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The failings of this unit have been well documented for years. Early
on in the tube revival, David Manley pointedly rejected the ST70 as an
improveable chassis in his writings, and other manufacturers whose
early start consisted of reworking them showed by example what was and
wasn't useable about hte design (i.e., not much).

I found this on a cursory search:


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Dynaco Stereo 70 issues and how they are addressed in my amp

The Dynaco Stereo 70 first came out in 1959 as a response to the
creation of, and overwhelming acceptance of, stereo records which came
out in 1958. When Dynaco designed the Stereo 70 they made it to sell
at a price point (about $100 for a kit and $130 for a wired unit). As
delivered, the amp was blessed with an excellent Mullard rectifier and
great sounding Mullard output tubes. The A-470 output transformers
were of fine quality also. In all fairness though, serious compromises
were made in other areas that affected its overall sound quality and
long term reliability.

1. The power transformer was big enough to run the amp but had no
reserve and as the years went by many ST-70's started to be plagued
with hum, buzzing and mechanical vibration issues that emanated from
the power transformer. The new USA made power transformer in my amp,
although looking near stock, has a higher stack lamination and a much
greater power handling capacity. On the old transformer both tube
heater legs could output 6.3 volts AC @ 3 amps per channel (6 amps
total) while the new transformer is rated at 6.3 volts AC @ 5 amps per
channel (10 amps total). The old power transformer was rated at 115
volts which was the AC voltage in homes in the 1950’s while the new
one is rated for modern AC voltage levels of about 120 volts.

2. The A-470 output transformers are brand new USA made custom
production transformers. They use the same interleaved/layer winding
technique as the original cloth lead A-470's and incorporate the same
high quality M-6 grain oriented laminations. They are dimensionally
accurate to the original design and have a durable heat resistant
black finish.

3. The multi-sectioned electrolytic capacitor on the old amp was a
30, 20, 20, 20 Mfd cap (90 Mfd total). This amount of capacitance was
enough to run the amp at lower power levels but would cause the audio
to break up prematurely at higher volume levels. The “35 watts per
channel” was really about 25 watts per channel before the amp would
start to run out of gas. The replacement quad cap is an 80, 40, 30, 20
(170 Mfd total) with the 80 Mfd section arranged to power the output
transformers. This cap upgrade allows the amp to play at higher volume
levels with less strain.

4. The driver board on the original ST-70 was a brown board with
open solder traces made of a very inexpensive phenolic material. As
the years went by, this board would not stand up to the many hours of
hard use and would actually start to burn from the heat. On the area
around and under the 7199 tubes the board would turn a blackish color
on many ST-70’s. Expansion and contraction from the heat would cause,
at times, cracks in the open solder traces. When a crack occurred, the
usual symptom was noise and/or intermittent output usually in just one
channel. The 7199 pentode/triode driver tubes were a compromise by
Dynaco to use just two tubes on the driver board. They worked OK but
some feel (and I concur) they were just not great sounding tubes.
Dynaco also used an excessive amount of loop feedback (20db) in their
design in an attempt to reign in and even out some circuit
compromises. Most modern preamps and driver boards use an all TRIODE
design which nearly always results in better sound. In 1964, five
years after the introduction of the 70, Dynaco came out with its
little brother the ST-35 power amp. In this amp Dynaco used a
7247/12DW7 driver tube which is a dual triode design. Some Dynaco
aficionado’s claim that (in stock form) the ST-35 was the best
sounding amp that Dynaco ever produced. The VTA (Vacuum tube Audio)
driver board used in my amps was designed by Roy Mottram of Puyallup,
WA. This board uses an all triode design and is a double sided
military spec printed circuit board made of epoxy fiberglass. This is
a very high quality material and will not go bad over time. This board
features three 12AT7 dual triode tubes. Only 13 db total loop feedback
is used. This board features an onboard bias system and separate bias
controls for each tube. The original amp had one bias control for each
PAIR of tubes. This newer bias system makes setting bias easier. The
use of matched pairs or quads of output tubes is not absolutely
necessary with this bias system.

5. The tube sockets on the older amps were a phenolic plastic
material and could burn and cause intermittent contact on the tube
pins as the pins slowly oxidized with time. The 7 tube sockets on the
new amp are modern quality Celanex sockets which are much more durable
than the original stock tube sockets.

