This module is Randall's take on a boosted Vox AC30TB.
Mods
A well documented mod over on grailtone to change the Top Boost back to something like an ordinary Vox:
- R7 change from No Part to 100K Ohms (1/4 watt)
C7 change from 22u to No Part - simply cut this part out
C13 change from 22u to No Part - simply cut this part out
Switchable Vox/Top Boost
I decided to make the above mods switchable for maximum flexibility. Here's what you need to do:
- unsolder negative leg of both C7 and C13;
- solder wires from C7 and C13 negative leg to one side of a DPDT (do not connect both wires together as we want to keep the signals separate when disconnected);
- solder a ground wire from the ground side of R7 (right hand side when looking at module from front) to the middle of the DPDT switch;
- bridge both middle pins of the DPDT with another small piece of wire so they are both grounded;
- solder 100k Ohm resistor into left side of R7 and then to the opposite side of the DPDT (either pin is OK).
What this means is that with the switch in one position, C7 and C14 are connected to ground and R7 is left floating (Vox Boost mode). With the switch set the other way, C7 and C14 are brought out of the circuit and R7 is connected to ground, lowering gain and giving us the Vox mode.
What does C7 / C13 ?
C7/R17 and C13/R29 are part of the module's common cathode circuit. Each pair form a cathode bias resistor and cathode bypass cap for one gain stage. The resistor creates a local negative feedback effect which lowers the tube gain's stage. This works at all frequencies. The bypass cap then forms an alternate route for bypass of certain frequencies, dependent on the value of the cap. Lower the value, more treble. Higher the value, more bass.
Once again, there are programs that can help you visualise changes in values - www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/
There is also a fantastic video here that explains all about common cathode circuits - www.youtube.com/watch?v=HrkYUPmEUhk