PRINCETON RECORDING AMP®™
(This is the model name for warranty claims)
9
bias circuit on the TZ/FX PCB. This is fed by the or-
ange-orange secondary. Trimpot R110 allows for
this adjustment. By observing the voltage across a 1
ohm resistor on the Main Tube PCB, the bias current
drawn by the 6V6 power tubes can be monitored and
set. Typically, a voltage setting of -36.8VDC at
TP7 will result in 20mA being drawn by each of the
two 6V6s. So, 40mV is read across the 1 ohm resis-
tor.
SILENT ADAPTOR
The SILENT adaptor circuit provides an analog
modeled response that accurately mimics the PRA’s
Jensen C10-R speaker. This circuit is found on the
upper left hand side on page 2 of drawing
0069346000. This circuit is not field serviceable.
L is the forward path output voltage from
power amp U11. When the silent adaptor is em-
ployed, this node is disconnected from the speaker
(Relay K1) and only drives the Silent adaptor circuit
(Relay K4A). The Silent adaptor’s output,
LS_TZ_OUT, feeds the reverse path in place of U23-
A (Relay K4-B).
TZ ATTENUATOR - USER INTERFACE
The Mode Selection circuitry is found on the lower
portion on page 1 of drawing 0069346000. A 16-
position (4-bit) encoder (S4) provides the manual in-
terface for the guitarist to select the configuration of
the TZ amp. Its state is read by four digital inputs of
microprocessor U24 through a ribbon connector.
This microprocessor is especially programmed for
the Princeton Recording Amp (p/n 0069439X00). It
outputs a serial data stream (SER_DATA_01) and
control signals to load two serial-to-parallel shift reg-
isters. One register holds data that feeds the DAC
(U25 to DAC = U18); the other register controls 4 re-
lays (U29 to relay drivers = U27 & U28). Other U24
digital outputs assist in controlling & configuring the
DAC. The last microprocessor output switches a
mute control circuit comprised of Q34 and Q35.
MUTE_TZ switches a JFET (Q33) to kill the signal in
the TZ amp’s forward path.
When the encoder is fully CW, the amp is configured
so the tube power amp feeds the speaker directly.
Moving CCW, the next 14 selections places the TZ
amp between the tube power amp and the speaker.
The selection that is next to the most CW setting
configures the TZ amp in its unity-gain mode (the
tube amp outputs and TZ amp would ideally be the
same at this setting). The speaker volume reduces
with each step CCW, but the level at output trans-
former remains the same. (This accounts for the
same level out of the Headphone and Line Out at all
TZ amp settings).
The most CCW of the encoder’s position configures
the PRA in its SILENT mode. With this, the speaker
is disconnected and replaced by the silent adaptor
circuit. When this is employed, output signals are
only observed at the Headphone & Line-Out in the
SILENT mode.
SPECIAL NOTE FOR SERVICING THE ENCODER
& MICROCONTROLLER: Two different types of En-
coders (S1) & Microcontrollers (U24) can be found
installed in this product. The encoder can either be a
binary code type or a Gray code type. The part
number of the microcontroller installed in the product
(see its label) allows the service person to determine
the encoder that is installed. Refer to the TZ/FX/IO
PCB parts list where p/n 0073684X00 is found to de-
termine the proper microcontroller & encoder
pairings.
MISCELLANEOUS CONTROLS
There is a pair of footswitch control outputs – P4_OD
& P1_COMP - that feed current from U5-A & B on
the IO PCB to the TZ/FX PCB. Their mating signals
on the TZ/FX PCB are OD_FS & COMP_FS (respec-
tively). Reference the middle of page 2 on drawing
0069346000. Both of the interfacing control circuits
on the TZ/FX PCB are the same.
