LumberJocks Woodworking Forum banner

1964 Dynaco ST-70 Stereo Tube Amp Rebuild- System Update

3K views 33 replies 12 participants last post by  knotscott 
#1 · (Edited)
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!

Circuit component Passive circuit component Hardware programmer Audio equipment Electronic component

Electronic instrument Audio equipment Gadget Gas Font

Rectangle Grille Audio equipment Gas Composite material

Circuit component Passive circuit component Hardware programmer Audio equipment Gas

Electricity Building Liquid Gas City

Musical instrument Gas Rectangle Musical instrument accessory Audio equipment

Gas Toy Space Audio equipment Engineering

Automotive lighting Motor vehicle Entertainment City Musical instrument

Circuit component Passive circuit component Hardware programmer Electronic instrument Electrical wiring
 

Attachments

See less See more
18
  • Love
Reactions: bandit571
#24 ·
Great job… but where on earth do you find vacuum tubes these days?!?
Unfortunately the Golden Age of getting them cheap has passed. And since 2020 all the Russian sourced ones have been embargoed. That leaves Chinese for mainstream ones and Czech Republic for some of the esoterics. Chinese make good ones but price is premium for those. Lower priced ones have lower priced results, sometimes good, sometimes early failures. So the only real alternative for "good" is to pony up to a reseller with a solid rep in case you have trouble with your purchase. Russian tubes were generally good across the board. Their market absence has definitely thrown the guitar world into a tailspin with China the main beneficiary.

The other route is get lucky at an estate or yard sale, hamfest, flea market, or auction and find NOS USA, Euro, or Japanese tubes. You can also salvage them from old non-audio gear like oscilloscopes. Definitely a crap shoot but once in awhile you get lucky. NOS US & Euro can still be had from tube vendors but the prices keep going up as the supply shrinks. Just another example of you don't know how good you had it until it's gone. None of which would stop me from getting into any vacuum tube gear today. When properly set up & maintained tubes should last decades. I've had many instances where original tubes in gear tested exactly as they did 50+ years ago.
 
#4 · (Edited by Moderator)
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.
 
  • Like
Reactions: bandit571
#5 ·
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.
 
#25 ·
These amps if you can find one are now selling for $800 and more. I wish I had kept mine.
Of note, all the parts to build an ST70 from scratch exist (although on backorder now). Tube amp magic is in the quality of the output transformers and the ST70 ones are great. Plenty of documentation on how they were wound and they've been made by various companies almost non-stop since Dynaco folded. By all accounts I've read new ones equal to originals. With a complete kit costing that of a working original, I'd likely go with the kit. Half the work already done in that you don't have to strip it down.
 
#7 · (Edited by Moderator)
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?
 
  • Like
Reactions: bandit571
#9 ·
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!
 
#10 · (Edited by Moderator)
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.
 
  • Like
Reactions: novice
#11 ·
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.
 
#13 ·
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!
 
#14 · (Edited by Moderator)
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.
 
#18 ·
Great rebuild and thanks for bringing it back to life. There's just something about tubes, ya know? The warmth, the glow…. I appreciate all that digital audio has brought to the table but I don't want something that sounds "just like" tubes. That's like saying something tastes "just like" chicken. :)
 
#19 · (Edited)
Yep, this is an old thread, but there has been enough changes to these amps and the whole system that I thought I'd give an update for those interested in this sort of thing. This will be a little long winded, but bear with me, I'll try to be concise (pictures to follow)

I've been loving these old Dynaco tube amps with the VTA circuit mods since upgrading them in 2020. They've evolved a bit since then, and is largely where I've been focusing my time and energy instead of woodworking. I've been experimenting with various brands and types of tubes to tailor the sound just how I like it (aka "tube rolling"), tweaking the system, and going after the low cost weak links. A year or so ago, I removed the rectifier tube from the amplifiers completely, and installed some solid state diodes to perform the same task...theoretically, this doesn't impact sound quality, but since these were considered budget amps when originally made, and were built to a price point, one of the weak links has always been the power supply...it's "adequate" but not great. Now the power supply doesn't have to supply power to a rectifier tube. There's also zero voltage sag from the rectifier tube now, and the solid state diodes do the job consistently without drawing much from the power supply.

