SR70A Driver Module for Dynaco ST-70 Installation instructions

Mapletree Audio Design SR70A “Special Red”
Driver Module for Dynaco ST-70
Installation instructions
Rev. Jan. 9/13
The “Special Red” SR70A driver module is a drop in replacement for the original driver board of the Dynaco
ST-70 power amplifier. Unlike the original, all wiring on the SR70A is point-to-point with connections to the
amplifier made to turret terminals. The schematic diagram of the SR70A module identifies the turret connections
to the ST-70 circuitry. The instructions identify which of the original wires and components are to be removed
and discarded and the new wiring that is required using the silver-plated Teflon wire supplied. When making a
new connection from one of the SR70A turrets to a point in the amplifier, first route the wire as shown in the
diagram in order to obtain the correct length. Then strip ¼ inch of insulation from each end and melt a small
amount of solder on the exposed wire by heating it with your soldering iron and melting the solder on to the
wire (pre-tinning). The best connection to a turret is made by inserting the pre-tinned end of the wire into the
hole in the top of the turret, leaving a bit of wire exposed, then heating the turret with your soldering iron while
adding solder so it runs down the wire inside the turret.
Note: It may be necessary to remove the power take-off sockets on the front apron of the ST-70 to allow room
for the SR70A board. If this has not already been done in previous modifications to your amplifier, the
following instructions should be followed.
1. Unsolder both ends of all wires connected to the lugs of the two power take-off sockets on the front
apron of the ST-70.
2. Remove the sockets.
3. The stainless steel filler plates supplied can be attached to the inside of the chassis using the original
socket machine screws or the #6 sheet metal screws supplied.
4. If it is desired to provide external access to the bias test voltage for the EL34s, you can mount suitable
test jacks in the filler plates, making sure you maintain clearance for the SR70A board. These jacks
should be wired to lug 1 of V3 (L) and V6 (R). The bias test voltage (1.56 V) is then measured from
each jack to the chassis (ground).
Refer to Figure 1 showing the original wiring to the printed circuit board, with the power take-off sockets
removed.
1. Unsolder all wires and resistors from the input RCA jacks and stereo-mono switch on the front panel. This
assumes that the stereo-mono switch will not be used.
2. Unsolder both ends of all wires connected to the eyelets on the original board that are marked with an “X”. The
two wires connected to the 16 Ohm taps of the output transformer should only be unsoldered from the eyelets on
the circuit board.
3. Remove the original board from the chassis.
4. Install the SR70A board using the original screws/nuts or the 4-40 screws and hex nuts supplied.
5. Unsolder the 22K resistor between lugs 3 and 4 of the original 4-section filter capacitor and discard.
Refer to the wiring diagram for the SR70A board connections (Figure 2).
1. Solder the twisted pairs of wires pre-soldered to lugs 7 and 8 of the 6SL7 tubes V4 (left) and V5 (right) on the
SR70A board to lugs 2 and 7 of V3 (L) and 2 and 7 of V6 (R).
2. Solder the wires coming from the 16 Ohm speaker terminals on the rear panel to the FBL and FBR turrets on the
SR70A board.
3. Measure, cut, and strip silver-plated Teflon wire lengths and make the following connections:
i.
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
xi.
xii.
xiii.
RCA left input jack ground lug to RCA right input jack ground lug
RCA left input jack ground lug to the SR 70A ground (GND) bus
RCA left input signal lug to INL turret
RCA right input signal lug to INR turret
Filter capacitor lug 4 to turret B1
Filter capacitor lug 3 to turret B2
Chassis ground lug (next to filter capacitor) to the SR70A ground bus
Center lug (2) of the right bias potentiometer to the BiasR turret
Center lug (2) of the left bias potentiometer to the BiasL turret
Lug 6 of V6 (EL34) to turret G6
Lug 6 of V7 (EL34) to turret G7
Lug 6 of V2 (EL34) to turret G2
Lug 6 of V3 (EL34) to turret G3
This completes the installation of the SR70A module. Re-check all connections using the wiring diagram as a guide.
