Why not try the newest iteration? http://www.dynaco.com/st70.php Back when I was modding these(1980s), Joe Curcio providing numerous tips that helped me maximize their performance. ie: much more capacitance in the power supply(two 5AR4s wouldn’t have hurt, but- would have required, at least, another filament trans). http://www.curcioaudio.com/st7upg_3.htm When the original went out of production, some 350,000 had been sold. Certainly, a major feather in David Hafler’s hat, if not the most popular stereo amp, in audio history.
I have heard, and owned, quite a few, and still have a couple in the closet. Rather than answer with a brand, I will suggest the areas that I feel bring the greatest benefits.
Although most concentrate on the front end, the basic Dyna front end, using 7199 or, with different wiring, 6GH8A is quite nice. Not that it cannot be bettered, by boards like the Tubes4hifi.
No I think the greatest weakness in the amp is the power supply, and would look for an upgraded power transformer, such as those made by Triode Labs and Dynakitparts, allied with increased, and lower esr capacitance, such as the Curcio board. IMHO, if you buy the best power supply that you can it will give the best result. And you can always play with input boards later.
One thing that I always do from the get go is strap the output tubes into triode, which is cheap as it only involves soldering in four resistors.
If you want something pre built the Latino ST70 and ST120 are great amps, with highly upgraded power supplies. The 120 should not be used with the 5AR4 tube rectifier, so I would stick to solid state rectification of the B+ voltage with that one:
Best of luck with the hunt. I am sure that other members I’ll forward other, equally valid, perspectives.
I'm partial to Bob Latino's version but the only other one I've heard is the original and I'm going from memory there.
I have built a pair of Mark IIIs from the Triode Electronics kit and the power supply upgrades are nice but the chassis is more utilitarian than Bob's.
There's enough stuff out there you could build any kind of 70 you want.
I was searching for the link that rodman provided. I have not heard the new series 3, but it is selling for $1,500 when its price when it was introduced just a couple of years ago was $3,000, so it was obviously not too popular.
So bear in mind I have not heard it or any of the modded ones, but it is the one I would purchase. I do have the original ST70 with the desirable cloth lead transformers (I think that what they were referred to as), an original with the Japanese sourced transformers and a series 2 made by the Panor corporation. I just purchased another amp and will be putting these up for sale when I get around to it since I don’t need them all.
And while bdp24 is surely right about design flaws, I love mentioning that the ST-70 is the greatest selling amplifier of all time. I will sheepishly mention that mine are in storage at this time.
Per the Absolute Sound article titled - The Ten Most Significant Amplifiers of All Time - "Along with Avery Fisher’s 500c, David Hafler’s ST-70 put high performance audio in the American home. Introduced in 1959, the ST-70 is the world’s most prolific stand alone amplifier with over 300,000 units manufactured. Amazingly, its clear, three-dimensional sound is comparable to many amplifiers made today."
I've had a couple of stock Dynaco ST70's, and there have been some mods or even redesigns which addressed the flaws in that very historically-important and influential amp. Some of Bill Johnson's early ARC designs were variations on those of David Hafler (particularly the SP-3 pre amp)---even at the time of the SP-3/D75/D51, Johnson was selling modded ST70's, and Frank Van Alstine at one time offered an extensive mod which eliminated the ST70's major flaws.
In contrast, Roger Modjeski started with a blank slate for the RM-10, creating a completely new design, not an updated version of a design from the 1950's. Why anyone would want an ST70---old or new, stock or modified---rather than an RM-10 is a mystery to me, especially when an RM-10 can be had for the same price as an ST70. What if a Porsche cost the same as a VW? ;-)
To view the historical significance and number of Dynaco ST70's sold as a reason to consider it worthy of consideration today is akin to doing the same for the Acoustic Research 3a in the field of loudspeakers. Good for it's time, but far surpassed today. I regularly see pairs of AR 3a'a offered on Audiogon at a price that can also buy one a pair of Vandersteen 2c's. Which loudspeaker would you rather own?
FYI: Bob Latino sells his wares on http://www.tubes4hifi.com/, along with Roy Mottram who sells pre-amp kits and customizations to original Dynaco 70s.
Regarding recitfer tube vs SS, Bob’s amp can run either without issue. In addition, there are a lot more upgrades to the amp beyond the power supply (output transformers, better caps, upgraded chokes, high precision resistors, driver circuit changes, a fully modern tube compliment, upgraded chassis, to name but a few). So IMHO Bob’s amps are really just modeled after the Dynaco ST70--his VTA 120 is a fully modern amp that is much closer to the Rouge Magnum and other modern tube amps that can run KT88s and KT120s (and now even includes an auto-bias board option.....)
Full disclosure--I own Bob’s VTA 120--super product with super support (something you might want to consider). Great supporting community as well.
Agreed that the ST120 can use a tube rectifier and produce full output, just not a 5AR4. Mr. Latino makes it pretty clear in the post below:
4 KT88 or 6550 are just going to require more current than the little rectifier can put out at full voltage. If you don’t need full power than save some money and buy the ST70. Note that the original Dynaco MKiii amps use two 5AR4s to provide the B+ voltage to the same 4 KT88s.