Selecting a new ST-70 amp kit

As you've guessed, it's mostly the iron. Circuits and chassis are pretty much identical otherwise with some minor tweaks here and there to handle the higher voltages.

From Roy's website:

- The power transformer instead of 360-0-360 secondaries like the ST-70 has 420-0-420 secondaries and puts about 485 volts on the plates (pin 3) of the output tubes. The ST-70 puts about 420 volts on the plates. The stack lamination on this transformer is about 2.70 inches tall.

- My ST-70's power transformer stack is about 2.05 inches high. This transformer weighs about 4 pounds more than the ST-70 transformer and is rated at 425 milliamps of current flow.

- The output transformers have 4200 ohm primaries, 40% ultralinear screen taps and are 3 7/8" tall, 3 1/4" wide and 3 1/8" deep (about 1/2" taller, 3/8" wider and 1/4" deeper than a stock A-470 output transformer). Each output transformer weighs about 2 pounds more than a stock A-470 output transformer.

- The power storage caps have a higher capacity and voltage rating than the VTA ST-70.
 
How about speakers?

I imagine this is even harder to quantify than the merits of a particular tube amp design, but what are some speakers that seem go especially well with this type of amp?

Would the Klipsch RP-260F be a good choice?
 
I've been using Warfedale W90's, Large Advents, Imperial 6's with my ST70.
The RP-260F has a sensitivity rating of 97dB, which is on the high side. It should be plenty efficient for a 35 or 60 watt amp https://f072605def1c9a5ef179-a0bc3f...com/product-specsheets/RP-260F-Spec-Sheet.pdf With this kind of efficiency, I would think a Stereo 70 would have all the power you'd need, unless you like to play music crazy loud.

Aside from known technical issues like a speaker's efficiency in relation to amplifier power output, again, these amps don't all sound the same. Identifying what kind of sonic character is preferred will help to guide a synergistic sonic match between amp and speaker to suit the listener's sonic taste.
Good point. The three sets of speakers I use with my ST-70 are in the 90+ db range. They get plenty loud. But there isn't anything wrong with have some extra head room if you find less efficient speakers that you like.
 
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I'm just wondering...
What amp(s) are you using now?
What's your skill set and experience?
Have you heard any different tube amps using different output tubes? Any El34, EL84, KT66 amps? It's not all about power, different tubes have their own sonic signature and reputations. EL84 was called "The little tube that roared". The Beatles used EL84 amps. EL34 is was once considered the most popular tube used in home HiFi.

Why exactly are you considering an ST70 as your next project?

I chose the ST70 because I thought it was an excellent platform to learn from. I took my time, learned a lot and really enjoyed the experience.
 
Good questions. Here is some background: I basically have the equivalent of an ST-35 now. It is an EL84 based pp amp. It was originally a Magnavox 9300 console pull, but it has evolved a lot. Not sure if you are familiar with Dave Gilespie's thread on rebuilding these, but I have done everything he has on mine:

http://audiokarma.org/forums/index.php?threads/more-fun-with-magnavox-the-9300-series.687735/page-3

Rewired phase inverter, replaced OPTs with Dynaco ST35 transformers, added EFB bias circuit, Added input buffer and vol control, Rebuild feedback loop, Rectifier mod. There are no original parts now except the PT, tube sockets and chassis. It was so much fun I thought next time I'd like to start with a kit of all brand new components. The technical part and building are easy for me. In fact, that's probably equally enjoyable to listening. I am currently running 4 Baldwin branded EL84's. They are black plate Sylvanias circa 1960 in NOS condition. I've got Mullard 6EU7s in the front if that matters. They came with the amp.

I'm currently getting 15 watts RMS continuous per channel into 8 ohms at 1KHz with this amp, and it is sufficiently loud with the Advents, but a little reserve would be nice.

For speakers I have a set of Advent Laureate because I got them for free. I recapped the crossovers and added an L-pad on tweeters. Still they are a bit harsh for my taste in 1-3khz range. I like smooth, rich, warm sounding tunes. I hate harsh. Musically I like stuff that sounds good. I don't care what type of music it is. Well recorded, not complicated sonically, I like to hear single instruments and dynamics. Definitely *not* metal. Jazz/rock/country/indie as long as it's well recorded, clear dynamic, fresh.

Wow it's hard to explain sound ;)

Thanks for your help.

