Sept. 9, 2002
Part 9: Input Stage
The input stage uses a 6K7 variable-mu (also known as "remote cut-off") pentode V19.
This is where the automatic gain control takes place during compression.
I was quite amazed at how well this "legacy" remote-cutoff pentode performs. The JAN
metal version is very quiet from a microphonic as well as hum standpoint; the later glass 6K7G
does not fare nearly as well in either category.
To help in reducing hum, the filament for this stage is powered by DC (from the
Variable-mu Input Stage
A direct input is provided for use with guitars. This has an input impedance (for signal) of
470k, as determined by R145. If this input is disconnected, the jack's internal switch
bridges a 47k resistor (R147) to ground, to help prevent noise (and oscillation at high
gain) if the input is left floating. This also forms a voltage divider along with R147 to
enable the high-level input, with an input impedance of about 270k and attenuation of about 6:1
for use with external FX unit "preconditioners" (such as, for instance,
The Real McTube).
Capacitor C64 provides a ground path for signal, and resistor R144 passes the DC
control voltage from the compressor amplifier/rectifier to the control grid of V19. A wide range
of signal gain is afforded by the design of this tube, in which a control grid with variable
spacing allows gain variance while remaining reasonably linear.
Another feature of the 6K7 and similar pentodes is that a simple screen supply consisting of
a rudimentary voltage divider (R139 and R140) is completely adequate. It is,
however, well bypassed for signal by both an electrolytic (C61) and a disc ceramic
capacitor (C60), in order to assure AC stability.
R141 is the plate load resistor, and C59 couples the amplified signal to the
following stage (line preamp). Resistor R143 scales the cathode
voltage for metering the compression level, and diode D28 adds a bit of a "knee"
to help make meter response somewhat more representative of actual system gain.
End of Article