Page 18 - Triwatt Custom Lead Amp Builder's Guide

The grid ("leak") resistor, typically sets the impedance of the stage and biasing. It is interesting
because it and the previous stage's plate resistor form a voltage divider on the signal. What this
means to you is that the grid leak resistor can be used to control the level into the stage. Low grid
leak values will attenuate the signal into the tube stage. If you look at different tube amp
schematics, you can see where they control the level into the stage by using different values for the
grid leak resistor. There is a maximum value that you need to adhere to. Check the datasheet for
the tube you are using to see the typical value of the grid leak resistor. This particular circuit is
called cathode bias which you can read about here.
In summary, the cathode resistor, plate resistor and grid resistor, determine the biasing of the tube
stage. The cathode bypass cap controls the degree of bass reduction - generally 25uF passes all
frequencies - commonly used in Fender amps, 1uF an 0.68uF are used in Marshall amps. A
capacitor can be placed in parallel with the plate resistor to roll off highs and you see this in bass
channels of amps sometimes. The plate receives the voltage from the power supply through a plate
resistor, the grid receives the AC signal as input and the cathode is grounded through a cathode
resistor.

TrinityAmps TRIWATT Builders Guide Ver. 2.31.docx- 18 -