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1964 Dynaco ST-70 Stereo Tube Amp Rebuild

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Forum topic by knotscott posted 01-23-2020 12:29 PM 432 views 0 times favorited 17 replies Add to Favorites Watch
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knotscott

8365 posts in 4012 days


01-23-2020 12:29 PM

Hi gang – There was zero woodworking on this project, but nonetheless am happy enough about doing it that I wanted to post it here.

This is a Dynaco ST-70 stereo tube amp built from a kit (Dynakit) by my uncle in 1964. He used it for many years, and gave it to me sometime in the late 1980’s. Shortly after, I ran into a 1961 factory wired version of the same amp, and ran both amps as mono blocks in my stereo setup for around 25 years. Wonderful sounding amps. A couple of years ago the amps started acting up, as any 50+ year old electronics do, so I pulled them from service to give them an overhaul.

My uncle’s 1964 amp is now finished. I stripped it down quite a bit, gave it a good cleaning, bought and soldered up a new and upgraded circuit board for the driver stage from VTA electronics, replaced all 8 vacuum tubes, installed a new quad capacitor, gave it a new power cord, new inputs, new outputs, added some toggle switches that allow me to switch between pentode and triode mode, and added an “on” light indicator to help my wife remember to turn it off! ;-) I got it wired up, biased the output tubes, and watched in glow!

The amp uses a GZ34 rectifier tube, (3) 12AU7 driver tubes, and (4) EL34 output tubes. It puts out 35 watts per channel in pentode mode, or about 20 watts per channel in triode mode. I’ve got new 12AX7 tubes ordered for my 1990s Lazarus Cascade Basic preamp, and will continue to use a nice pair of custom speakers I built in the mid 90s. For now, I’m going to stick with one stereo amp, and use it in triode mode that will put out a beautiful sounding 20 watts per channel. Hope to get another 25 years of service from the old Dyna!









-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....


17 replies so far

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garlandkr

58 posts in 2141 days


#1 posted 01-23-2020 12:39 PM

Very cool, thanks for sharing!

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MrUnix

7699 posts in 2836 days


#2 posted 01-23-2020 12:42 PM

Great job… but where on earth do you find vacuum tubes these days?!?

I remember the time when you could run down to the local Radio Shack and use one of those fancy tube tester machines! One of lifes little joys that the kids today will never get to experience :-O

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

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knotscott

8365 posts in 4012 days


#3 posted 01-23-2020 01:07 PM


Great job… but where on earth do you find vacuum tubes these days?!?

I remember the time when you could run down to the local Radio Shack and use one of those fancy tube tester machines! One of lifes little joys that the kids today will never get to experience :-O

Cheers,
Brad

- MrUnix

They’re still around! Millions were made in the USA and England, and are still available as NOS….expensive, but available. The Russians and Chinese are still making new tubes, as they’re still quite popular in guitar amps, and there’s still a niche market for audio buffs that’s growing in Asia.

It’s funny you mentioned tube testers. My dad tinkered on old tube televisions when I was young, and from the time I was old enough to read numbers and letters, he let me use his tester and sort through boxes of tubes. Probably one of the reasons I gravitated to tubes as an adult.

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

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MrRon

5829 posts in 3880 days


#4 posted 01-23-2020 06:17 PM

I built a Dynaco ST70 back in 1963 along with the (PAS 3?) pre-amp. I remember the kit cost less than $100 and is highly sought after on the used hi fi market. That model amp was tuned up by a famous electronics leader in the hi fi sound field (can’t recall the name) and made to sound as good as a high end amp selling for 10x more. He used a hand full of parts costing just a few dollars. These amps if you can find one are now selling for $800 and more. I wish I had kept mine.

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Kaleb the Swede

1918 posts in 2606 days


#5 posted 01-23-2020 06:36 PM

Now that is a nice piece of equipment! Us guitar players love tubes, if you know whats what about them

-- Just trying to build something beautiful

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knotscott

8365 posts in 4012 days


#6 posted 01-23-2020 07:22 PM


I built a Dynaco ST70 back in 1963 along with the (PAS 3?) pre-amp. I remember the kit cost less than $100 and is highly sought after on the used hi fi market. That model amp was tuned up by a famous electronics leader in the hi fi sound field (can t recall the name) and made to sound as good as a high end amp selling for 10x more. He used a hand full of parts costing just a few dollars. These amps if you can find one are now selling for $800 and more. I wish I had kept mine.

