Cost is about 120 USD using brand-new parts, including the case.
Note well: This circuit operates on lethal voltages (200V DC). The voltages are present for several minutes after the circuit is switched off. I am not responsible for your safety.
Schematic:
The circuit can operate between the guitar and the amp or preferably in the amplifiers "Effects loop".
All the components are rated at 250V or more.
The power supply uses a 262B6 transformer (Hammond Manufacturing) which is designed for single tube pre-amps: it has both a plate secondary and a heater secondary sufficient for only one tube (300ma).
The 110v AC household current is fused and switched. The ground wire connects to the chassis (metal box). The plate winding of the transformer is rated 120v AC, but I measure 145v AC and all the voltages are stated as measured. A silicon bridge rectifier and smoothing capacitor provides 208v DC. The 18k resistor and second smoothing capacitor results in 182v DC for the two 100K plate resistors. The 470k resistor eventually drains the plate voltage when the power is turned off. A 100nF capacitor mounted close to the plate resistors reduces high frequency noise on the power supply.
The 12AX7 contains two triodes each with its own heater. So the 6.3v winding powers them in parallel (pins 4&5 and 9). The 47K pot very much reduces hum.
The pre-amp is two stage. The idea is that the first stage provides enough gain to overdrive the second stage and generate lots of desired tube distortion. The output is attenuated to avoid overdriving the guitar amp- which would produce undesirable transistor distortion.
The input is via a switched 1/4 socket. The 22nF capacitor isolates DC from the guitar, and the 470k resistor provides a 0v DC bias to the first stage Triode grid. The 2k resistor and bypass capacitor sets the triode to operate at about .57ma with a gain of about 100x. The signal is decoupled and fed to the "Drive" potentiometer which controls the level applied to the second stage.
The second stage is the other half of the 12AX7. The 1k resistor operates the triode at about .8ma. This value was chosen to reduce the output impedance of the Triode to better match the return impedance of the effects loop input. The bypass capacitor gives a gain of about 100x. Output from the second stage feeds a 100K potentiometer. The 220k and 100k resistors allows for more control of the output level. Output is to a 1/4 socket ready for a patch cord to the amplifier.
Circuit Layout:
For safety, the chassis and lid connect to ground. A phantom ground avoids a ground loop. Input and output is through mono 1/4 sockets. These are the plastic insulating type to avoid connecting 0v to the case.
The vacuum tube is mounted on a bracket inside the case and without ventilation. This was done to avoid breakage. The tube generates about 1watt of heat when operating, which is barely enough to warm the outside of the case.
0v reference is a bus-bar derived from the solid copper ground connector from an electrical cable. A 0v "star" design did not noticeably reduce the circuit noise, but did make for a messy circuit.