Install the four tubes with the 6SJ7s toward the front of the chassis. It is recommended that you re-adjust the bias for the
output tubes in the usual manner.
Notes:
1. The SR70A can be supplied with a heater voltage of 6 VDC, 0.6 A for each channel if desired.
2. If non-stock Dynaco output transformers are installed, it may be necessary to adjust the value of C1 (270
pF) in each channel for optimum transient response (as evidenced by the best square wave reproduction
and/or the flattest high frequency response).
2
power take-off socket
and wiring removed
power take-off socket
and wiring removed
X
XX
X
X X
Unsolder and remove all wires and components from
input jacks and stereo-mono switch
X
X
X
X
original driver board
X
X
Unsolder both ends of all wires
and components marked X and discard.
.
unsolder at eyelet end only
unsolder at eyelet end only
X
X
X
X
X
X
SR Board Installation FIgure 1
3
stereo-mono switch
input jacks
power take-off socket
filler plate
INL
INL
INL
INL
R
INR INR
V9
V8
LL
FBL
V8
R R
V9V9
B2 B2B1B2
B1
B1 FBR
GND
GNDBUS
BUS
gnd bus
88
G6 G7
V5
TWISTED
WIRES
G6
BiasR
8
777
G7
G2 G3
BiasR
V4
BiasL
8
BiasL
77
8
7
G3
8
G2
TWISTED
WIRES
L
power take-off socket
filler plate
FBL G2 G3 BiasL BiasR G6 G7 FBR
SR Board Installation FIgure 2
4
Mapletree Audio Design
SR70A “Special Red” driver module for Dynaco ST-70
© Copyright Lloyd Peppard 2005-13
rev. Jan. 8/13
RIGHT
V5
6SL7GT/5691
V9
6SJ7/5693/6SJ7GT
1
R. Input
INR
R1
10K
8
260 V
2
3
45 V
6
4
R2
3.5K
R6
10K
R5
100K
BiasR
R. bias pot
-32.8 V
81 V
6
R10
150K
4
C4
1/400
5
C1
270 pF
2
6SJ7
8
R12
221K
260 V
7
C6
0.1/400
7
6SL7
FBR
To 16 Ohm tap
on right OPT
Pin 6 of V7 (EL34 with
white/blue plate lead)
R11
221K
R8
54.2K
R7
475K
R13
3.5K
G7
3
C3
200p
R3
100
Gnd Bus
R9
150K
1
C2
0.047/400 R4
475K
5
R14
475K
69 V
C5
0.1/400
G6
Pin 6 of V6 (EL34 with
blue plate lead)
To R. heater voltage
(Lugs 2 and 7 of V6)
220K 1W 5%
LEFT
V4
6SL7GT/5691
V8
6SJ7/5693/6SJ7GT
1
INL
R1
10K
L. Input
8
3
R9
150K
1
45 V
C2
0.047/400
5
260 V
2
6
4
R14
475K
69 V
C5
0.1/400
2.3 V
R2
3.5K
R6
10K
R4
R5
475K 100K
R8
54.2K
BiasL
R10
150K
C4
1/400
C1
270 pF
2
6SJ7
5
R12
221K
G3
260 V
7
C6
0.1/400
7
8
6SL7
FBL
To 16 Ohm tap
on left OPT
L. bias pot
-32.8 V
6
4
R13
3.5K
R11
221K
81 V
R7
475K
Gnd Bus
Pin 6 of V2 (EL34 with
white/blue plate lead)
3
C3
200p
R3
100
G2
B2
Pin 6 of V3 (EL34 with
blue plate lead)
B1
To L. heater voltage
(lugs 2 and 7 of V3)
428 V
121 V
3
+
Gnd lug on chassis
397 V
4
remove
22K
+
6.8K
2+438 V
1
+
+
Quad filter capacitor
Plate voltage = 425 V
Screen voltage = 423V
5