Dynaco ST70 Tube Amp Restoration: Final Objective Testing!!

[@jerrybromenshenk1971]

79 yrs now. Caught the audiophile bug in 1960s after hearing dual MacIntosh tube amps connected to top line Bozaks, crazy bass and lower mid-range w 4 12” woofers, 2 mid horns and 8 tweeters in each speaker, pair of Bose 901 stacked on the Boz to boost mid and high hadrange, equalizers to sort it all out. I could not afford, so went to Dynaco kit, built 3 way acoustic suspension speakers 12” bass, large horn mid-rage in large cabinets, Akai cross-field reel-to-reel tape, and an Empire turntable. The Empire was a steal - rich college kid sold for Penny on the Penny - didn’t know what he had. It wasn’t a cool brand. It’s still running with an updated carbon fiber tone arm. 10 pound, aluminum platter, belt drive, floating on a cushion of oil. There were some surprisingly good gear in 60s, early 70s, and a lot of junk such as Gerard, idler wheel, spring loaded tone arm. My Empire and a pair of early 70s AR speakers survived, The rest wore out. Tubes became almost extinct until the Berlin Wall came down. Great to hear that a renovated Dynaco is still competitive today w modern equivalents. Thanks for confirming what some of old timers knew.

[@samb3706]

Thanks Steph, for a great review of this amp. You have definitely proven, again, that the quality of the transformers is the key ingredient of a tube amp's performance.

[@stephenkeever6029]

Wow those graphs are incredibly impressive for a 65 year old amplifier/design! David Hafler really knew how to properly design circuits and build good output transformers. So glad you did this series.

[@SkunkieDesignsElectronics]

This is actually someone else's "design" as the driver board is completely different.

[@Richard-Isley-Bigsley]

When junk and the Skunk collide we all know what happens ............. brilliant! Bravo!

[@tubes4hifi]

Great job on the rebuild of the ST70, glad to hear you are pleased with the results!

[@SkunkieDesignsElectronics]

This is a great board!!

[@ChrisHornbeck]

Great, great stuff as always. You'll get a bazillion opinions about ST-70s because there are almost half a million out in the world and everybody has an opinion. Gotta say, from my POV, your take is more solid than I've seen anywhere else. All good fortune.

[@SkunkieDesignsElectronics]

Yes, everyone has an opinion. I've gotten dozens of emails already about how I am wrong this and should be doing XYZ lol

[@sajyho1988]

Impressive results! 4:30 There's a very little crossover distortion there, but not significant. 5:50 those F response bumps might be due to resonances, low F bumps around 20 Hz are mainly due to undamped resonances caused by PSU LC filters, and Hi F bumps might be caused by output transformer resonances. Some people think they are Hi F or Lo F boosts, but I don't think so!

[@GiguereSylvain]

Thumbs up!! I remember my St70 of yesteryears as a great sounding little amp, but that was 'falling apart' when asked to drive my then inefficient speakers.. I knew little then, and should have replaced the speakers, not the amp! I know better now and am happy with the 15w class A of my Quads II and Tannoys.. I won't do the same mistake again. Efficiency is the first condition for musical fun with tube amps..

[@SkunkieDesignsElectronics]

Absolutely!

[@NebukedNezzer]

Great results.

[@RobertKohut]

Nice to discover what has been known for decades.... LOL There is a reason this amp sold so many and lived for 50 years....🙂

[@timessquarerecordscom1469]

Some great stuff made in that error Before solid state got very popular

[@gregj.gotham4402]

Sure looks good from here. Congratulations Stephe. How did it sound in your system. I’ve heard it takes 10-20 hrs of burn in before amps really sounds great. 50 watts is a hell of a lot of power.

[@jackstanley3529]

As to gaining a little warmth to the ST70's sound, I have found that the resulting elevated tube heater and B+ voltages from today's 122+ line voltages adds some stress / harshness to the ST70 sound (could it be due to excessive tube heater voltage - I don't know). The clear top 12AU7s as you mention and I agree can be a bit harsh as well (I have no experience with the JJ EL34s so can't comment on them). Also, operating the output tube current at 50mA as per the stock ST70 also adds warmth to the sound. You mention that you are running the O/P tubes pretty hot (at 85% dissipation) at 42ma per tube. Per Dynaco ST70 specs, plate voltage should be 410 @ 50ma per EL34 which is 20.5 watts / 82% dissipation - of course that is at 117V line voltage. So if you can get that line voltage down you can enjoy the sweetness of the 50mA tube current (as well as destressing the tube filaments).

