The best part of this video is showing audiofools the amp’s dismal output capability. The euphonic nature described is actually the amp distorting trying to handle impedance changes of the loudspeaker.

👍👍

I like your grounding approach. The red and white wires - I’d twist each pair together;)

Just picked up an A12. Rewatching the series as I’m planning on doing the full mod. I was surprised how good the amp sounded out of the box - suppose I was expecting it to sound worse given what I remembered from your videos. Having said that, it was a bit grainy and shrill for sure. Started off putting some Psvane EL34PH Phillips replicas, a pair of 12AD7’s and a 5V4. Helped, but still had the issues listed. Just popped some 6V6’s in it, and that certainty helped. Not nearly as fatiguing. As I’m currently listening to Cornwalls in a smallish room, bass is not an issue. I’m looking forward to ordering parts and doing the mods.

I do believe after listening to a few of your videos today that you’ve saved me some heartache! I’m new to even the idea of tube amps so listening to the myriad of designs and constructions it’s clear I should continue to learn before taking a plunge. I like myself some music and I’m excited to watch more of your content. You are a KZbinr in the truest sense- I appreciate your perspective, candor, clarity, snd time!

I see a SM cap near the input pot. Psionic Audio has flagged the ones that say "SM" on them as being prone to eventual DC leakage.

Could you please advice if Hammond 1628SEA is good loudspeaker transformer for Single Ended EL 84 tube amp? Is there anything better on the market in similar price? And what about amorphousaudio transformers? Is this good direction? Thank you.

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.

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).

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

Great results.

Eric A at Tekton is a jerk. I've lost lots of respect for the YT reviewers after learning they never publish negative reviews (i.e., Steve G, Andrew R).

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..

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

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

Looks like an old 50w M&K from the 90's.

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.

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).

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.

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.

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!

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!!

Yeah, the reality is that most people who can afford good stereo gear can't HEAR good stereo gear! ;)

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

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

I’m an old guy. 68. Haven’t had a good system since my 20’s and I wanted to replicate the sound of a huge speaker box store Technics system I had back then. A year ago, I ended up pairing the original 600 M’s with a cambridge AXR85 receiver and a Klipsch sub. I could not believe the amazing sound quality. Recently added a Wiim Ultra streamer and now I’m just completely blown away. I’ve been so impressed with the 600M’s that I’ve spent the last year trying to talk myself out of them. Thanks for the review!

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

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

HI !! Hello, please advise what kind of wire do you use to route components inside the amplifier?

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.

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.

Thanks for sharing and enjoy that amplifier! :-)

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..

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.

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.

Thank you for your review. Nice Reel to Reel machine that is some nice analog equipment ❤ Sounds like you have had audio as your work/hobby for some time !

I'm with you regarding the musical sound of the 12AU7. I've used it as the driver tube in two amplifier designs plus I have an ST70 with the 12AU7 VTA board which sounds great. The key, as you say is to run it at the right current. Speaking of operating current, info with the VTA board suggests operating the EL34s at 40mA vs the 50mA run in the stock ST70. I think that this recommendation is to preserve both the power transformer and the rectifier tube given the higher B+ current draw of the VTA board and higher present day line voltage. In both the ST70 and other P-P EL34 amps that I own, my experience is that 50mA is the sweet spot soundwise for the EL34s. To deal with this in my ST70, I'm running a reissue, 120V rated input, larger stack replacement transformer plus a second rectifier tube.

Just wanted to comment on what I recently did with the bias pot resistors. I replaced my pots using your older video which used a 5K resistor before the pot, a Vishay 10K pot, and an 18K resistor after the pot. When I tried the EL34 and KT77, I couldn't get enough current through the pot with it wide open. What works was to replace the 18K resistors with 13K one's. Now I have plenty of room to spare on the pot with KT88, KT77, 6550, and EL34. Can bias all of them no problem. When using the EL34 or KT77, 20 on the VU will give about 30mA. The KT88 and 6550 is where 40 is 40mA. So, I am satisfied with the resistor network now. Also, the 22mm Vishay pots are now in stock at mouser so ordered those to replace my 16's. Thanks so much for figuring this all out for this caveman. lol

Hi Stephe, I’m just curious why you chose the vta board vs the original dynaco board?

I just discovered the Earl Klugh Trio. I didn't know!

Best tube channel on KZbin

12au7 is a great audio tube

On a side note, I recently bought an octet of KT77. Of the eight, 7 were good. One tube had an internal short across pins 5 and 4 that I didn't catch before placing in my X7. I powered up and noticed smoke coming out of V3 vent. Luckily I killed power immediately. Damage was only to the grid resistor between 4 and 6 (took the paint off but casing was still intact). No other resistors were affected. I ordered upgraded Vishay resistors for all power tubes and the amp is fine, in fact great. From now on I will be checking each new tube I receive for shorts using DMM. Really easy to do and will prevent a big headache. The EL34 and KT77 are great tubes for certain.

To restore this thing back to stock would have been absurd. With the know how to make huge improvements, no other course should be taken. You improved the R8 tremendously and now this ST70. I really hope people appreciate what you do because the info is invaluable. Great work!!!

Doesn’t connecting in parallel your speakers and the sub affect the total impedance towards the amp?

Nice work, i’m not sure why, but I’ve always found a really good preamp gave more bass drive a deeper Sound stage. A crappy preamp will have Opposite effect😊.

Last month I sent a link and a note to the British audio file, I may not be the only one, but he did mention you in the video that I watched this morning. I hope this helps grow your channel. I really like what you’re doing.

Glad you're liking the ST70. Interesting to track your journey with your plans for it. I also agree it's worthy of its reputation. It's been the little amplifier that so many people have enjoyed in so many ways. I still have my father's ST70, and his PAS2 and FM2 with the mulitplex integrator (effectively an FM3). They Dynaco tubed amps and some of the transistors amplifiers were very good. Some of the speakers were also notable. Preamps not quite up the same reputation. The FM3 is an excellent sounding tuner.

Great work on a classic! Question: since the power tranny was meant for lower a/c voltahe (117v) and it's now runningva bit high, 'could' choosing another rectifier, or going for 'choke input' power supply instead of attacking a capacitor as the first component bring the high voltage to a more desirable, lower value? Just asking myself..

12AU7 tubes are indeed very good sounding. Audio Note uses them in their gear including the DAC that I have. Any tube can be made to sound poor in a badly designed circuit.