Fender TONE-MASTER Service Manual - Theory of Operation
TONE-MASTER
THEORY OF OPERATION
The input to J1 feeds pin 7 of V1B. This is the first preamp stage for both the A and B channels. The
signal exits through pin 6, and from there the signal splits, one side goes to R1, the A channel VOLUME
control and the other side goes to R5A, the B channel GAIN control. SW1 is the FAT switch for the A
channel. SW1 connects the Capacitor C1 to ground to provide a 3dB increase in gain, mostly in the lows
and mids. Relay RLY 1A activates that circuit automatically when switching to the B channel. Resistor
R15 is there to prevent “popping” of the A channel FAT switch when actuated. NOTE: The FAT switch
may pop the first time it is switched. This is due to the tubes warm-up/stabilization cycle.
For the A channel, the signal goes from the VOLUME control to pin 2 of V1A. From here it exits via pin 1
and then feeds the tone control driver section. The signal leaves the tone controls through the TREBLE
pot, R2. The signal then goes to the LIne Transformer T5, then to the Effects-Loop SEND. The signal is
then routed to the EFFECTS RETURN jack, then to Line Transformer T6 and on to Relay RLY 1B, which
provides the channel switching from A to B. For the B channel, after the signal leaves the GAIN control
R5A, it goes to V2B pin 7. The signal feeds to the tone control driver section. Switch SW3 is the B
channel FAT switch, operating on the same principle as the A channel FAT, only this time across R26.
The signal leaves the tone controls through the TREBLE control R6. The signal then goes to the Line
Transformer T3 and the Effects Loop SEND, just like the A channel, then back in to Line Transformer T4
and then to Relay RLY 1B which performs channel switching. Relay RLY 2A is used to mute the channel
that is not in use so bleeding of channels is eliminated. Relay RLY 2B is used to light LED 1 in the B
channel mode. From the channel switching Relay RLY1B, the signal then goes to the Phase Inverter V3A
and V3B. It is a classic Fender-style split load type, used without feedback. Capacitors C16 and C17
isolate the high voltage of V3’s plates from the B channel master VOLUME control R9. Relays RLY 3A and
RLY 3B switch the B channel master Volume control out of the circuit when the amp is in the A channel
mode. Capacitors C18 and C19 isolate R9 from the negative voltage (Bias) supplying the 5881 power
tubes. The 5881s are used in a push-pull configuration that will produce 100 watts into 4,8, or 16 ohms.
The IMPEDANCE
Ω switch is labeled “Low, Med, Hi”. In normal operation of the Tone-Master, this switch
serves two functions. One, to set the output impedance of the amplifier to better achieve the cranked-up
sound at a lower volume level. On the B channel, with a single Tone-master enclosure, this switch should
be set according to the setting of the B channel VOLUME control. For volume setting of 3.5 and lower, “Lo”
would be the appropriate setting. For settings of 3.5 to 7, “Med” is best. For settings above7, “Hi” will
produce full power and tone. The impedance switch will not damage or reduce tube life at these settings.
What is does do is increase the gain in the “Lo” and “Med” settings and thereby better approximate the
“Full Volume” sound at a lower level. Diodes CR7 and CR8 are used to protect the output transformer from
arcing due to flyback voltage in the event of a no load output condition. The speaker output jack will short
to ground to protect the output transformer in case of no speaker cord plugged in.
SPEAKER ENCLOSURES
All Tone-Master speaker enclosures are configured at 16 ohms. The 212 enclosure (P/N 0813000000)
with tilt-back legs is loaded with Celestion G12-80 loudspeakers. This cabinet can produce a creamy blues
tone, sounding much like it looks. The 412 enclosure (P/N 081300100) features Celestion Vintage 30
loudspeakers. It offers a bright, full bodied sound, great for Hard Rock or Country. Another 212 enclosure
(P/N081300300) features a Vintage look and is loaded with two Celestion Vintage 30 loudspeakers.