[@SkunkieDesignsElectronics]

See I don't agree lower voltage and high ma is "better" sound. My experience has been the opposite and even then it's more about % of dissipation than one voltage/ma combo vs another to end up at the same %

[@NebukedNezzer]

I have a beautiful pair of acrosound ultralinear II amps. I put KT77s in them and get about 60 watts rms without pushing the original EL34s to hard. I do not have an EL34 amp but would grab on to one of these st70s if It comes buy(not likely as they are not easy to come by).

[@MojoMedicineMan]

👍👍

[@sopasadena1499]

Wow. Thanks for tackling this amp. The st-70 seems to live up to its reputation. Am I correct in believing this is one of the best tube amps ever made? ASR tested a new production st70 (don’t know if it was from this supplier) and liked the original better. Would be interested to check out.

[@SkunkieDesignsElectronics]

IDK about "Best ever made" and this driver board is very different from the original board, but I'm impressed with this performance.

[@rickg8015]

Hi Stephe.. It’s more of a subjective performance aspect in my case that I prefer the bigger power trafo.. Sounds less strained with more demanding types of music IME.. FWIW, I have 30uF as an input PSU cap for my ST70.. Anyway, thanks for this, It’s great to see favorable test results.. I would try 5814A’s for the LTP’s if it’s possible..

[@SkunkieDesignsElectronics]

I'm sure later I will be trying other stuff with this amp, but like I said earlier, I don't plan to make a career out of this amp. There are a ZILLION opinions on how to make "The best ST70", and I know what I did here isn't very far down that rabbit hole. It's just a video series on taking a dead ST70 and bringing it back to life.

[@318ishonk]

Thanks for sharing and enjoy that amplifier! :-)

[@philsanalogbasement]

Maybe I missed it but was that 50w number with both channels driven or just one?

[@SkunkieDesignsElectronics]

Just one. I'm pretty sure it won't do both channels this well, my second probe is broken, a test for another day. I'm only using less that 10W so personally don't care :P

[@philsanalogbasement]

haha all good thanks. I was just curious because I always test with both channels driven and sometimes have to explain to people why their amp doesn’t make as much as other people’s, when they see vids or posts on forums etc. I would agree that it won’t, limited by the plate voltage falling off a cliff faster

[@georgeswanson9483]

It'd be cool to see how it performs wired in triode.

[@SkunkieDesignsElectronics]

I'm not a huge fan of beam power tubes triode strapped.

[@asx1248]

Hi Stephe, I’ve been eagerly waiting this one! I feel like part of the magic of valve amps are those 2nd order harmonics. Is there any ‘tuning’ or tweaks you can make to turn down the 3rd? Thanks.

[@SkunkieDesignsElectronics]

It's just part of being a class AB amp.

[@asx1248]

@ Hi, What about when it’s operating in class A, can anything be done to reduce the 3rd harmonic there? Thanks again.

[@SkunkieDesignsElectronics]

I think it's still related to being a push pull rather than a pure single ended amp. Possibly some adjustments could impact this? I'm done with this amp for right now.

[@asx1248]

@@SkunkieDesignsElectronics well I enjoyed following its journey with you. Thanks for sharing and all the other great content.

[@DaveEdwardsDesigns]

Looking at your thd vs power it looks like the stock circuit performs better until you get to up there in the power.

[@SkunkieDesignsElectronics]

Anything under 1% THD is splitting hairs IMHO and won't be audible in a tube amp.

[@DaveEdwardsDesigns]

@ agreed from a subjective approach. Objectively speaking.1% is better than 1%

[@1mctous]

50 watts below 2% THD, and full bass response below 20 Hz! I wouldn't exceed 10 W, so I'd keep it in class A. BTW the kits cost $1250 with a full set of tubes.

[@RobertKohut]

Throwing this amp in with the China junk is blasphemy!!! LOL Keep in mind the new production ST-70s do not have the original output transformers. Also I believe the new release ones have a larger power transformer, I think! Still will sound great but they are not the original ST-70s. 🙂 I had a VTA ST-70 and it was amazing!!