6. The input jacks on the original amp would oxidize with time and
cause intermittent contact. They were spaced very close together and
some modern, thicker interconnects could not be used on the original
amp. The newer input jacks are much higher quality, are gold plated
and have a wider spacing to allow for more modern design thicker
interconnects.

7. The output terminals on the original amp were simple screw
connectors. On the new amp modern gold plated 5 way binding posts are
used.

8. The new chassis is identical to the old chassis in size, shape and
all chassis markings but instead of nickel plated steel like the old
chassis, the new chassis is made of mirror polished type 304 STAINLESS
STEEL. This chassis will never rust.

Bob Latino
508-347-
####

[Peter Wieck]

On Oct 28, 8:27*pm, wrote:

Stuff.

And?

Peter Wieck
Melrose Park, PA

[Sal]

If you want to get into vintage tube audio at a reasonable low cost, no
doubt in my mind the Dynaco ST-70 is the way to go. The only thing where
I would agree would be the power transformer, all of my ST-70's have the
original transformers with no problems.

Like you mentioned, it was designed for 115 volts nominal, if you are
worried, put a variac and drop the voltage a bit.

The Dynaco ST-70 is a simple tube amp to repair with replacement parts
if needed still being manufactured today. What other tube amp can you
say that for?

Sal

[tubegarden]

On Oct 29, 7:13*am, Sal wrote:
The Dynaco ST-70 is a simple tube amp to repair with replacement parts
if needed still being manufactured today. What other tube amp can you
say that for?

Sal

Hi RATs!

I have played with about a hundred.

None of them were hard to fix.

Just fun

Happy Ears!
Al

[Engineer[_2_]]

On Oct 29, 8:13*am, Sal wrote:
If you want to get into vintage tube audio at a reasonable low cost, no
doubt in my mind the Dynaco ST-70 is the way to go. The only thing where
I would agree would be the power transformer, all of my ST-70's have the
original transformers with no problems.

Like you mentioned, it was designed for 115 volts nominal, if you are
worried, put a variac and drop the voltage a bit.

The Dynaco ST-70 is a simple tube amp to repair with replacement parts
if needed still being manufactured today. What other tube amp can you
say that for?

Sal

A variac is an expensive solution for this (but you still need one on
the bench.) Take a $10 (or spares box!) 6.3 VAC, 3A, filament
transfomer and use the 6.3 VAC as a series "buck" to the high mains
supply. Put it in a metal box with a two-hole, 3 pin receptical
mounted on it. Good up to 330 watts. Use permanently for all 110/115
VAC vintage radios and audio.
Cheers,
Roger

[John Byrns]

In article ,
Engineer wrote:

On Oct 29, 8:13*am, Sal wrote:
If you want to get into vintage tube audio at a reasonable low cost, no
doubt in my mind the Dynaco ST-70 is the way to go. The only thing where
I would agree would be the power transformer, all of my ST-70's have the
original transformers with no problems.

Like you mentioned, it was designed for 115 volts nominal, if you are
worried, put a variac and drop the voltage a bit.

The Dynaco ST-70 is a simple tube amp to repair with replacement parts
if needed still being manufactured today. What other tube amp can you
say that for?

Sal

A variac is an expensive solution for this (but you still need one on
the bench.) Take a $10 (or spares box!) 6.3 VAC, 3A, filament
transfomer and use the 6.3 VAC as a series "buck" to the high mains
supply. Put it in a metal box with a two-hole, 3 pin receptical
mounted on it. Good up to 330 watts. Use permanently for all 110/115
VAC vintage radios and audio.

Or if you want to be slightly easier on the transformer, or increase the voltage
and current ratings slightly, connect the secondary and primary in series, with
the line across the whole composite winding, and take the reduced voltage output
from the "tap" near the top of the composite winding.

--
Regards,

John Byrns

Surf my web pages at, http://fmamradios.com/

[BretLudwig]

Dropping the line voltage fixes one out of many, many problems with this
perennial time and energy sink.

--
Message posted using http://www.talkaboutaudio.com/group/rec.audio.tubes/
More information at http://www.talkaboutaudio.com/faq.html

[[email protected]]

On Nov 1, 10:07 pm, "BretLudwig" wrote:
Dropping the line voltage fixes one out of many, many problems with this
perennial time and energy sink.

--
Message posted usinghttp://www.talkaboutaudio.com/group/rec.audio.tubes/
More information athttp://www.talkaboutaudio.com/faq.html

They are finally starting to get scarce since the Asians started
buying them.