Fender Bassman 300 Service Manual - Circuit Description
BASSMAN 300
(This is the model name for warranty claims)
CIRCUIT DESCRIPTION
Preamp
The preamp section employs two 12AX7 tubes. Channel one (V1) is fairly conventional, while channel two (V2) uses
a second tube with additional op-amp gain (U2B) to provide a 60's style overdrive sound. The op-amp is used as
straight gain and is never allowed to become non-linear.
In typical operation, the drive channel and the clean channel are presented to the channel 1/2 mix control (R17) so
that a "blend" can be established. The channel select button (S9) on the front panel (or it's foot switch counterpart)
selects between this channel mix or channel one only. This selection employs JFETS Q1-Q4.
Signals are then presented to a second - order Sallen and Key filter set to F0 of 36.6 Hz and a Q of 2.1. This filter
provides about 6.5dB of boost at 41 Hz.
The signal path is routed to U17B where low-boost (S3) , mid-notch (S2), and high-boost (S4) equalization switches
can be used. These circuits are followed by conventional Baxendall low (R58) and high (R62) equalization. The
Baxendall circuit output is then sent to a graphic equalizer which can be selected by the footswitch or graphic EQ
button (S7) on the front panel. The far-right graphic EQ slider (R71) is a +/- 10dB trim for graphic EQ gain
compensation.
Next, the signal is presented to a second-order state-variable filter before being presented to the compressor circuit.
Said filter is set to 420 Hz. At the compressor, low-frequency compression ratio (R100) and high-frequency
compression ratio (R119) are adjustable to suit the user. Audio is presented to the "compressor EQ balance" (R102).
The compressor EQ balance controls the mix ratios of the compressor outputs. A compressor gain trim (R80) is
provided to compensate for the amount of amplification lost due to using the compressor. The compressor-gain trim
control is able to compensate for a compressed signal of up to 10dB.
The FETs Q5 and Q6 select between pre- and post-compressor via the front panel switch (S8) or footswitch. After
passing through op-amp U9B and master-gain control R107, the signal is sent to output jack J2, effects-loop send.
The signal input at J3, effects-loop return, goes to differential receiver U11A.
Heater voltage (24VDC for 2 series-connected 12AX7s in the preamp) is derived from the 32VDC supply
(+/-16VDC).. Excess voltage is dropped across across paralleled ballast resistors, R354-R356, on the preamp PCB.