A new company with a strong reputation in professional music has acquired the Hafler and Dynaco brands and intellectual property. I will not say who as that may be considered advertising.
Our question is:
For an ST-70 amplifier would the preference be for:
1. An amplifier as faithful to the original ST-70 but incorporating the improvements of that have been done to it by the audio community but using no New Old Stock parts. Sorry, no 7199 input. But following the original Pentode-Triode input circuit with feedback.
2. The best possible amplifier that will still use the EL34 output but where the input/driver uses new topologies for what we feel will be better sound quality. We have spoken with people who knew David Hafler and we would basically do what he would have done today if he were around. However, the signal path will still be through tubes.
Either of these made with all new parts will have an MSRP a little north of US$1200
We ask because there are so many old and rebuilt ST-70 amplifiers out there, that some here have felt that a person who wants the original 1950s sound quality will just buy one of the original or reproductions for a lower price. That a person buying a new unit wants something that is competitive with a current well rated tube amplifier.
Note that the lead engineer on this project received their basic electronics education in the 1960s on tubes and has been involved with the audiophile community for decades. This is not just some rebadging exercise.
Regardless, as a long time DIYer, I would appreciate the input of the DIY community on the approach to take. And your comments do not need to be limited to the ones I have given. For your suggestions, all I ask is that you only recommend tubes that are still in production.
Our question is:
For an ST-70 amplifier would the preference be for:
1. An amplifier as faithful to the original ST-70 but incorporating the improvements of that have been done to it by the audio community but using no New Old Stock parts. Sorry, no 7199 input. But following the original Pentode-Triode input circuit with feedback.
2. The best possible amplifier that will still use the EL34 output but where the input/driver uses new topologies for what we feel will be better sound quality. We have spoken with people who knew David Hafler and we would basically do what he would have done today if he were around. However, the signal path will still be through tubes.
Either of these made with all new parts will have an MSRP a little north of US$1200
We ask because there are so many old and rebuilt ST-70 amplifiers out there, that some here have felt that a person who wants the original 1950s sound quality will just buy one of the original or reproductions for a lower price. That a person buying a new unit wants something that is competitive with a current well rated tube amplifier.
Note that the lead engineer on this project received their basic electronics education in the 1960s on tubes and has been involved with the audiophile community for decades. This is not just some rebadging exercise.
Regardless, as a long time DIYer, I would appreciate the input of the DIY community on the approach to take. And your comments do not need to be limited to the ones I have given. For your suggestions, all I ask is that you only recommend tubes that are still in production.
So what would Hafler have done today?
Sorry but buying new tube amps is not in my DNA.
I'd want a pentode LTP with local Schade FB....preferably done with 10-15% E-Linear taps on the primary. The A470 opt is wound by lots of folks; and for reasonable money. Might as well keep that...
though with the E-Linear modification...and maybe drop the U-L rigging; local FB will reduce the source impedance far below that of U-L, and with less input capacitance.
cheers,
Douglas
Sorry but buying new tube amps is not in my DNA.
I'd want a pentode LTP with local Schade FB....preferably done with 10-15% E-Linear taps on the primary. The A470 opt is wound by lots of folks; and for reasonable money. Might as well keep that...
though with the E-Linear modification...and maybe drop the U-L rigging; local FB will reduce the source impedance far below that of U-L, and with less input capacitance.cheers,
Douglas
Staying with the original pentode voltage amplifier DC coupled to a "concertina" phase splitter is problematic. ALL of the suitable "tri-pents", not just the 7199, are out of production. Using separate bottles runs afoul of other folks intellectual property rights. Refer to the Triode Electronics replacement driver boards, for old Dyna ST70 specimens.
The party that acquired the Dynaco and Hafler rights might have to license just about anything that's "out there", in the way of "new" driver circuitry.
I'm very dubious about using A470 "iron" with Williamson style circuitry. Mullard style circuitry is definitely safe, but high gm small signal devices are (IMO) strongly indicated in circuitry with GNFB loops. High gm small signal pentodes are, AFAIK, out of production. A 6922, with its sections in cascode, is a reasonable substitute for a high gm small signal pentode, but poor PSRR requires that regulated B+ be employed. JJ's ECC99 and New Sensor's 6n30p are reasonable candidates for the LTP job.
If JJ can be coaxed into making them correctly, their E34L would be excellent as "finals". New Sensor's EH 6CA7 and "reissue" GEC KT77s are additional possibilities for the O/P tubes.
A single 5AR4, with its 250 mA. capability, is somewhat overstretched feeding 4X EL34 class tubes plus small signal circuitry. As high PIV Schottky diodes are every bit as quiet as vacuum rectifiers, using them seems obvious. Several techniques for "softening" B+ start are available.
The party that acquired the Dynaco and Hafler rights might have to license just about anything that's "out there", in the way of "new" driver circuitry.
