VTA70 mod installation
photo of stock ST70 before mod
1 - remove all tubes from amplifier
2 - turn amp over and remove bottom cover
3 - unsoldier all wires from the old PCB and mark them
with the corresponding number or function
4 - the wires to pins 3 & 18, 6 & 21, 11&14, and pin 19 will no longer be used,
so these wires can be completely disconnected on the other ends.
wires removed from original PCB
5 - the old bias components (2 - 50uF caps and 2 - 10K 2w resistors) can be removed
and thrown away. Also the connections to the 2 original bias pots can be removed
and thrown away. The selenium rectifier can be removed and thrown away.
6 - the 6.8K and 22K resistor on the quad cap can be removed and thrown away.
The 2nd and 3rd section of the quad cap should be connected together
with a piece of wire, and then a new 2.2K 2w resistor added between the 3rd and 4th section.
The choke will remain connected to the 1st and 2nd section of the quad cap.
The end of the new 2.2K 2w resistor on the 4th lug will connect to the PCB B+ connection
( see photo below just before step 12 )
7 - the front panel components will need to be (temporarily) removed so the old PCB can
be easily taken out and the new PCB installed.
If you are going to keep the octal sockets the wiring to them can remain, just remove
the screws and move them out of the way. Same with the stereo/mono switch and input jacks.
Alternatively, you can remove them all and insert a thin aluminum panel to cover all
the empty holes, and install new RCA input jacks and test jacks if you prefer (see photo).
8 - remove the old PCB
original PCB removed
9 - put a new 4-40 x 1/2" screw thru each PCB mounting hole from the top side of the chassis,
and on the bottom side attach a 4-40 x 1/4" spacer to the screw. Then mount the new PCB
and thread a nut onto the end of the screw. The four screws & spacers may need to be
slightly loose when you do this to align the PCB. Once the board is aligned make sure
the screws and hex spacers and nuts are all tight.
spacers installed to mount new PCB
10- begin reconnecting the wires to the PCB. I suggest placing the stripped and tinned wire
into the hole in the board and then quickly soldering the wire to the pad.
As you complete each wire, remove its masking tape identifier. Some of the wires may need
to be replaced if they are too short.
new PCB installed
11- here is a list of connections that must be made:
(a) attach the input jacks (was 7&17) to the L-input and R-input of the new PCB,
(b) attach the input ground(s)
(c) attach the 4 wires from the output tubes (pin 6) to the outer edges of the new boards
at the 270K resistor (was 1&2&22&23)
(d) attach the NFB wires (was #12&13) to the Left and Right NFB connections on the new PCB.
(e) attach the ground wire (#9) and the B+ wire
(f) attach the bias power supply wire (red/black) from the power transformer to the PCB
where it is labeled "-50VAC", This wire will need to be spliced to make it long enough
to reach the PCB at the striped end of the diode D1. This diode replaces the old selenium
rectifier diode on the ST70, and the old part can be removed and thrown away. (see #5 above).
(g) finally, the filament connections need to be made.
connect the original filament wires (#4&5) to the outer tube on the PCB (pins 4/5 & 9)
and do the same on the other side with the filament wires (#15&16) to the other outer tube
(pins 4/5 & 9) on the new driver PCB. (see photo).
connections made to new PCB
close-up view of new bias resistors on output tubes and 2.2K resistor on quad cap
12 - inspect each connection again and carefully look for signs of melted insulation near adjacent wires
or PCB traces to insure that no electrical shorts can occur.
Double check all steps above and your wiring.
Remove the old 15.6 ohm resistors connecting pins 1 & 8 on the output tube sockets, and replace with
the four 10 ohm resistors. The jumper between the pairs of tubes should also be removed.
You now have a 10 ohm resistor on each output tube instead of a pair of output tubes sharing the
old 15.6 ohm resistor. The tube bias can now be measured across each of these resistors
new bias resistors and overall view of new installation
New DC Bias Circuitry
The original Dynaco ST-70 used one bias potentiometer to adjust a pair of output tubes
(one bias pot for the two tubes in each channel). To measure the bias current flowing in
the pair of output tubes, Dynaco had placed a single 15.6 ohm resistor in the current path
for both tubes (from pins 1 & 8 to ground).
The new VTA driver PCB has one bias pot for each output tube which will allow you
to balance the currents in each output tube individually. This way you can measure the
voltage across each resistor and therefore the current thru each output tube individually.
With the 10 ohm resistors in place the proper bias adjustment voltage reading is now
0.40 VDC (NOT 1.56 VDC as in the original configuration) which corresponds to 40mA
of tube current. This way the output tubes can be perfectly matched and balanced.
This also lowers the current about 20% from the original which will give you much longer
tube life and less heat, with no loss of performance.
Dynaco conveniently routed a wire from the original 15.6 ohm resistor to the octal
socket mounted on the front of the amplifier. You will now need to add an additional
wire to another unused pin to permit both cathodes voltages to be present at the octal socket,
for a convenient place to measure while the amplifier is in use.
Final Checkout & Initial Power Application
1 - If you have a spare 1A quick blow fuse handy, replace the original power fuse (3A Slo-Blo) temporarily.
2 - If you havenít already, remove the rectifier tube. Plug in and turn on the amplifier.
Examine the filaments on the new replacement driver board. All three should be illuminated.
If any tube is not illuminated, turn off the amplifier (remove the plug from the AC socket) and
check the filament connections to the new PCB. Refer to the paragraph above regarding
Filament Power. Reapply power - all three driver tube filaments should be illuminated.
Remove power from the amplifier.
3 - Remove the three driver tubes and insert the rectifier tube and the two left channel
output tubes. Do not install the right channel output tubes right now.
4- Replace the original 3A fuse. Make sure all four BIAS pots are set to their center position.
Connect your DC Voltmeter to monitor the current thru the output tubes by
measuring the voltage across the 10 ohm resistor located at pins 1 & 8 of the output
tubes - connect the Negative lead (Black) to the chassis and connect the Positive
lead (Red) to the junction of the 10 ohm resistor and the output tube pins 1 & 8.
Monitor the voltage as the tubes reach operating temperature, which takes about 15-25 seconds.
If the voltage begins to exceed 0.60 volts quickly remove power. Otherwise adjust the Bias pot
to obtain 0.40 volts. Then check the other output tube and adjust for 0.40 volts.
Once they are the same, go ahead and turn off the power.
tubes 4 hifi |
"the path of least resistance is through a vacuum"