Fender Twin Amp 1994 Service Manual - Theory of Operation(Cont)
’94 TWIN AMP
THEORY OF OPERATION(CONT)
Reverb return circuit uses JFET Q1. Without a footswitch connected to the footswitch jack, diode CR17 enables the
reverb to function. V6A amplifies the reverb return signal. Note that V6A should provide a minimum gain of 49. The
output from V6A drives the Reverb control (R62), then sums with the dry signal through R63.
V6B feeds the Cathode Follower V7B, which drives the Preamp Out jack. The signal is normalled to the Power Amp
in jack and then feeds V7A. V7A contains the Presence control, accepts feed back from the output and feeds the
Phase Inverter V8.
OUTPUT SECTION
The output tubes (V9, V10,V11, V12, 5881/6L6WGC) are arranged in a push-pull configuration that will produce
100watts into a 4, 8, or 16
Ω load. The Impedance Selector (S10) will switch between the respective taps of the
output transformer secondary. CR1&CR4 are flyback protection diodes that prevent the output transformer from an
over voltage condition. This can occur when the output transformer experiences an open load. A separate
secondary winding drives an XLR jack (J13), which provides a balanced line out with a floating ground pin (pin1).
This can be used to drive a slave amp and additional speaker cabinets. It can also be used as a send to a mixing
console. However to get a useful sound, the signal should be pre-equalized before the console input. A low-pass
filter set a 5 to 6 kHz with a minimum 18 db per octave slope works well.
BIAS
The Bias is set by measuring the voltage drop across the Flame Proof 1
Ω resistors R89 & R90. If the voltage drop
reads 80mV, then the current through the resistors will be 80mA (E/R=1) = (80mV divided by 1
Ω = 80mA). CR2 & 3
are protection diodes for R89 & R90. If an output tube shorts, the fault current will shunt through the diodes instead of
R89 & R90.
BIAS ADJUSTMENT
The Bias Adjust and Balance Adjust controls, along with the Bias and Balance test points are located on the rear
panel for easy access. Allow the amplifier to warm up for at least 2-3 minutes with the Output switch set to High, and
the Standby switch in the On position. With a digital voltmeter set to its most sensitive DC voltage scale; connect the
test probes to the test points (J11 & J12) labeled “Bias”. Adjust the Output Bias Adjust control (R94) to read .08 VDC
(80mVDC). This will Bias V9 &V10. Now connect the test probes across the “Balance” test points (J10 & J11). The
meter is now referenced to the 80mVDC instead of ground. Adjust the Output Balance Adjust control to obtain 0VDC.
This precisely matches V11 & V12 to V9 & V10. For optimum sonic performances, set Bias to 80mVDC, then set
Balance.
For optimum sonic performance with increased tube life, set Bias to 60mVDC, then set Balance.
For optimum tube life, set Bias to 40mVDC, then set Balance.
Note: If a bias measurement of 40mVDC cannot be obtained, it may be time to replace the output tubes.
CHANNEL/REVERB SWITCHING
A 27.5VAC signal is tapped off from the Brown secondary of the power transformer. This signal is presented to the
Footswitch jack. By rectifying the positive or negative half of the waveform, a DC control voltage is created. This
voltage is used to control several opamp comparator circuits.
TROUBLESHOOTING TIP
Most channel switching problems will probably be related to intermittent or sticky relays. However troubleshooting the
switching circuitry is easy. All test point voltages are on the schematic. First verify the reference voltage at the
comparator. The reference voltage is applied to the non-inverting input of the opamp. Then measure the output of
the comparator. If it doesn’t toggle properly, check the control voltage. If the control voltage is not correct, hang a
scope probe on the footswitch jack. Look at the AC waveform, activate the channel select switches, and look for the
rectified waveform. The Reverb is switched by rectifying the negative side of the waveform. The Gain Select and
Channel Select are switched by rectifying the positive side of the waveform at two different voltage levels. Diode
CR16 and Zener CR18 set the two voltage levels.
POWER LEVEL SWITCHING
Via the output switch (S9) the output power of the amplifier can be switched between 25 and 100 watts. S9A selects between the
center tap and full winding of the high voltage secondary of the power transformer. This causes the B+ and Z supplies to