Playing with cathode temperature on electron tubes

It reminds a bit experimenting with temperature for baking croissants, the pastry made with real milk butter, (instead of the usual vegetable butter or palm oil which has a reputation of clogging arteries).


It is great fun playing with cathode temperature by adjusting the heater voltage when the tubes operate at low anode potentials as on the Pleiades amplifiers. The easiest before doing this is just disconnecting the heater circuit and listen to what happens. On low level pre preamplifiers such as the Pleiades V series the sound output increases and noose decreases. A similar principle must be used on the Neumann U47 electron tube condenser microphone which has the VF14 heated at a much lower than specified by the manufacture voltage.


The change of grid potential with respect to cathode can be monitored with a voltmeter and the music output of the amplifier can be listened to in real time.


The cathode is usually connected to ground on most Pleiades amplifiers so the voltmeter black lead is just connected to chassis or battery minus.


It is also very interesting to carefully connect (always use fuse) just a heater voltage on any tube and observe the grid becoming progressively negative wrt cathode as the cathode heats up. And this without connecting anything else to any other terminal. This shows that nature internaly biases the grid negative or the cathode positive as electrons escape from the heated cathode.


This possibly explains why a tube is cut off at low anode voltage.


On Pleiades amplifiers a tube operates normally at very low anode potential by compensating this internal negative bias effect externally with a high Megohm resistor from anode to grid.


The grid bias may still be negative but less so and the electron tube operates in an unheard off low noise and sound quality.


References:


Operating features of the Audion - Edwin H. Armstrong


On preserving transconductance of an electron tube at anode potentials as low as 3.6V - euroelectron blogspot


Fringe Knowledge for Beginners - Montalk - free PDF