Since I had two amplifiers at my disposal, I was using them in a "vertical" bi-amp configuration for a while. Vertical is where two identical amplifiers are used....one for each channel. In this case they were two stereo amplifiers, so I was using one channel of each amp to power the woofers, and the other channel of each amp to power the mids/tweeters. In February of 2023, I made the decision to add an old Kenwood integrated solid state amp to the system to drive just the 8" woofers of the speakers...vacuum tube amps can have incredible clarity through the mids and highs, but they tend to have a fairly low damping factor, which means they don't have as much control of the woofer movements, so bass tends to be more dynamic with a solid state amp. Thus the addition of the Kenwood integrated amp to do what it does best, while leaving tubes the do what they do best with the mids and highs. All this use of multiple amplifiers is possible because each driver of my speakers has it's own input jack. The addition of the Kenwood to the woofers is considered "horizontal" bi-amping, but I'm still using one channel of each tube amp up top, so the system is really sort of a hybrid of vertical and horizontal bi-amping. Some of the benefit of this comes by way of the change in the impedance load to the tube amplifiers from a 4 ohm load, to an easier to drive 8 ohm load, and it made them sound notably clearer.

The next significant change came when I installed an "active low pass" crossover for the woofers, and bypassed what had been their stock "passive low pass" (essentially an inductor). An active crossover gets installed in the low level signal path before it gets amplified, and has some advantages. There's also lots of flexibility to fine tune the bass to suit the room. There's undeniably tighter bass with more slam and control with the SS amp and active crossover to the woofers, and boosting the bass no longer muddies up the vocal range. Sweet IMHO!

There have been a few other minor tweaks and changes here and there, but I won't get into them unless someone has a specific question. Suffice to say I've been extremely pleased with how the system sounds these days. It's not state of the art by any means, but it ain't bad! I've waited a long time to say this with confidence, but it sounds spectacular IMO every time I fire it up. Loving it. I initially put together the layout diagrams below so my boys will know how this thing is configured once I'm long gone, but they also help illustrate the whole system configuration better than all these paragraphs do. Thanks for reading this far!
-Scott

Photograph Product Black Font Electronic device

Rectangle Slope Font Parallel Diagram

Table Wood Wood stain Hearth Hardwood
 

Attachments

#30 ·
I believe your configuration with (2) identical amps was set up as a horizontal biamp. Horizontal is usually done with dissimilar amps, solid-state amp driving woofers, tube amp driving top end. A vertical biamp is when one amp drives one entire speaker, one channel on bass and other on top end. This is an advantage as top end channel not working very hard, bass channel is though. If they share the same power supply (like most stereo amps) there is more dynamic power available to the bass channel. Whereas when used that way in a horizontal biamp the bass amp is basically running flat out the while the top end amp is loafing. But set up this way you could have an 8WPC tube amp running the top end as it doesn't need much power and a monster solid-state amp running the woofers. This whole setup only works effectively if you are also using an active crossover sending bass info to 2 channels and top end info to the other 2 channels. It also does not apply to monoblock amps with their unshared power supply. Or, I misread you and in that case never mind.....

novice (also audio nerd)
 
#21 · (Edited)
Nice job on the rebuild. I have an ST70 awaiting same. Looked at the VTA boards so I can get rid of the 7199 front-end tubes. I've got NOS ones but am saving those for another amp (Scott LK-150). May convert ST70 to 6U8s, very linear oscilloscope tube from back in the day. Requires cutting traces and moving connections but pretty basic. Have some Mullard copy (National Panasonic) EL34s to drop in, sitting on them forever. Seeing your work is motivating.
 
#22 ·
Do it! Can't say enough good things about the VTA mods. It was definitely next level, and responds wonderfully to other tweaks and tube changes. Support from Tubes4hifi.com was excellent too (dynacotubeaudio.forumotion.com). Good luck, and please post when you finish it!

p.s.: What speakers do you use with the Dyna amp?
 
Top