Modifications For The Dynaco™ ST-70 Basic Amplifier

Stock amplifier shown. (I
no longer have pix of a modified amp!)

Schematic

The downloadable schematic has been replaced with a high resolution pdf file as
of 2007.01.23 The power supply schematic is not shown, but is generally assumed
to be identical to the original except as indicated in the amplifier schematic.

Parts
List

Download
board artwork
Use at your own risk! No Warranties of any kind. That
said with some care boards can be made using this information.

Installation
Instructions
For those of you who might have acquired a used board
and want to reuse it.

Note: This information is provided for personal use only. All information and
design remains the property of KTA/Kevin Kennedy and may not be used for
commercial purposes without permission. Licensing for commercial use available
upon request.

KTA ST-70
Driver Board shown for informational purposes. Not currently available for
purchase.
Layout orignally taped by hand back in late 1989. (All genuine
KTA pcb based on this artwork.)

Introduction

The modifications detailed here really amount to a virtually complete gut
job on this amplifier. Only the transformers, chassis, and cage are
retained.. If you are like most, you will probably wonder why someone would
develop yet another driver circuit for the venerable Dyna ST-70. The reasons
are numerous, but not the least of which is the widespread availability of
this amplifier at a reasonable price, and the good quality of the output
transformers.

The ST-70 represents a very desirable package for the modifier as it
lends itself to a large number of modifications from the most simple to the
most complex, witness the AR ST-70-C3 that graced the pages of the Audio
Amateur about 20 years ago. This modification is rather less complex, but is
capable of exceptional performance nonetheless.

The new driver circuitry is fairly uncritical in terms of its layout
and lends itself well to pcb, perf board or point to point wiring, but very
high voltages will be present in operation, so exercise caution in the
construction, debugging, and operation of this amplifier.

If you have not yet purchased a Stereo-70 to modify you want to look
for one equipped with the A470 output transformers, as models with the
later Z326 (Japanese) transformers may not sound as good. The part number
can be found stamped on the transformer end bell facing towards the output
tubes, and you can also tell by whether or not the output transformers have
cloth insulation on their wiring, the Z326 has plastic insulated wires. In
addition the earlier PA060 power transformer is considerably larger and runs
somewhat cooler.

GENERAL DESCRIPTION

This design encompasses improvements in a number of areas besides
the obvious changes in driver circuitry. Linearity in the output stage is a
crucial issue to the overall sound of an amplifier, and for this reason I
highly recommend triode output operation unless you really need the
additional output power. The amplifier may be wired for ultra-linear or
triode mode operation or alternatively switches may be installed allowing
instant selection of either mode.

Operating the 6CA7/EL34 output tubes in triode mode results in very
low plate resistance as compared to pentode or ultra-linear operation as well
as very little harmonic distortion beyond the second harmonic. Low plate
resistance results in less phase shift in the output transformer, and better
power bandwidth in the output stage. Power sensitivity is reduced by about
6dB, and power output is reduced to 16 watts RMS.

Examining the circuit diagram you will notice the absence of the
original "Biaset" circuitry as well as the presence of several resistors in
the cathode circuits of all three stages. These resistors are important to
the design of the amplifier and should not be left out. They provide a
limited amount of current feed-back to linearize the individual stages. The
effect of this current feed-back is quite audible even in the output stages,
and results in a cleaner, more detailed and relaxed (unstrained) sound.

POWER SUPPLY

The new power supply is infact identical in most respects to the old
except in the area of supply decoupling on the driver board, refer to the
driver schematic for the values. (R20 and R22 are located on the Quad cap as
in the original circuit.) The bias supply topology is unchanged except that a
silicon diode should be substituted for the original selenium stack, and larger
values of electrolytic capacitor can be employed in the supply. Use 100V
parts here.