Here is a pic of my amp
ampMagComplete.jpg
 
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Your story sounds familiar to mine, including our taste in music.
I bought New Large Advents back in 1980 and had them until 2 years ago. They served me well through the decades as I gradually upgraded the rest of my system. I thought I would be buried with them.
A couple of years ago I replaced them with a set of Snell E-III's because the Advents began to sound harsh to my ears. Too much Glare. I loved the Advent's tight bass and clear highs, but that glare became too much, maybe it's age related. I had replaced every other component in my system including the Advent's crossovers until I decided the problem might be with my speakers, and it was.
I heard the Snells at a friend's house and enjoyed them on his system. So I found a pair for around $200 that needed a re-foam and tried them on my system....there was no comparison. The Snells set up a more enjoyable image. Not just left and right, there's now a depth of stage in the recordings. The Advent's were too directional. The Snells are "airy" clear, not bright and never harsh. The bass is full and deep but it's a different sounding bass as the Advent's were sealed and the Snells are ported.

For my room.....the Snell's are a better speaker.

What are your sources and preamp?
 
My primary source is my iphone and Spotify on highest bitrate.

No preamp other than the 1-tube input buffer built into the Maggie which allows for the volume control without messing up the high frequency response. I see no benefit in one. I tried messing with tone control circuits for a while but they just made things sound better in some areas and worse in others so I decided to just leave things flat.

Oh, I do occasionally play some vinyl. The turntable has a small Pile preamp inline. I have a collection of older monaural vinyl, but I play those on an old Voice of Music HiFi console from 1956. It has a 6V6 PP amp and big speakers in a big cabinet and sounds wonderful with those old recordings.

I'm not familiar with Snell. I'll look into them. Thanks.
 
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In this thread, I see mention of a difference between the original ST70 and the current Dynakit version, in the power supply section. Some sort of upgrade or redesign which fixes a PS problem which the original ST70 had. What changed?
 
Some ST70s might benefit from nursing home care, and some might need surgery or worse. ;) I totally agree that my ST70 has been having problems with performance and not design. I was not imagining its problems were related to a design flaw. I just was curious about repeated mention by the OP of a power supply problem fix which I, and now you, guess is related to that first (40 uf) filter cap. Yes, I am looking forward to the day, following a proper repair, when this amp is working reliably.

Once that has occurred, I intend to run it into a 4-Ohm speaker load. This, presumably, will tax the amp a bit more but its design specifications allow for a 4-Ohm load. It will not be asked to run anywhere near full power. The speakers are fairly efficient and the volume will easily allow for dinner conversation. It just need to be operating in-spec. It's as simple as that, I believe.

:)
 
I built Bob's M125s - choice was easy - so many great reviews. It has been about 10 years and still going strong. Love the flexibility you get with the 125s.

HYivhKN.jpg
 
Dave G. pretty much covers the few problems in the big thread. Comparing that thread to some of his others, what I get is how solid the basic design engineering is and how few improvements/ enhancements it takes to fulfill its potential.
 
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Quick question. Will the st70 power magnapan mg IIIa?
Recommendation from the manufacturer that I found says 50 watts minimum, 200 watts max.
The ST-70 tops out at 35 watts per channel max. The MG3's are rated at only 83-85db efficiency. So a more powerful amp would be much better. Like 100wpc or more.
 
My opinion:
The 70 used a pentode input stage directly coupled to a triode split load inverter in order to get rid of a coupling cap, and stuff more feedback around the amp.

If I was going to build another ST70 I would do my very best to use that topology.
Pentode-direct to split load- capacitor coupling to outputs with global feedback to the pentodes cathode.
That's the cheap but good design work that Hafler was famous for. We can continue in that tradition by using the available stock of tv tubes instead of the 7199.
The 6an8 and 6gh8 tubes can be used in this topology for low distortion drivers. It's still an affordable easy to construct driver, and it makes no sense that so many people trash it for a pair of triodes.
Is it even a ST70 once you've totally redesigned the circuit into something else entirely?
If you want a ST70, try an actual ST70, not some other amp in a ST70 chassis.
 
Recommendation from the manufacturer that I found says 50 watts minimum, 200 watts max.
The ST-70 tops out at 35 watts per channel max. The MG3's are rated at only 83-85db efficiency. So a more powerful amp would be much better. Like 100wpc or more.
Thank you
 
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