipei, so I'll to check further into those. Maybe I'll go visit this year, with an ulterior motive.
I cannot find any official information about O'Netics. Just lots of kudos and whatnot. Does he do strictly OEM sales, or does he have dealers, or sell privately, or is he just a very vivid figment of everyone's imagination?
..Todd
Is it the sound quality (via ultra linear P-P) of original ST-70 that interests you or is it the looks, price or is it something else?
If it's the sound, I can't think of a better option than to build the exact thing. Dynakit has the original design with newer parts. What I've done was to get a used ST-70 (mine was beat up) cheap, take it apart, then restore it with replacement parts here and there. I've learned a lot about this amp during that process. Dynaco Doctor was a big help.
Excellent question. I'm not really sure what the answer is, however it's a great opportunity to babble endlessly. (Something I learned from Destroyer X in the SS forum.) Sorry, no photos of my meals to accompany the text though.
My dad was a TV repair man in the 50's and 60's. I grew up in his workshop and was always up to my eyeballs in boxes of tubes and selenium rectifiers (which we learned would could stink up an entire school, causing it shut down for a day, if you plug its leads into an electrical outlet.) He had a big collection of antique (even then) radios. I fondly remember his VTVM and giant tube tester. He built me a 6L6 push-pull bass guitar amp for our band when I as a kid. He thought he was building a hi-fi amp, but it got re-purposed. I wonder what ever happened to that amp...
Recently (he's about 90 now) he passed along his tube caddy, still full of TV tubes, a box of remaining "keeper" parts (including the caps below), and huge box of Popular Electronics mags. So, I'm fully equipped to repair someone's black and white TV now, as long as it doesn't have any of those newfangled integrated circuits or flip-flops.
So, to answer the question, I just chose the ST-70 as a target circuit because there is so much information about it everywhere. I would have an endless supply of reading material that would apply to my project. And parts seem to be very commonly available.
I honestly have never seen one in real life. (I have an ST-80 SS amplifier that I'll probably gut and retrofit one day). I haven't heard a tube amp in many years. It's time to re-visit.
..Todd
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They are so ugly, they're oogly to look at, but sound pretty darn fine
They are also one of the most easily tweaked amps... a good deal of that has to do with so much info out there and they did start off with a helluva great tube lineup.
Their OPT iron is also nothing to sneeze at - quite well done, especially for the era.
Cheers!
IMO, you should consider working with your dad's caddy, instead of "cloning" a ST70. Develop a list of tubes in your possession and post it. For instance, horizontal O/P types can be used for audio O/P, with only a little ingenuity.
A photo of the ST70 is here.
BTW, your dad's caddy may have money in it.
Plenty of TVs used 6SN7s. NOS RCA 6SN7GTBs command hefty prices. Good 6CG7/6FQ7s are also valuble. 6AU6s can be conscripted as audio signal pentodes and they are needed in tube FM tuner IF strips.A couple of comments from a long time ST 70 owner. I really have a soft spot for an amp that really delivered for years.
There are good things and bad things about your plan. As you pointed out, there is a lot of information out there, and the transformers and chassis are readily available. Also, the amp was a solid performer.
A couple of things you should know. The amps were re-rated for 25 W RMS. This was due to a marginal power supply.
Second, the input tube is a 7199 Pentode / Triode that is difficult and expensive to buy today. Many of the modifications substitute other tubes and topologies for the 7199.
If I were looking for a reasonably priced amp, I would look at one of the EL84 PP amps on the forum. The one I personally admire most is Red Light District. If it were available, the Tubelab Simple PP would be worth its weight in gold just for the quality of the instruction manual.
The forum is great about answering questions. Good luck to you.
Doug
Hi Doug & Eli,
The caddy is in storage currently, but when I drag it out I'll list its contents here and you folks can tell me what's workable. I thought of the same plan actually (to build something with caddy parts), but I don't understand tube electronics enough (yet) to tackle anything remotely exotic. And I'm not expecting half those tubes to be suitable to amp building. I was visiting Dad this evening and he said all the tubes in the caddy are new (as in never used, not as in age). He also said he kept a really nice set of output tubes for an amplifier project, but they have long since disappeared. Too bad. In the meantime, I need a keep-it-simple project to start with: Something ST-70-ish.
As for the 7199, thanks for mentioning it. I didn't realize that. I was planning to buy a more recent blank driver PCB from the many available, so hopefully that won't be a problem. I'll keep that point in mind.
And I'm not concerned about the power output. I will probably design a set of speakers to work well with it anyway (one day). And 25 watts is still plenty for all but the bigger block rocking parties. And for those I have plenty of big-watt amps laying around pining for abuse.
And in case anyone misunderstood, I've seen PLENTY of ST-70 photos. Just real no units to plug in and listen to.
