Hacking an electron vacuum tube to operate at low battery anode voltage

How it is done:
At your own risk, all safety precautions should be taken anyway.


1. If it is not a triode, connecting all grids except the control grid directly to anode, (triode connection).


2. Connecting the cathode directly to ground or battery -.


3. Connecting one (a few Megohm) resistor from anode (plate) to control grid.


4. Underheating the heaters.


5. Coupling the signal to be low noise amplified to the control grid through a capacitor so as not to upset the DC conditions. For example a microphone input transformer's secondary is connected through a capacitor to the control grid.


6. Using an output transformer directly to anode will make operation possible at even less supply voltage or Vb.


For example the Pleiades V6 microphone pre preamplifier operates with 3.9 volts on both anode and heater circuit. It uses an EF183 high gm, variable gm electron vacuum tube triode connected.


Another way is connecting the input transformer secondary to grid. A low variable cathode resistor, Rk, in order to find the sweet spot, max gain spot exoerimentaly. For example the Pleiades V-1 with the 7025 electron vacuum tube. Heater supply is 5V, anode supply is 18V.


References:


On preserving electron tube transconuctance (at low anode voltage or potential) - euroelectron blogspot


Pleiades V6 schematic