I'm very dubious about using A470 "iron" with Williamson style circuitry. Mullard style circuitry is definitely safe, but high gm small signal devices are (IMO) strongly indicated in circuitry with GNFB loops. High gm small signal pentodes are, AFAIK, out of production. A 6922, with its sections in cascode, is a reasonable substitute for a high gm small signal pentode, but poor PSRR requires that regulated B+ be employed. JJ's ECC99 and New Sensor's 6n30p are reasonable candidates for the LTP job.
If JJ can be coaxed into making them correctly, their E34L would be excellent as "finals". New Sensor's EH 6CA7 and "reissue" GEC KT77s are additional possibilities for the O/P tubes.
A single 5AR4, with its 250 mA. capability, is somewhat overstretched feeding 4X EL34 class tubes plus small signal circuitry. As high PIV Schottky diodes are every bit as quiet as vacuum rectifiers, using them seems obvious. Several techniques for "softening" B+ start are available.
You have too much competition to not do this option. I agree that a ss rectifier and a regulated power supply
would be key. At that price you'd have to sell direct, though.
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The real question is - by the time all the various "improvements" were made to the circuit topology, parts selection, mechanical layout, etc., would it remain an ST70 in anything but name? While I've never heard an "authentic / vintage" example myself, coming from folks whom I trust, that'd not necessarily be a negative.
A retail price circa $1200USD for an assembled unit seems very reasonable
A retail price circa $1200USD for an assembled unit seems very reasonable
The original circuit would not be competitive today. It wasn't even in the seventies,
and that's why so many were modified.
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I have. The original ST70 is a mediocre amplifier. The DynaMutt that was built from the 2 failed examples i owned is superb. It does not use the UL taps at all, being able to run in pentode or triode (used mostly in triode)
dave
Minor point, and personally opinionated, but I have to point out that even the best upgrade driver board designs out there are at least somewhat compromised in that they are have been designed with a criteria that they must retrofit to the ST-70 chassis.
In other words, if you told someone to design the best EL-34 input and driver stage they could, even if it had to use commercially available tubes, it would probably be very different, with lots of SS parts, etc., have a much larger footprint, and therefore look nothing like an ST-70 circuit.
If I had $1200 to spend, I would much rather know that it was a good EL-34 design than know that it was an upgraded ST-70 and carrying some original ST-70 design baggage.
In other words, if you told someone to design the best EL-34 input and driver stage they could, even if it had to use commercially available tubes, it would probably be very different, with lots of SS parts, etc., have a much larger footprint, and therefore look nothing like an ST-70 circuit.
If I had $1200 to spend, I would much rather know that it was a good EL-34 design than know that it was an upgraded ST-70 and carrying some original ST-70 design baggage.
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If you ever get to the production stage you should call it something totally different. It won't have any Dyna DNA in it. It won't be an ST-70. Buying a name isn't going to recreate any links to the past, just create a false marketing front. Hafler went SS as soon as he could so you can't say what he would do today... He'd be a smartphone company by now. If you can create a new high quality product, put your own name on it.
For 1961, the ST70 was WAAY better than the fine wood RCA/Magnavox/Admiral/Philco/Motorola consoles that were the competition for "hifi". One usually needed a ground guide to find the McIntosh/Klipsch dealership, they were so out of the way. ST70 was WAAY better than the speakers selling at the same price point. I heard a lot of AR and KLH bookshelf toy speakers, and the AR3 and KLH 5? cost more than an ST70. On modern high output speakers, see my signature, my original unit is still quite good sounding.
In the current market, with Best Buy dominating the "listen before you buy" market outside of cities of 5 million population, the 1961 ST70 sounds WAAY better than any speaker actually sold in stores.
In the impersiousness to lightning strikes through the power line feature, my original 1961 ST70 is WAY better than any SS amp up to the $1000 price point I've encountered. Not even my semi pro Peavey amps have the 30 mm MOS supressors on the power lines backed up by burnout resistors required to protect SS gear from power line strikes, the way Siemens motor drives are protected for example.
Furthermore as a transformer coupled amp the updated ST70 will not damage speakers after a wiring fault the way 99.999% of the "superior" amps discussed and touted on here will do without further parts installation. And hard contact relay contacts as SS speakers protectors, I have to laugh. I've been personally through the gyrations the ineffective "crowbar" circuits cause. Crowbars so far have been great at burning PCB lands, total failures at protecting the speaker from DC.
I think a 70 watt stereo tube amp with RCA in and Screw terminals out is a great idea. Parallel Speakon connectors at the 4 ohm tap would be a useful accessory. 16 ohm spreakers are a dead duck and need not be supported. The sound should be compatible with the better of the updated units, ie <.5% THD, IMHO.
I see a 4 output tube unit called a ST70 as being quite successful, as long as the store selling it didn't specialize in watt soaker acousic suspension speakers. Hint, a compatible efficient horn/woofer speaker, maybe a private label Peavey SP5 or SP2, would make a nice saleable combo.
