No I meant really cheap, the 100 bux could be enough or even less for true rms dmm.
you'll need to measure the voltage on the cathode resistor(white box) to ground read and write the value down, then measure the voltage drop on the plate resistor(across resistor) not to ground, divide this voltage by the value of the plate resistor.This gives you the amount of current being drawn by input tube in milliamps. Write this value down.
Knowing the voltage value on the cathoade which could be closest value of your AA rechargeable cell.
Remove the "white box"place the battery between the cathode and ground so we have now constant pure DC source, battery cathode bias.
Batery that can by use, NiMh, NiCad or NiZn, do not use alkaline battery can explode! The cathode circuit itself keeps the battery charged so you don't have to be wary, they will last as long as the battery will. I pull out my battery after 3 months of use and check with the DMM and the voltage is keeps steady.
Wish u would live closer to me..u for sure could help in modifying my 337 for sure!