Some of you may want to utilize solid state rectifiers, although I don't
recommend this - in place of the 5AR4 rectifier, but if you elect to do this
you must use capacitors with a voltage rating in excess of 600 volts, or
series connect two capacitors of 300 volt rating or better with voltage
sharing/bleeder resistors in parallel in place of each of the individual
capacitors. The unused 5 volt winding could then be used to drive a full
wave voltage doubler, in turn supplying a LM317 wired to provide 6.3 vdc to
the input tube filaments....

DRIVER CIRCUITRY

The driver circuitry is relatively straight forward and may be built
on perf board. The design consists of a single 12AX7A used as voltage
amplifier shared between both channels, driving a pair of 12AU7A's connected
as cathode coupled inverters. See schematic.

Use of current feedback in amplifier stages has the effect of
reducing the stage transconductance resulting in higher slew-rates, higher
bandwidth and in addition lower distortion in a given amplifier stage. It
may also be used to control the overall gain in a stage. I prefer to use
this type of feedback when possible in place of or in conjunction with
limited global feedback.

The input stage provides an open loop gain of about 50, while the
cathode coupled phase splitter provides a gain of about 5-6 for an overall
gain of between 250-300. (approx. 48dB) Open loop bandwidth is well in
excess of 50KHz prior to loop compensation. Closed loop gain is approx. 27dB
and the input impedance is 100K ohms.

FEEDBACK

The overall loop feed-back is quite low at approximately 20dB, but in
this application global feedback confers some advantages in the areas of
distortion reduction, lowered output impedance (increased damping factor) and
most importantly reduces voltage output dependance on load impedance.

I feel that high levels of global feedback may make a device measure
better, but is ill advised in many applications because interstage delays
result in a delayed feedback signal which causes smeared transient response
and a loss of detail. Further, although global feedack may result in a
reduction of harmonic distortion, it also results in additional small
quantities of higher order harmonic distortion products.

Tube power amplifiers generally utilize fairly low levels of global
feed-back because output transformer phase shift at the frequency extremes
can cause oscillation if excessive feedback is employed, and in addition high
feedback levels usually require rather severe loop compensation to assure
that gain is less than unity at the point at which phase shift becomes 180
degrees. Failure to adhere to these criteria usually results in an
oscillator.... I try to design circuits with fairly constant feed-back
margins in the audio pass band, but this does imply careful control of gain
across the audio bandwidth because excessive gain levels require
progressively earlier roll-off of the voltage amplifier stages in order to
prevent oscillation at one frequency extreme or the other...

"The one last thing you need to know is the impedance of your
loudspeaker because in some situations performance will be enhanced by taking
the feedback from the actual tap in use rather than from the more traditional
16 ohm taps." Edit 2007.01.23: I no longer recommend this, and instead
suggest always using the 16 ohm tap of the A470 for feedback. I have noticed
a definite difference in sound quality in favor of the 16 ohm tap regardless
of the actual tap the speaker load is connected to. This may or may not
apply to non-Dynaco transformers so I advise experimenting. In the case where
you have a transformer with 4.3K primaries and only 4 or 8 ohm taps use 499
ohms or 715 ohms respectively. Note that the values of compensation components
C1 and R6 may need slight adjustments for best squarewave response at 10kHz.

Edit 2007.01.23: Improved compensation values for C1 and R6 based on
feedback from a builder planning to use the amplifier primarily in UL
connection. These values are also recommended for use with triode
connection.

CONSTRUCTION

Start by completely gutting the candidate amplifier, except for the
transformers and the associated AC power wiring. You should probably start
by replacing all the tube sockets with new ones purchased from Sound Values
or your favorite broker. I would recommend replacing the quad cap with a new
one, or better still with one having three or four sections of 40uF at 500
volts or better. You can also build out the capacitors with the diminutive
Panasonic snap mount types. The bias pots are best replaced by components of
equal value. (Ohmite makes a very nice mil spec locking pot that can be had
for much less than $10.00 each. Check Digikey catalog.)

