The negative self bias of E88CC

Take all precautions.


The measurement readings below are very conservative. The actual self bias of grid is much more negative for at least too reasons:


1. An electrometer or pH potential meter was not used. Instead a digital multimeter was used with an input impedance of only 10MΩ to 1 GΩ.


2. The heater voltage used was 4.7V. The higher the cathode temperature the more negative grid becomes.


These measurements were done on Mullard E88CC electron tube in a Pleiades Electra 3 connection. Initially no voltage was supplied to plate. There was a plate to grid resitor of 10MΩ that makes operation of the amplifier possible at low anode potentials such as 12V-24V.


For more accurate initial measurements nothing at all should be connected to electron tube except heater voltage.


The negative terminal of the voltmeter was connected to chassis ground or cathodes.


The plus cable was connected to each grid in turn.


Firstly without connecting heaters, there is a potential at grid of about 200mV. This should be due to photo emmision as the electron tube was near the sun window.


Next 4.6V was supplied to heaters.


The grid gradually became -600mV. The other grid -550mV.


Anodes were respectively -400mV and -350mV.


A potential difference between anode and grid could not be measured (the 10MΩ resitor short circuits the high output impedance potential difference).


Next 24V were connected to anodes through the Sennheiser HD580 headphones. (See schematic, take all safety precautions (including ears of course) and suitable fuse in series with any battery).


Music was connected to grids through 22nF capacitors.


The sound was very nice. The 06 track of Fame soundtrack was being played.


Bias at grids was -50mV due to Pleiades bias (from the 10MΩ resistor, anode potential, and underheated cathodes).



At peaks bias was becoming more negative possibly due to the grid rectification effect [Armstrong].


At large peaks, bias became -100mV indicating reduction of gain or automatic gain control or compression.


Operating features of the Audion - E. H. Armstrong