One further quibble. The ST70 was made outside Philidelphia, which I was quite proud of. I don't buy anything new made in the orient with oil imported from a country famous for supporting 3 proxy wars, and building an atom bomb to vaporize a US ally in the region. Assembling it the the western hemisphere is a requirement to market to me. As few *****ese components as is possible is a plus, which certainly could lead to consistent units coming in. Look what the production engineers in the orient have done to the P****** amplifier designed by John **** our fearless leader. Much cheaper construction than original is being shipped now, IMHO.
At least now, if you sort through the trash, JJ is making tubes in a place I wouldn't mind working or living. By all means dump the pentode/triode input stage for something more accessible, four 12AX7 if you can manage it.
Compatibility and power support for the transformerless PA preamp (those octal sockets on the front) is no longer a necessity. I never knew anybody that owned a pair of those, anyway. I found the dynaco line through the college library, that was using them 18 hours a day 7 days a week in the LP listening rooms, where they sounded great to my RCA/Top Value stamps abused ears. Bias set on the front is still useful, and I have modified the socket for a B+ test point since the capacitors and rectifiers had usually a 3-4 year life. Both useful features.
Best wishes to the entrpreneurs that bought the rights and name. May you design build and test well. live long and prosper.
I think that the Dynaco name is still of some value, but the amplifier should be a clearly modern design,
maybe named something like the "Stereo 75" to emphasize that.
indianjo - cool your jets, man - no-one said the ST70 didn't represent great value and the Dynaco team deserves as much credit for their contribution of making DIY audio accessible to the public, as for the designs themselves.
There were other companies of that era with products in similar power range - Heathkit WM6, and Stuart Hegeman's masterpiece the Citation II ( $229 in 1959 dollars ), either of which could be argued attained at least the same level of performance, but neither sold over 350,000 units.
The 3 items in poll are fine enough, but how the hell would we know what Dave Hafler would do today - that might even not include ultralinear.
There were other companies of that era with products in similar power range - Heathkit WM6, and Stuart Hegeman's masterpiece the Citation II ( $229 in 1959 dollars
), either of which could be argued attained at least the same level of performance, but neither sold over 350,000 units. The 3 items in poll are fine enough, but how the hell would we know what Dave Hafler would do today - that might even not include ultralinear.
Citation II about 1850 in 2015 dollars still a bargain at that price.indianjo - cool your jets, man - no-one said the ST70 didn't represent great value and the Dynaco team deserves as much credit for their contribution of making DIY audio accessible to the public, as for the designs themselves.
There were other companies of that era with products in similar power range - Heathkit WM6, and Stuart Hegeman's masterpiece the Citation II ( $229 in 1959 dollars
The 3 items in poll are fine enough, but how the hell would we know what Dave Hafler would do today - that might even not include ultralinear.
What and who would that be?
It strikes me that the weakest point after the power supply was the output transformers. A pentode-to-cathodyne input stage is simple, elegant, and can be very high performance.
The 7199's variability in the ST70 often caused quite asymmetric swing on the driver outputs. The only simple fix was a variable screen resistance.
I used a 499k in series with a 1M cermet pot. As it turned out, the best adjustment was for about 110VDC on the pentode plate/triode grid.
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Triode Electronics offers 2 versions of separate (3) bottle pentode voltage amplifier/triode "concertina" phase splitter ST70 driver boards. A version uses 2X 6AU6s and a twin triode, while the 2nd uses 2X EF86s and a twin triode. IMO, the obvious choice is the 2X EF86 board and an ECC99 as the twin triode.
SY, you'll get no argument from me about pentode voltage amplifier DC coupled to "concertina" phase splitter as having the potential to yield excellent performance. The ability to use high value grid to ground resistors at the I/Ps, which makes 50 Kohm passive control centers quite viable, is but 1 reason.
I'd like to see high gm be present in all small signal devices, to provide resistance against slew limiting induced by the NFB HF error correction signal. I'm unaware of any current production small signal pentode whose gm is high. As I stated previously, a 6922 cascode is a possible candidate for the job.
the 12at7 concertina stage was well liked in our own listening tests...
can's tell it apart from a mulard 5-20 input stage....
i have done a 6j9 pentode input and a 6CG7 LTP front-end and it too was well liked..
so far a well regulated G2 amp sounded better than ultralinear amp in another listening test..
btw, the comments about the sound was not mine, those were from people who came and listened
to the amp, i ussually keep mum during listening tests and just listen to what people had to say..
can's tell it apart from a mulard 5-20 input stage....
i have done a 6j9 pentode input and a 6CG7 LTP front-end and it too was well liked..
so far a well regulated G2 amp sounded better than ultralinear amp in another listening test..
btw, the comments about the sound was not mine, those were from people who came and listened
to the amp, i ussually keep mum during listening tests and just listen to what people had to say..
OK, so it's not actually an IP issue. I had trouble imagining that, since people have been doing this for 40 years or more.
Past that, since this is a commercial venture, I'm not interested in giving free design advice other than "basic Dynaco input stage topology can be made to work very well if that's what they want to do."
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