CUSTOM Dynaco ST-70 aka The DynaHerb Get ready for hi-fi fun!

Some months ago, I came across a really clean
example of a Dynaco ST-70. The original owner still had this amplifier in his

equipment rack, sitting below a recently purchased
surround sound receiver. I gave the amplifier to master tech Herb Ward

and asked him to come up with something different
than the usual refurbished offerings.

 

Herb was very pleased to find that this chassis was
a 1960 vintage (date codes on bottom of transformers) which means that the
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 the versions with the shorter lamination stack, often found on
many refurbished ST-70's. 

 

This is an amazing amp that comes with a one-year
warranty (output tubes not covered in the warranty. The driver-board-tubes are
sources from Jim McShane and are warrantied for 6 months.)

 

Here’s Mr. Ward’s description of the work done to
the amp (in quotation marks):

 

“The driver board is from Greg van der Sluys  from Classic Valve in Canada., and it is a pure triode octal
driver with a long tailed pair phase inverter having a FET equipped constant
current source (CCS). The CCS also incorporates an AC auto balance circuit,
which was developed by Fred Nachbaur and used in his famous design for the RA-100
reference amplifier. (www.dogstar.dantimax.dk/tubestuf/driver02.htm)

 

The driver board uses a section-matched pair of new
Tung Sol 6SN7 tubes, and a new 6SL7 tubes—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. 

 

I’ve installed the Dynaco Designs bias board for
individual adjustment of output tube current. Topside test points make the
biasing procedure easy. The Dynaco Designs bias board has pull down resistors
on each trimmer potentiometer. In the unlikely event a trimmer potentiometer
goes bad, the output tubes will go into cut off, and NO damage will be done. (This
is a fairly common failure scenario.  Several
 refurbished Dynaco's have ended up
at my shop with blown tubes or worse--all due to a lousy bias adjustment pot.)

 

I 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.

 

With the amp driven to clipping before the
rectifier tube output starts to sag in a big way, maximum cathode current per
pair of output tubes is about 50 to 175 ma . I chose this value to minimize the
inconvenient need to replace fuses if the needle is dropped on the record.
Better protection will be offered if a 150 ma fuse is installed (but, however,
this may be the cause of more frequent fuse replacements.) 

 

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. The
output tube plate supply has an additional 330 uf Kemet ALC10 series capacitor
mounted where the original quad cap was installed. The ALC10 series is a very
high performance electrolytic having characteristics similar to that of a 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 amplifier has multiple personalities. I
recently serviced a Mesa Tigris integrated amp, and I was inspired by the
Mesa’s multi-mode functionality. Similar to the Mesa, this custom Dynaco ST-70
now 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.

 

There are also modifications to the output
transformers. 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. I added a bolt

to each corner of the output transformers and
secured them “Linn tight.”    

 

 

 

 

Operating
instructions (see accompanying photos)

 

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
WPC.

 

The triode / ultralinear switching incorporates
relays positioned in between the output tube sockets. This scheme reduces the
need for extended output transformer wires, and allows for one switch to switch
both channels from the ultralinear mode to the triode mode of operation.

 

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 up 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
distortion. 

 

It should be pointed out that with the amp in the
triode mode, (also reduces feedback slightly)

the feedback source switch in the 8-ohm tap
position, and the feedback ratio switch in the mono

position, that there will be little feedback
applied ( about 6 to 8 dB if my math is right ) . The distortion will increase
to that which is typical of a SET amplifier ( a few percent ) I will also point
out that the amplifier will remain a push pull amplifier so that much desired
2nd harmonic will 

not be at SET levels. What I will say is that I
think I have successfully mimicked the Mesa Boogie Tigris operation with its
multiple modes and feedback adjustments. “

 

 

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
audio source/s.

 

With the exception of the tubes, Herb Ward
warranties this unique amplifier for one year from date of purchase. If
anything goes wrong with the amp during the first year, send it back to Herb
(on your dime) and he will repair the amp.

 

When you buy this amp I will send along the simple
biasing instructions written by Herb Ward. Biasing takes all of 5 minutes,
probably less.

 

Note: If you have a Dynaco amp that you want to
send to Mr. Ward for improvements and modifications let me know. Know that this
particular custom Dynaco ST-70 required about 40 hours of labor and $500 in
parts

 

I really don’t want to ship this amp outside the
US. Tubes will be removed and labeled and packed with the amp in a heavy-duty
double box.

 

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.

 

Thank you.