Dynaco ST-70. Dual mono! Master Tech Herb Ward's amazing Dynaco. Get ready for hifi fun

You have never heard a Dynaco amp like this. Master technician Herb Ward has been building, repairing, and modifying audio equipment for almost 40 years. He's seen it all, heard it all, and improved it all. (And he can work wonders on your tired audio components, including your stock ST-70. Contact me for his contact info.) 


Here's what Herb has to say about this Dynaco ST-70 (in quotations):

"For those of you that think
that Ed Laurent and David Hafler got more right than wrong with the design of
the Dynaco ST-70, here’s my version of the Dynaco ST – 70  enhanced with a Dual-Mono power supply.

The rebuild of this amp
features the following ;

Triode/ultralinear switches
were added. Screen stability resistors were added for both the triode and
ultralinear modes of operation which can be switched while the amp is on. In
the triode mode , a little more than 15 watts per channel is produced before
clipping. (Forward for rock & roll... move it to rear for jazz.)

Vintage Electron glass epoxy
circuit board ( 7199 ) version.

Tube socket adapters,
allowing use of other triode/pentode tubes such as 6GH8a or 6U8a. 7199 tubes
may be used if the socket adapters are removed.
Tube rolling is now possible with the addition of these socket adapters.

IRC and Stackpole film types
resistors.

Sprauge 716P .22 uf film type
coupling caps.

New quad cap is a German made
unit from Dynakit parts.

This amp has the AVA input
bandpass filters with adjusted component values making the 1db down points at
20 Hz and 20 kHz.

Interstage RC filter has been
eliminated.

This amp has a highly
upgraded dual mono power supply. Each channel has a separate output stage plate
supply with its own choke and 100 uf
of capacitance.

Each phase inverter stage has
its own 100 uf of capacitance. Each gain stage has its own
30 uf of capacitance.  

The power supply grounds are
all wired to a star ground configuration. The chassis of this amplifier is not
used for any ground returns.

The output tubes each have
there own precision matched 10 ohm cathode resistors.

The biaset test points for
all four tubes have been brought out to the front panel sockets. Tube balance
and matching can now be verified by means of this circuit addition.

The correct bias setting is
now 500 millivolts for 50 ma of tube current as each tube can now be
individually measured.

I suggest 400 millivolts for
modern output tubes. The old Mullard tubes can safely handle 50 ma current.

The global negative feedback
loop of this amplifier has been modified in that the feedback is now taken from
the 8 ohm speaker tap. The feedback resistive divider network was slightly
changed as a consequence of this modification.

The amp comes with four
Mullard 6CA7's that are healthy based upon in circuit test, and measurement in
a recently calibrated Hickock tube tester. The
pairs are marked accordingly.

The rectifier tube is a fresh
Sovtek, selected for minimal voltage drop under load.

The amp has a new power cord,
a new power switch, and a pair of inrush current limiters to minimize the cold
turn on current spike in the transformer
primary.

This amp exceeds published
specs due to the improved power supply. Unlike most ST -70's this amp REALLY
puts out 35 watts rms per channel prior to clipping. The typical distortion
rise at frequency extremes is greatly reduced by the power supply upgrades.

As so many have observed, the
stock ST-70 power supply simply significantly hinders the real capabilities of
the  original  circuit design. Having listened to a variety
of the ST- 70 driver board mods, I feel that this amp performs as well as any
of the  popular multi-tube driver board
offerings. The low level detail, the soundstage, and imaging of this amplifier
are greatly enhanced when compared to a stock,
properly functioning Dynaco ST - 70."

Paypal users add 3.1% to the final price, including shipping and insurance fees. Paypal users with funds that originate in a overseas bank, add 4.1 % to the total.  

Please ask any and all questions. Nothing to hide here, just good clean hifi fun. 
The fall selling season is beginning. In the next few weeks, I’ll be listing lots of cool components. Up next is a Herb Ward/McShane modded HK Citation II