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EJJ amps
PHILLO AMPLIFIERS
Vacuum Tube Amplifiers
Volume Control Circuit Effect on Amplifier Performance
EJ Jurich
The circuit to the right shows a common volume control
arrangement. The volume control R2 serves two
purposes, first as the volume control and second as the
vacuum tube control grid resistor. This is not the best
arrangement for two reasons. First, grid current,
although small in value, can cause R2 to become noisy
as current flows through the wiper moving across the
resistance. Second, performance of a high impedance
vacuum tube stage can vary depending on the input
load.
As R2 rotates, the series input resistance varies between 0 ohms to the full value of R2. Common
values for R2 are 500K or 1M and C1 a value from .01uf to .47uf. In essence, C1 and R2 form an
adjustable series tuned circuit to the tube control grid. Because the grid is high impedance the
tuning effect of C1 and R2 can resonate at audio frequencies and alter the input frequency
response. Test done by this author has shown that as R2 is rotated clockwise towards full
volume, frequency response varies as R2 passes through its center rotation. This tuning effect
can be minimized by using a lower R2 value of 100K.
A better arrangement is shown to the left. In this case
C1 is between the R2 wiper and the tube input grid,
C1 blocks DC grid current from flowing through the
R2 wiper. An additional R4 grid resistor is required; a
R4 value of 100K will lower grid input impedance
reducing the effect of grid circuit components. To
minimize the chance of R2 - C1 tuning effects use a
R2 value of 100K. It should be noted that this tuning
phenomenon is more pronounced on high gain stages.