Results 1 to 12 of 12

Thread: Amp bargain

  1. #1
    Forum Regular
    Join Date
    Sep 2006
    Posts
    506

    Amp bargain

    By a rather coincidental set of circumstances yesterday, I ended up with a Dynaco ST-70 tube amp for $80. It was a local deal so didn't even waste money on shipping. The fellow's house wasn't more than 5 minutes out of my way from a trip I already had to make that morning.

    The guy said it was "As-Is" because it was blowing fuses. It turned out to be a pair of leaky caps in the power supply (about $4) and a shorted tube.

    Of course I can't leave well enough alone. The thing was pretty much original except for a replacement to the "can cap" so I've decided to completely overhaul it. I've already got it completely disassembled and the chrome polished. Need to repaint the transformers but the big change is going to be a new driver board and power supply overhaul. That and the new tubes will run a bit over $200 but I'll end up with a great amp. (The chrome base on this thing is in as good of shape as I've seen on a Dynaco that hasn't been rechromed.)

    I've still got some decisions to make. Can't decide if I want to try a conversion to 6L6 outputs or stick with the EL34s. Same thing with whether to stick with the original configuration or go with the lower power triode configuration. The latter choice can wait but I need to decide on the tubes before I order parts tomorrow or Monday.

    When everything is done I'll post a picture of the results.

  2. #2
    Sure, sure... Auricauricle's Avatar
    Join Date
    Jun 2008
    Location
    Yonder
    Posts
    2,886
    Congartulations, misstl! That is truly a great find, and more to you for figuring out the problem!

    Send before and afters when you do the photos.

    You've done a man's work, sir!

  3. #3
    Forum Regular
    Join Date
    Sep 2006
    Posts
    506
    It helps to have test equipment sitting in the basement. About 10 years ago I went through a phase and picked up a Tektronix scope, quite a bit of Heathkit test equipment such as capacitor checkers, distortion analyzers and frequency generators. and a Superior TV11 tube tester. Amazing what you can buy in this area for a few hundred dollars.

    It didn't take much more than 15 minutes to find the problem. The slow part is doing the complete disassemble so I can rebuild it from scratch. But, I've always like that kind of tinkering.

  4. #4
    Forum Regular
    Join Date
    Sep 2006
    Posts
    506
    Just finished the mods on the amp and thought I'd share a picture. This is a complete gut with serious modifications. The output tubes are now 6L6GCs and the driver circuit has been changed to a Triode Electronics board with the 6AU6/12AU7 tube combo. I reverted back to a GZ34 rectifier instead of the diodes that had been installed, but kept the Curcio capacitor board. The transformers were stripped and painted. Sadly the power transformer had been clearly and severely stressed at some point in the past so it had to be replaced.

    Things are looking good on the initial tests (left channel is under 0.3% IM distortion but I've still got to work out some crossover notch distortion on the right) but hope to have its listening trial run here in the next day or two.

    All in all, a fun project.
    Last edited by mlsstl; 07-22-2009 at 07:16 PM.

  5. #5
    Shostakovich fan Feanor's Avatar
    Join Date
    Jun 2002
    Location
    London, Ontario
    Posts
    8,127
    Quote Originally Posted by mlsstl
    ....

    All in all, a fun project.
    Great work, misstl.

    I envy you your electronics savy. I have built some kits in the past and might in the future, but I'd have no clue about diagnosing or remedying problems, much less designing improvements.

  6. #6
    Music Junkie E-Stat's Avatar
    Join Date
    Dec 2003
    Posts
    5,462
    Quote Originally Posted by mlsstl
    Just finished the mods on the amp and thought I'd share a picture.
    Wow, cool pic. Unit looks brand new. That's still a great amp when used with the right speaker.

    rw

  7. #7
    Sure, sure... Auricauricle's Avatar
    Join Date
    Jun 2008
    Location
    Yonder
    Posts
    2,886
    Good job, misstl! Boy, I didn't know I was this green! Very nice, indeed!
    "The great tragedy of science--the slaying of a beautiful hypothesis by an ugly fact."--T. Huxley

  8. #8
    Forum Regular
    Join Date
    Sep 2006
    Posts
    506
    First listening report.

    I finished testing this evening. It came out at 45 watts a channel at under 0.2% IM & THD, either channel. The increase in power from 35 watts/channel was probably half due to the change to 6L6GCs and the rest from the new transformer. (The original power transformer was one of the weak points of the ST-70.)

    I put it in place in my =very= small computer office/studio (8' X 7.5') just off the 2nd floor landing. I used the analog outs of a Squeezebox Duet, my Spendor SP3/1Ps and listened to a couple of tracks.

    This thing has a great, gutsy sound. I was in an R&B mood so the first cut was Taj Mahal's "River of Love". What a great set-up for this type of music! It is easy to see why a lot of guitar players like the 6L6 tubes. The back-up bass vocals on the cut were just perfect. It wasn't as refined as the bigger Spendor SP1/2Es with the Lavry DAC and the bigger Image 65i tube amp with 6550 outputs, but that's a whole different setup.

    I did uncover a couple of issues. I've got a very slight residual buzz. I'll need to try experimenting with the input wiring. Also, the sensitivity is much too high for use with the straight output of the Squeezebox. I had to run the volume on the Duet far too low for my tastes. As such, I'll probably knock the gain down 6 dB or ever a bit more. That's one really neat aspect of a project like this - you can fine tune things as you see fit.

  9. #9
    Forum Regular
    Join Date
    Sep 2006
    Posts
    506
    After much additional experimentation the final version of my modded ST-70 is now playing as I type this post ("Vivaldi's Cello" with Yo-Yo Ma).

