Audio Research ST-70

Audio Research ST-70-C3

Offered for sale: The Audio Research version of the famed Dynaco Stereo 70.
Some history; back in 2004 I purchased this as an unassembled upgrade kit on
eBay and then wrote Audio Research asking about modifications and upgrades to
the original unit. I received a reply from William Z. Johnson (ARC founder and
designer of the revised amp) with a list of things I could do to improve this
design. The rebuild was finally accomplished in 2012. The following changes were performed, most per Mr. Johnson’s
recommendations:

Replaced all carbon composite resistors with metal films, mostly Dale Vishay
RN65Ds

Replaced the two electrolytic caps on the circuit board with Kimber Kaps
(film) and changed resistor values to maintain the same time constant

Replaced the 6 coupling caps with AuriCaps

Replaced the original power supply diodes with larger, faster Fairchild
Stealth diodes

Replaced the original Dynaco power transformer with a Triode Electronics PA
060S with more capacity, heavier construction and which runs cooler and gives
better regulation (one reason the original ARC version used 6L6 tubes was to
reduce the 6.3 volt load due to adding 5 small tubes on the circuit board on
the original, note this unit does not have that limitation).

Replaced the Dynaco A470 output transformers with MagneQuest upgrade
replacement units

Replaced the 1977 vintage power supply electrolytics (475 volt) with new,
larger 500 volt or 525 volt units

Added a high voltage delay relay to allow the output tubes to warm up
before high voltage is applied.

During build, the wire originally supplied with the kit was replaced with silver plated, Teflon insulated wire with the correct color codes (except one wire that I could not find an exact replacement for).

Per Mr. Johnson’s specific recommendations, the unit was converted from a
pentode based design to an ultralinear design and resistors were added to the
output tube screens. This costs a few watts of output but gives tighter sound
and adds to tube life.

Finally the chassis was black powder coated and a large cut out was added
to the bottom to give better airflow through the unit.

Note that you can run 6550 or EL34 tubes in this unit with one small change
which will be detailed in the documentation. The original documentation will be
included as well as copies of the letters and emails between myself and Mr.
Johnson detailing the changes.

I will include a Dynaco tube cage (not pictured); it is for packing and
protection in shipping rather than for daily use as it impedes too much airflow
around the amplifier.

I’ve enjoyed this amplifier but now that I’ve moved and retired, I no
longer have multiple sound systems so this is one of the pieces I’m offering
for sale.

Please note: the 8/10 AGS rating is very conservative, more for the age of the unit rather than the physical condition.

Thanks for looking.