The old input jacks and output terminals should be replaced with something
better. I favor the Edison Price MusicPost™ for the outputs and Vampire™jacks
for the input. Be certain to mount the input jacks so that they are isolated
from the chassis. Remove the old octal sockets on the front apron and replace
with pcb material drill to accept a pair of pin jacks or similar..

Depending on condition consider replacing the octal sockets for the
rectifier, and output tubes.

Now you should have a chassis that is basically empty, and ready for the
construction of the new amplifier. As I no longer have a pcb available for this
modification you can either layout the circuit on a pcb, or build it on Vector
Board™. Building on vector board is quite easy. I recommend that the board be
sized very carefully to the existing opening, and that boundaries be marked.
The socket for the 12AX7A should centered, but slightly forward towards the rca
jacks, the sockets for the 12AU7A's should be symettrically placed about 1/4th
of the way in from the edges at either end. Allow room for the coupling caps to
the EL34's by each 12AU7A, and for the grid rtn caps nearby. (Place in a C
around the 12AU7A's.)

I constructed my prototype on perf board and have had no reliability
problems at all. The power supply I elected to use is basically similar to
the stock supply with the exception that it has higher capacitance than when
it was stock. I used metal film resistors from Dale, IRC and Resista, the
coupling caps are CDE WMF series parts which I recommend highly, although
some of you may object to them I find them to be quite good, and one of the
few that will fit (barely) in the space available, but if you feel inclined,
bypassing them with 0.01uF MIT Caps would be a nice enhancement...

I mounted conventional chassis mount tube sockets on the perf board
upon which I built my prototype. Almost all of the parts were mounted on
the board, but clearance is a little tight. The 12AX7A should be mounted
near the input sockets and the two 12AU7A's were mounted centered about one
inch from the edge closest to their respective output tubes. To avoid the
possibility of ground loops, only one point should be grounded to the
chassis. In addition the input grounds and feed-back grounds should be one
and the same for their respective channel.

Edit: I strongly recommend better quality coupling caps than fitted in the
old kit boards. Caps by REL or Clarity Cap are recommended.

TUBES

I recommend the Tungsram 12AU7A for the cathode coupled phase
inverter circuit and a Sovtek 12AX7WXT+ for the input stage. I use vintage
Mullard EL34 matched pairs in the output stages and an Amperex 5AR4
rectifier in the power supply.

Edit 2007.01.23 Current production JJ 12AX7A, 12AU7A, and EL34
sound great in this design. The EH EL34 is another well liked type. I recommend
all power tubes be burned in and matched for at least 24 hours prior to
purchase. Don't buy untested, unmatched tubes for this or any amplifier
project - this one thing will save you a lot of grief. Removed reference to
ersatz 5AR4 as I doubt this is a problem any longer. Use a reputable current
production tube from JJ, or Shuguang. To improve reliability and longevity
install a single diode (HF4007 or eq.) in series with each plate (banded end
connected to socket) which will reduce or prevent the occasional arc during warm
up or under transient conditions.

THE SOUND

I have never been impressed with the performance of any of the several
Dynaco tube amplifiers I have owned, having owned McIntosh MC-30's, a H&K
Citation II and assorted other tube power amplifiers, against which to
compare them, but I am impressed with the performance of this one. This
amplifier is detailed, clean, and open, with good depth and imaging.
Performance is excellent to both frequency extremes, with a very sweet, but
not euphonic mid-range. Voices and piano are reproduced with great realism
and clarity. The amplifier is overall very neutral and detailed without the
stridency present in many other amplifiers sharing these virtues.

Page Maintenance Policy (New)

This page is regularly updated and maintained. Several hundred of these
amplifiers have been built by enthusiasts from scratch as well as from the
board kit I used to sell.

I would like to note that the way I develop circuits has evolved quite a
lot in the 17yrs since I designed the mod and wrote this article. This
represents one of my earliest serious efforts at attaining better tube sound
and is a reflection of what I knew at the time. It has also been one of my
greatest long term successes, people are still building the original design
today.

©1989, 1994, 1998, 2001, 2007 By Kevin R. Kennedy