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Dynaco ST-70 Series III

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Poll: ST-70 Series 3

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close on 6th June 2015 at 04:48 PM

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7th April 2015, 04:46 PM

#1

dmfraser66

diyAudio Member

Join Date: Mar 2015

Location: Vancouver BC

Dynaco ST-70 Series III

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.

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7th April 2015, 05:01 PM

#2

Bandersnatch

diyAudio Member

Join Date: Jul 2003

Location: Ann Arbor, MI

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

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7th April 2015, 06:32 PM

#3

Eli Duttman

diyAudio Member

Join Date: Apr 2004

Location: Monroe Township, NJ

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.

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7th April 2015, 06:38 PM

#4

rayma

diyAudio Member

Join Date: Apr 2011

Location: Upper midwest

Quote:

Originally Posted by dmfraser66

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.
However, the signal path will still be through
tubes.

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.

Last edited by rayma; 7th April 2015 at 06:47
PM.

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7th April 2015, 06:42 PM

#5

chrisb

diyAudio Member

Join Date: Aug 2002

Location: victoria BC

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

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7th April 2015, 06:46 PM

#6

rayma

diyAudio Member

Join Date: Apr 2011

Location: Upper midwest

Quote:

Originally Posted by chrisb

While I've never heard an "authentic
/ vintage" example myself, coming from folks whom I trust,

that'd not necessarily be a
negative.

The original circuit would
not be competitive today. It wasn't even in the seventies,
and that's
why so many were modified.

Last edited by rayma; 7th April 2015 at 07:15
PM.

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7th April 2015, 07:41 PM

#7

planet10

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Join Date: Oct 2001

Location: Victoria, BC, NA, Sol III

Blog Entries: 5

Quote:

Originally Posted by chrisb

While I've never heard an "authentic
/ vintage" example myself

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

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7th April 2015, 07:51 PM

#8

leadbelly

diyAudio Member

Join Date: Dec 2002

Location: Calgary, Alberta

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.

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Last edited by leadbelly; 7th April 2015 at 07:56
PM.

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7th April 2015, 08:00 PM

#9

planet10

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Well said Tony.

dave

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7th April 2015, 08:10 PM

#10

20to20

diyAudio Member

Join Date: Jun 2010

Location: W-S, NC

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.

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