- MrRon

I think David Hafler was the guy who designed the amp, then went on to form Hafler amplifiers. The chassis or the cover alone is now worth nearly what the amp cost new!

So do you still have your Dyna Ron?

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

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knotscott

8365 posts in 4012 days


#7 posted 01-23-2020 07:23 PM



Now that is a nice piece of equipment! Us guitar players love tubes, if you know whats what about them

- Kaleb the Swede

My boys each have a tube guitar amp that I built for them…they love ‘em.

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

View Joe Lyddon's profile

Joe Lyddon

10898 posts in 4689 days


#8 posted 01-23-2020 08:40 PM

Great Scott!

You are full of surprises!

You did a BEAUTIFUL job on that amp! Did you replace all of the Caps or just that one?

It’s a wonder how you found all of those great old tubes… sure brings back memories.

Sounds like you have a solid background in electronics, etc. Are you a Ham also? I am WB6UFX since mid 60’s.

I remember, as a kid, in about 1951-53, I built a Hi Fi amp. kit… Don’t remember the name… it was NOT a HeathKit… something else; popular at the time… I followed all of the directions soldering all of the parts, etc. and it worked great… I used it with a record phono top with a Fancy GE pickup / needle! Then one day, it decided not work… I looked under the chassis to see if I could find anything obvious… Well, I saw where some of the solder joints were just falling apart! I had no idea how something like that could happen… Later, I found out the difference between Rosin Core vs Acid Core solder… I didn’t know sheet from shinola… I had used Acid Core solder! So much for that project… I NEVER bought or used Acid Core EVER AGAIN! Will never forget it… I was using the amp to play good ole 33, 78, & 45 records and playing my trumpet along with them… We have come a LONG way… A few years later, Hi Fi STEREO made the scene… What a wonderful development… Loved it… still do!

In the mid 1960’s my wife and I put a large (25” or so) HeathKit color TV together… believe it or not, we still have it! It’s not working… It is being used as something to place the Larger Flat screen TV on. It worked, but adjusting colors, etc. was a real PITA… very flaky… It was fun… a great experience… I remember when there was a high voltage problem… I was in the back poking around looking for bad/loose connections, etc. and POW that big bully JUMPED A SPARK across to my hand! Knocked me backwards onto my butt… Never got that close ever again! Heathkit replaced the high voltage section to get it working again. More fun.

Now, I have discovered the Arduino micro-controller! Just an awesome system using all kinds of sensors to talk to the outside world! Lots of fun!

OK enough about the memories you help dig up for me…

Thank you for showing us such a great job you did in resurrecting that old beauty!

-- Have Fun! Joe Lyddon - Alta Loma, CA USA - Home: http://www.WoodworkStuff.net ... My Small Gallery: https://www.ncwoodworker.net/forums/index.php?media/albums/users/joe-lyddon.1389/

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knotscott

8365 posts in 4012 days


#9 posted 01-23-2020 08:57 PM

LOL Joe! I got zapped pretty good last week with that amp…it wasn’t a particular strong shock, but both hands were on the chassis when I got hit, so it went through my body. Didn’t feel quite right for a few days, but could see better in the dark thanks to the glow!

No electronics training to speak of, but I was heavily into the audio years ago, and started a high end speaker company in the late 80s. I have lots of experience soldering with resin core! One step at a time with the tubes and electronics got me to this stage..just been at it for a while, so you pick things up.

I did replace all the caps in the amp, along with all the resistors, and any other electrical components except for the transformers.

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

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sansoo22

625 posts in 291 days


#10 posted 01-23-2020 09:57 PM

Very awesome. Thanks for sharing. I’m not old enough to remember using these. I think Hi-Fi would have been the earliest stereo equipment i got into.

I’m getting pretty big into micro controllers these days. I’m in the planning process of a retro stereo console table I want to build as my entertainment stand. With some hidden goodies like a raspberry pi and nividia shield. Can’t decide if i want to build my own tube amplifier or not yet. Some of the of the shelf solutions come with bluetooth, phone, and aux IO ports built in.