    I tried running the outputs in triode mode (wasn't worth the loss of power) but did end up making one change. I dropped the 6AU6 drivers and went to EF86 tubes. Much better.

    I've set this up as a second system in a spare bedroom that we've converted to a sitting/reading room. I moved the Spendor 3/1Ps into this room (and put my LS3/5a clones back on the computer system). I still need to get some speaker stands. This amp is a great combo with these speakers but the room is currently a bit too live. However, nothing really more to do now until the wife decides on the final layout.

    The source is a Slimdevices Duet receiver that I hadn't been using (I bought it for the remote.) This is the first time its been plugged in and have to say I'm fairly impressed with the output quality. Emminently listenable for this type of system.

    All in all, a great find, a fun project and a great second system.

  10. #10
    Shostakovich fan Feanor's Avatar
    Join Date
    Jun 2002
    Location
    London, Ontario
    Posts
    8,127
    Quote Originally Posted by mlsstl
    After much additional experimentation the final version of my modded ST-70 is now playing as I type this post ("Vivaldi's Cello" with Yo-Yo Ma).

    I tried running the outputs in triode mode (wasn't worth the loss of power) but did end up making one change. I dropped the 6AU6 drivers and went to EF86 tubes. Much better.
    ...

    All in all, a great find, a fun project and a great second system.
    Once again, nice going, misstl. In my limited experience tube rolling can pay off nicely.

    I'd really like to try restoring an ST70 or Mark III someday. I'll be looking for advice when the time comes.
    Last edited by Feanor; 08-16-2009 at 04:03 PM.

  11. #11
    Forum Regular
    Join Date
    Sep 2006
    Posts
    506
    The ST-70 is a solid design that is easily made even better with some modern parts.

    However, the Mk III is the basis for a very fine system IMO. Upgraded with a more modern driver board and power supply caps it is the buy of the century from a sound quality standpoint. Lots of power, good bass, finesse, and a great retro look.

    I used a pair of those as my main amps for several years and still somewhat regret having sold them. I went through three amps after selling the Dynacos only to settle back with a power amp with 6550 outputs. I really like the Image 65i (looks classier and better parts) but it really doesn't sound any better than the old Mk IIIs, which was pretty dang fine.

    Sometimes we take the long route to get back to where we were... ;-)

  12. #12
    RGA
    RGA is offline
    Forum Regular
    Join Date
    Nov 2003
    Posts
    5,539
    The only addition to what i can say is that make sure that you buy foil caps not plastic ones.

    Here is one of the top Solid State amplifier designers and electronics engineers and what he had to say about the ST-70 (IF YOU upgrade the caps). And convert it to triode.

    "A little anecdote might illustrate what was happening on a larger scale. At the time I glanced at the first promotional issue of Glass Audio, I was also working on an advanced 200 watt MOSFET amplifier with two friends from Tektronix. This amp represented the pinnacle of the high-end art: fully differential, all-cascode, all-Class-A, zero-TIM, 200V/microsecond, fully regulated, and 120 watts/channel. The same month, I went to the second Oregon Triode Society meeting, and one of the members brought a rusty old Dynaco Stereo 70 that first saw the light of day when Dwight D. Eisenhower was President. The sum total of his "tweaks" was to convert the EL34's to triode (cut and tape two wires), and replace two coupling caps. About 2 hours with a soldering iron. We're not talking aerospace engineering here.

    The OTS guy turned it on, and we compared the Stereo 70 to everything in the dealer's showroom. It was plainly superior not only to any transistor amp in there, it wiped out the latest $3000 Audio Research all-tube confection that had received a glowing review in the latest Stereophile. Say hello to humble, and good-bye to price, power, and prestige. (That dealer did not invite the OTS back - gee, wonder why? Buncha troublemakers if you ask me.)

    If you stay in audio long enough, that kind of experience can make you do some deep thinking about cherished assumptions. I set aside the transistor project, stopped laughing at the "tube nuts" and subscribed to Glass Audio (Vol. 1, Issue 0). Two years later, I reviewed the Herb Reichert Silver 300B and the Audio Note Ongaku for Positive Feedback (on the newly-designed Ariels).

    As the speaker designer, I felt I knew my speakers inside and out. Or so I thought. The Ariels were transformed from a pleasant speaker to near-electrostatic realism and "you-are-there" quality. All from changing an amplifier! David Robinson later called this my "Road to Damascus" experience. That ended any idea that amps were pretty much all the same, or if they weren't, mainstream high-end gear was pretty close to perfect. I was surprised to discover that speakers were better than I thought, and that amps had a long long way to go.

    As articles from Japanese and French magazines appeared in Sound Practices, Glass Audio, and Vacuum Tube Valley, the notion of craft, or "artisan" audio began to emerge. People started to design and build exotic amplifiers that cost thousands of dollars in parts alone, and then told others about their experiences in new magazines, the Internet, and regional vacuum-tube audio fairs. The subversive notion grew that quality was something you built, not something you bought.

    As people built more, it became apparent that mainstream prices were grossly inflated relative to parts cost ... the transistors in a $15,000 high-end amp cost less than a hundred dollars, while fancy metalwork, 4-color ads, and a dealer network add zero to the sound. On the other hand, hand-made power triodes, point-to-point chassis wiring, and custom-wound transformers are obviously expensive and labor-intensive ... and the difference from mainstream high-end to triode are obvious to the most casual listener. Oddly enough, many audiophiles are relatively deaf to the experience, but non-audiophiles can hear it right away.

    The full article can be found here http://www.nutshellhifi.com/library/tinyhistory1.html

    At $80 with a couple of hundred in upgrades betters top Audio Research gear and any Solid state amplifier - That would not be terribly surprising to me as was indicated by the folks who listened in that article.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •