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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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ST-70 Series 3

As original as possible

What Hafler would have done today

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7th April 2015, 09:55 PM

#11

indianajo

diyAudio Member

Join Date: Jan 2010

Location: Jeffersonville, Indiana USA

Quote:

Originally Posted by planet10

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

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.

__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2
ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's,
Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

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7th April 2015, 10:15 PM

#12

rayma

diyAudio Member

Join Date: Apr 2011

Location: Upper midwest

Quote:

Originally Posted by 20to20

If you ever get to the production
stage you should call it something totally different.
If you can
create a new high quality product, put your own name on
it.

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.

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7th April 2015, 10:26 PM

#13

chrisb

diyAudio Member

Join Date: Aug 2002

Location: victoria BC

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.

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7th April 2015, 10:32 PM

#14

Bandersnatch

diyAudio Member

Join Date: Jul 2003

Location: Ann Arbor, MI

You put the W6m and Cit.II in comparison to
the St.70 when it would be more appropriate to put them up against the
Mk.iii....not that either of them should be very threatened by such a
comparison.
cheers,
Douglas

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7th April 2015, 10:34 PM

#15

Triodethom

diyAudio Member

Join Date: Dec 2009

Location: Newnan GA

Quote:

Originally Posted by chrisb

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.

Citation II about 1850
in 2015 dollars still a bargain at that price.

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7th April 2015, 11:11 PM

#16

SY

diyAudio Moderator

Join Date: Oct 2002

Location: Chicagoland

Blog Entries: 1

Quote:

Originally Posted by Eli Duttman

Using separate bottles runs afoul of
other folks intellectual property
rights.

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.

__________________
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power of putting two and two together was annihilated."- EM Forster

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Yesterday, 12:57 AM

#17

rayma

diyAudio Member

Join Date: Apr 2011

Location: Upper midwest

Quote:

Originally Posted by SY

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.

Last edited by rayma; Yesterday at 01:01 AM.

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Yesterday, 01:06 AM

#18

Eli Duttman

diyAudio Member

Join Date: Apr 2004

Location: Monroe Township, NJ

Quote:

What
and who would that be?

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.

Quote:

A
pentode-to-cathodyne input stage is simple, elegant, and can be very
high performance.

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.

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Yesterday, 01:34 AM

#19

AJT

diyAudio Moderator

Join Date: May 2003

Location: Palatiw, Pasig City

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..

__________________
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is the number of diy'ers wanting to build it after all the years....never
the say so of so called gurus....

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Yesterday, 09:28 AM

#20

SY

diyAudio Moderator

Join Date: Oct 2002

Location: Chicagoland

Blog Entries: 1

Quote:

Originally Posted by Eli Duttman

Triode Electronics offers 2 versions
of separate (3) bottle pentode voltage amplifier/triode "concertina"
phase splitter ST70 driver
boards.

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."

__________________
"Pity, wrath, heroism, filled them, but the
power of putting two and two together was annihilated."- EM Forster

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