OFF - FOR SALE: Super Dynaco ST-70. An amazing amp by master tech, Herb Ward. Triode/UL switch. Adjustable feedback
Item #649403705Info: Super Dynaco ST-70. An amazing amp by master tech, Herb Ward. Triode/UL switch. Adjustable feedback
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Jan 02, 18 5:29am (PST)
Edited: Jan 17, 18 7:57am
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This is the Ultimate Dynaco ST70 amplifier modification, built by master tech, Herb Ward in New Canaan, CT. One-year warranty, excluding tubes. If anything goes wrong with the amp during the first year (it won’t) send it back to Herb (on your dime) and he will repair the amp.
According to the date code on the bottom of the transformers, this amp is from 1960.
Output transformers are the cloth lead A-470's , and the power transformer is the taller
lamination stack version of the PA 060 , a cooler running, more reliable
transformer than trannies with the shorter lamination stack.
The driver board is from Greg van der Sluys from Classic Valve in Canada. It uses a section-matched pair of new Tung Sol 6SN7 tubes, and a new 6SL7 tube—purchased from by Jim McShane. The driver board caps are all 716P film and foils, and the resistors used are the series of 1% Ohmite and PRP metal films preferred by Levinson and other high-end builders.
The Dynaco Designs bias board allows for individual adjustment of output tube current. Topside test points make the biasing procedure easy.
Herb also installed cathode fuses to project the output transformers. Each output transformer is protected by a 250 ma fast-blow fuse in the event an output tube shorts.
The power supply is an enhanced version of the
Triode Electronics capacitor board, with additional components. The preamp tube
power supply has each channel having its own large value film capacitor. Total power supply capacitance for this amplifier approaches 800 uf.
All power supply and signal grounds are wired to
one "star" . The chassis is not used as a conductor. An appropriate
in-rush current limiter is installed to lower filament and high voltages to
spec as the early Dynaco power transformers are wound for 115 VAC which was the
nominal AC mains voltage of the late fifties.
This custom Dynaco ST-70 has a feedback source selector, a feedback amount selector, and a triode /ultralinear mode selector. All of the mode / feedback switches may be operated with the amplifier on. (See below for operation instructions.)
The power cord is a 16ga 3 wire cord, and a double
diode ground loop eliminator has been installed to preserve the proper
functioning of the ground pin of the power cord.
The laminations of the original Dynaco A-470 are secured by only
two bolts. Loose transformer laminations can interfere with the optimal
operation of an audio output transformer. Herb added a bolt
to each corner of the output transformers and secured them “Linn tight.”
The right toggle switch switches the amp from Ultralinear to Triode mode.
When the switch is in the down position, the green
LED comes on, and the amplifier is in the triode mode, outputting about 15
watts per channel (WPC).
When the switch is in the up position, the green
light goes off and the amp is in Ultralinear mode (U.L.) outputting about 35
The left toggle switch is the feedback source switch.
When the feedback switch is in the up position, the
user has selected the 16-ohm tap. With the feedback switch in the down position,
the user has selected the 8-ohm tap of the output transformer.
The vast majority of tube amps
get their global negative feedback (GNFB) from the
output transformer’s 16-ohm taps. Some people feel that the 8-ohm tap should be
used because it is the output transformer tap that is actually connected to the
speakers. By doing this you reduce GNFB, which slightly increases gain, and
distortion while reducing damping factor.
The center slide switch is feedback ratio switch. With the switch in the up
position, the total amount of global feedback is 18
dB. With the switch in the down position global
feedback is considerably reduced to 12 dB, increasing gain and
This amp is a gas to listen to and play around
with. If you always wondered about the sonic differences between a UL and
triode amp or how more or less feedback
effects the sound of the music through your speakers, you can experience all of
that and more with the simple flick of a switch. Ultimately though, you can
custom tune this amp to the best mode for the best sound for your speakers and
ONE YEAR WARRANTY, EXCLUDING TUBES.
If you don't think this is the best 900 bux you've ever spent on a tube amplifier, send it back within 10 days of receiving it, and I will return you purchase costs, once I determine that I receive the amp in the same condition I sent it in . You are responsible for to-and-from shipping costs.
When you buy this amp I will send along the simple
biasing instructions written by Herb Ward. Biasing takes all of 5 minutes,
Tubes will be removed and labeled and packed with the amp in a heavy-duty
If you want to use PayPal, you’ll have to pay their
fees. Why not just send a check or money order?
Local pick up with cash payment is always
encouraged. Bring your favorite LP or CD and you can hear this amazing amp
before you buy.
This ad was originally posted on US Audio Mart.
Canuck Audio Mart is FREE!