Either way I doubt id be building any amp as beautiful as the one you posted.

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knotscott

8365 posts in 4012 days


#11 posted 01-24-2020 12:27 AM

Good luck whichever you decide!

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

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CaptainKlutz

2438 posts in 2131 days


#12 posted 01-24-2020 12:31 AM

Wow, blast from past!

Guess it’s time for some history?

Had a ST-70. Bought it used. Nice sound. Built several Heath kits as teenager.

Supported the college radio station transmitter and did Ham radio into my mid 20’s, but was too nerdy and RF just isn’t as cool as music. :)

My Sr. college project was 100W class D amp for automotive use based on a ‘Coffee Can’ amp I read about. It was stolen with my car 6 months after graduation. Today you can buy the same thing a single semiconductor package that I helped create.

College worked part time as speaker builder for band PA systems (folded horns, ported subs, etc).
Designed and built speaker boxes/electronics, and repaired all kinds of audio stuff for spare change; until I had to get real job after college. Never forget working on 19” racks each with Three Phase Linear 700 amps, wired into separate phases of 240v 3PH stage power, internally beefed up to output more power (claimed 1000W bridged?). Was always replacing blown transistors from previous weeks concert as the mix master got trashed and turned it up too high. Dread the smell of burnt capacitors when I detect these days? The PA rental company also had racks of McIntosh mono tube amps for bridge driving mid & highs that required lots of swapped tubes. The Japanese Marantz and other brands failed just as often. They kept several of us college sparky’s busy repairing stuff with a packed equipment warehouse and constant rental of semi-trailers full of equipment for major concerts.

Thought I was living a dream, working knee deep in music electronics industry at that time. The rental company offered me a 5-6 month gig as equipment manager for 3 band spring tour down/back the east coast to end up in Florida during the month long spring break my graduating year. Was soo tempting, but turned it down. Interviewed with Carver, Hafler, Peavey, Marshall, and others as I was graduating wanting to work in the audio field. Then I realized robotics industry paid bettter. :-)

Faded away from audio ‘hobby’ near turn of century, due family, and too many hobbies; not to mention digital audio was an insult to my ears. Have built a couple speaker boxes since then. Also have a modern 2000W home theater audio system to keep me warm at night, since damage from my early power audio days is starting to impact my hearing. :-)

They say ‘Life comes full circle’:
Got hired by guitar amp mfg ~20 years after college as consultant to help them improve mfg quality for electronics and cabinets. Had horrible issues with mfg defect rates, and could not compete with overseas mfg competition. Running a 10-12% scrap rate in world where 10ppm defects is norm.
Didn’t help that every Monday morning the mfg floor was filled with part time band members still hung over from weekend gig. They could barely stand or see straight sometimes due hair of dog remedies. Friday mornings were also bad due early weekend parties. Final quality control before packing was sound booth with musicians jamming, claiming they were looking for box buzzing or strange noises. But Coke cans in sound booth smelled like Jack Daniels and you realized the truth.
Was like I had stepped back in time with zero changes from college days.
Made me happy, I choose robotics as my start to earning a living.

Thanks for trip down memory lane!

-- I'm an engineer not a woodworker, but I can randomly find useful tools and furniture inside a pile of lumber!

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knotscott

8365 posts in 4012 days


#13 posted 01-24-2020 02:13 AM

Wow! That’s a heck of a journey! I remember the Phase Linear stuff was designed by Bob Carver, right? Before I was given my Dyna, I had Hafler DH500 that I built from a kit…both designed by David Hafler.

Cool story. Thanks for sharing.

-- Happiness is like wetting your pants...everyone can see it, but only you can feel the warmth....

View Joe Lyddon's profile

Joe Lyddon

10898 posts in 4689 days


#14 posted 01-24-2020 02:29 AM

YES! That was a super journey!
... and he’s still living it!

Great post!

Thank you!

-- Have Fun! Joe Lyddon - Alta Loma, CA USA - Home: http://www.WoodworkStuff.net ... My Small Gallery: https://www.ncwoodworker.net/forums/index.php?media/albums/users/joe-lyddon.1389/

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unclearthur

328 posts in 2424 days


#15 posted 01-24-2020 06:23 AM

Cool. Does it go to 11?

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