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.
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