I'm modding an old tape player aux amp (5w) for guitar. I'm having trouble with understanding basic tube circuitry.

Should the signal go from input to 12ax7 grid1(pin 2), then plate/cathode1(pin 8) to gain pot back to grid 2(pin 7) and then output from plate2(pin 3) to Volume,tone control stage then on to grid1 of el84 power tube? Thanks in advance, Terry

gmoon8 years ago

I suggest you look at the 5W Epiphone Valve Jr schematic, since it uses classic 12ax7 and EL84 tubes also.

Yes, the grids are the "control" structures, like the base of a bipolar transistor.

Plate and cathode are two different (opposite) things--the plate is the anode. To work as a voltage amplifier, a plate resistor (load) is necessary (R3 and R4 on the VJ.) On the power tube (el84) the transformer itself is the load...

A typical 5W amp is cathode biased--a resistor lifts the cathode above the GND to keep the grids at a negative voltage relative to the cathode. There are three cathode bias resistors on the schematic (R8, R9 and R14.)

The EL84 is a power pentode--these have three grids, unlike triodes which have only one. A control grid (like the triode), a suppressor grid which is connected internally to the cathode, and a screen grid.

It's important to include the coupling capacitors (C1, C2 on the VJ)--these block DC voltages from each successive stage, and work as high-pass filters, too (with the resistors following.)