..Todd
The caddy is in storage currently, but when I drag it out I'll list its contents here and you folks can tell me what's workable. I thought of the same plan actually (to build something with caddy parts), but I don't understand tube electronics enough (yet) to tackle anything remotely exotic. And I'm not expecting half those tubes to be suitable to amp building. I was visiting Dad this evening and he said all the tubes in the caddy are new (as in never used, not as in age). He also said he kept a really nice set of output tubes for an amplifier project, but they have long since disappeared. Too bad. In the meantime, I need a keep-it-simple project to start with: Something ST-70-ish.
As for the 7199, thanks for mentioning it. I didn't realize that. I was planning to buy a more recent blank driver PCB from the many available, so hopefully that won't be a problem. I'll keep that point in mind.
And I'm not concerned about the power output. I will probably design a set of speakers to work well with it anyway (one day). And 25 watts is still plenty for all but the bigger block rocking parties. And for those I have plenty of big-watt amps laying around pining for abuse.
And in case anyone misunderstood, I've seen PLENTY of ST-70 photos. Just real no units to plug in and listen to.
..Todd
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Yes, there are umpteen ST70 driver boards available, but the standout (IMO) is from Triode Electronics. TE's board retains the OEM topology, but implements said topology with available types. A TE board populated with 2X EF86s and an ECC99 is better than OEM. A strong case can be made for the EF86 being the very best small signal audio pentode, ever. The ECC99 is a low RP/high gm type well suited to "concertina" phase splitter duty. The high gm provides resistance against slew limiting, which is important in a circuit that employs GNFB.
.
The "replacement" ST70 power trafos currently available are not undersized and 35 WRMS is available, when sufficient energy storage is built in.
It's not a question of being "exotic". It's a matter of staying within the parameters contained in the published data sheets. "Sweep" types typically have fragile screen grids. So, regulated g2 B+ is MANDATORY. It is not (IMO) especially difficult to map the Mullard style circuit found in the Harman/Kardon Citation V onto "sweep" tubes as "finals". AAMOF, its highly likely your dad's caddy contains the 12BY7 and 6CG7 used in the 5's small signal circuitry.
I would recommend buying a used Dynaco Stereo 70 off ebay and upgrading it with a new driver board, individual bias adjustment for each output tube and a hefty power supply board.
Enclosed hopefully are three pictures showing a HOME MADE board as well as several other popular boards.
Enclosed hopefully are three pictures showing a HOME MADE board as well as several other popular boards.
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Here is another good sounding board.
To be perfectly honest there is a Dynaco Stereo 70 thread that I started a few years back in which I knocked the 70 quite badly. After receiving several emails from experts here on this forum I took their suggestions and put forth some effort to see how good I could get the 50 yr old amplifier to sound. I have 1/2 dozen of them to date which varous modifications and driver board combinations. The last one that I purchased will soon give up its rusted/pitted chassis in favor of two custom chassis that will match my Aikido linestage.
To be perfectly honest there is a Dynaco Stereo 70 thread that I started a few years back in which I knocked the 70 quite badly. After receiving several emails from experts here on this forum I took their suggestions and put forth some effort to see how good I could get the 50 yr old amplifier to sound. I have 1/2 dozen of them to date which varous modifications and driver board combinations. The last one that I purchased will soon give up its rusted/pitted chassis in favor of two custom chassis that will match my Aikido linestage.
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hey-Hey!!!,
If you're going to go for other-that-A470-outputs, consider raising the load when you spec them. I've installed clones of the Peerless 16446 outputs in my Dyna Mk.iii's at 6k6 a-a loading and am very pleased with their performance.
The front end leaves you a lot of room to move around in. Mine is a pair of 6AC7 pentodes arranged as a LTP. Two-stage amp with local FB; if you'd like a schematic jus' email me through the forum. The Hammond 1650R at 5k a-a is another option; very nice performance from that particular model.
I'd further suggest going Mk.IV-style and building monoblocks. They'll be lighter and easier to carry. Build their PS like it should serve a stereo amp...
cheers,
Douglas
If you're going to go for other-that-A470-outputs, consider raising the load when you spec them. I've installed clones of the Peerless 16446 outputs in my Dyna Mk.iii's at 6k6 a-a loading and am very pleased with their performance.
The front end leaves you a lot of room to move around in. Mine is a pair of 6AC7 pentodes arranged as a LTP. Two-stage amp with local FB; if you'd like a schematic jus' email me through the forum. The Hammond 1650R at 5k a-a is another option; very nice performance from that particular model.
I'd further suggest going Mk.IV-style and building monoblocks. They'll be lighter and easier to carry. Build their PS like it should serve a stereo amp...
cheers,
Douglas
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