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X-Amp Design
(Refer to schematic diagram)
Signal input to the X-Amp is by way of either the High or Lo input jacks. These are shorting type jacks
with input resistor arrangement (R62 & R63) such that the LO input is attenuated 6 dB with respect to the HI
input when either jack is used alone. If both jacks are used simultaneously then they each will have the same
sensitivity. The input impedance at the HI jack is 2.2 Megohm (R14): the LO input impedance is 20k ohms
(R62 plus R63).
The two triode stages of V1 serve as the input preamps for both the LEAD and RHYTHM channels. The
LEAD channel preamp is operated with a cathode bypass capacity for maximum gain whereas the RHYTHM
channel preamp employs no cathode bypass so that maximum headroom can be provided for clean playing.
Because V1 serves as the input preamps for both channels it is the most critical of the three 12AX7A's in the
unit. This tube should be selected for low noise and low microphonics.
After the RHYTHM channel preamp, the signal is processed through a fixed EQ network and then applied to
the RHYTHM volume pot. When the RHYTHM channel BRIGHT switch is actuated C14 is allowed to pass
high frequencies around the volume control. For this reason the BRIGHT effect decreases as the RHYTHM
volume control is increased past about "3". A4(A) serves as a level recovery amp following the volume control.
FET switch Q4 is used to mute the RHYTHM channel signal whenever the LEAD channel is selected. Note
that logic line Q goes high when the RHYTHM channel is selected and goes low when the LEAD channel is
selected. From A4(A), the signal is routed to A5(A) through FET switch Q5.
Following the LEAD channel preamp V1(A), the lead signal is processed through a fixed EQ network which
has two possible response characteristics depending on the setting of the HI LEAD switch. FET switch Q1
mutes the lead signal whenever the LEAD channel is not selected in order to prevent distortion products from
leaking into the (clean) RHYTHM channel. The two triode stages of V2 are used in series to provide high
levels of overdrive. The MASTER volume pot (P3) sets the amount of overdrive signal to be passed on to the
FET switch channel selector A5(A).
At the channel selector stage Q2 selects the LEAD channel and Q5 selects the RHYTHM channel. A channel
is selected when the control line at the gate of its FET is low. Logic lines Q and Q NOT toggle in response to
either the front panel PULL ON channel switch or in response to the foot switch; these lines also drive the front
panel LED's via inverted buffers at A6.
The signal selected at A5(A) feeds the effects loop drive amp A4(B). If an outboard effects is patched into the
loop then A4(B) drives the effects input and the effects output returns to A5(B) via Q10. If no effects is patched
in then the send signal feeds the return by way of R160. FET switches Q9 and Q10 toggle in response to the
EFF 1 button on the FS36 footswitch. The send and return levels are varied simultaneously by the effects level
switch.
The signal returned from the effects loop is processed through two tone control stages at A1(A and B) and then
fed to both the reverb drive amp IC3 (A and B) and the reverb return summing amp A2(A). When the front
panel tone controls are set to "5" these stages have a flat frequency response and unit gain.
The reverb unit is driven by the two op amps of A3 operated in paralleled for increased current capability.
Reverb drive equalization is provided by R58 and C31. A2(B) serves as the reverb return preamp and is operated
with 40dB of gain. Further equalization is provided by C32 and R56. FET switch Q6 turns the reverb on and
off. Logic signal conditioning for Q6 is provided by Q7 and one of A6's inverter buffers. The REVERB
control (P8) sets the amount of reverb signal returned to the reverb summing amp. The output of the reverb
summing amp A2(A) constitutes the output of the preamp section of the X-Amp and is tied directly to the input
to the power amplifier.
V3's two triode stages serve as a phase inverter for the power amps push-pull class A/B output stage. Power
pentodes V4 and V5 drive the output coupling transformer T2. Grid biasing for V4 and V5 (approx. -52V)dc is
determined by the voltage divider from by R126 and R127.
standby switch) is normally 100mA (50mA for 50 watt models) +-15mA. Power transfer to the loudspeaker is
maximized by selecting the appropriate output transformer tap at the speaker impedance switch. Loop feedback
around the power amplifier is provided by R67 ad C55.
(technical description)
Quiescent current (measured across the open
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