DYNACO ST70
RE-ENGINEERING.
In 1998 I bought an ST70 which was offered to me for $200 and which was
in appalling condition
after being parked in a garage for 15 years after a couple of
output tubes had died.
There were lots of problems within the amp and I didn't like the
design much, and figured that the power supply
could be a lot better and the driver amp could be improved so I decided
to strip the whole amp and start all over again.
I rebuilt the power supply but found the stock standard power
transformer became very hot, and since it was a
110V mains version and since I have 240V here in Oz then I decided to
rewind the power transformer but with
the core lamination stack height increased 25mm. The power transformer
then ran cooler.
Unfortunately, I didn't photograph it before I sold it easily when I
placed it in a local hi-fi store for sale on consignment.
I did post the schematic for the amplifier on the last edition of my
site, but it was a little untidy so I have re-drawn the
schematic to include the power supply and basic protection circuit, and
added a minor change to
the bias resistor values for the output tubes so that now this
schematic would be a fine schematic
which anyone could use to make a new circuit to use to make a stereo
integrated amp with EL34/6CA7, etc.
The schematic is a rather cluttered since it includes power supply,
protection circuits, and one channel of the integrated
amp. I have since learnt to create schematics in MS Paint, slow and
laborious, but better looking, although
I would use about 3 sheets to contain the same info as in the schematic
below.
Modified Schematic for Dynaco ST70 or other new amp.
This schematic is very close to what I used to replace what was in the
old ST70 I had
bought.
The tube rectifier was removed and silicon diodes used for rectifying
HT voltages in a doubler configuration.
People say voltage doublers have poor regulation but they havebetter
regulation than a full wave rectifier using a GZ34. McIntosh
used silicon diodes and doublers without any problem in their tube
ampos as soon as suitably rated silicon diodes
became available. Much bigger value
filter capacitors were fitted and placed in a box on the chassis top
and in the area available where the old caps and tube rectifier used to
occupy.
Instead of using the original circuit board with the 2 original
triode-pentode input/driver tubes, I replaced the
whole board with a small brass plate and with 4 ninepin sockets to
allow the use of four 6CG7.
Each channel was wired as above, V1 being the line stage pre-amp
with gain = 20dB, ( 10 x ) and V2/V3/V4 SET input and LTP driver stage.
The V5/V6 output stage is a normal cathode biased EL34 class AB1
ultralinear output stage.
There are LF and HF stabilizing networks shown which will suit owners
of ST70.
R13&C5 form a zobel network to reduce HF gain and phase shift
of the input triode V1.
R25&C15, and R29&C16 are zobel networks across the primary and
secondary of the OPT respectively to
also control HF gain, phase shift, and to lessen ring on transients.
In addition a phase advance cap, 1,000pF, C28, is used across the
global feedback resistor, R39.
LF gain and phase shift is controlled by C4 and R12A.
The ST70 OPT are not good enough to to give the amp unconditional
stability without these networks in place.
Warning, the values shown suited the
amp I modified but anyone should not assume the phase tweaking
networks shown here will work if the OPTs they may have are different
to what I found in this amp.
The LTP with 6CG7 has my typical use of a transistor, MJE340, constant
current source.
The amp has output tube cathode bias, and there are 39V zener diodes
in series with 50 ohms across each cathode R25&R26 to limit the
rise of voltage in the cathode bypass
caps when the amp
works hard in class AB. The 50 ohms in series with the zeners is a low
resistance path for
excessive cathode currents, and if there is a large enough voltage
across these 50 ohm resistors
for longer than a few
seconds, a voltage which is high enough at the top of R31 can turn on
the SCR which then opens the Relay 1 which interrupts the
HT supply winding on the power transformer and the B+ will then fall to
0V, and the amp will stay shut down.
Resetting is by turning off, waiting 20 seconds, and turning back on.
If the SCR latches on, the fault LED will light up. Under normal
music conditions, there is seldom any rise in the cathode
voltage,
and thus the bias voltage virtually remains fixed, but if a tube fails
or a there is a some other problem
then there is active protection against excessive tube dc currents and
the OPT will not be stressed.
Two years after the amp was sold, the owner had an output tube
develop a dry soldered joint in one of the output tube heater pins, and
this caused just one tube of two in one channel to do all the work,
thus making it work a lot harder.
The amp began to trip the active protection circuitry, so he brought
the amp to me,
and the problem tube was resoldered within 1/2 an hour, and problem
fixed.
Sorry, no pictures are available of this Turnerized Dynaco, which
was
sold before
there was a Turner Audio website.
The appearance was relatively unchanged except for absent tube
rectifier, capacitor box on top of the chassis,
increased size of power transformer, 12 x 8mm dia holes drilled
through steel chassis top around each output tube
socket, holes drilled in the bottom cover, brass subchassis for
more input tubes, selector switch, balance and gain controls
on the front, led indicators, and a black painted original cage over
the amp.
The new
owner said he liked to be able to hear the musicians turning their
pages,
and he liked the clear, warm, dynamic and untiring sound.
Back to Index
Page