The AM Forum

I have been working on a cathode follower for a screen modulation application ... nothing new about that and there have been many designs discussed on this forum with apparent sucess ... I was working on my breadboarded 6ea7 design and for convienence had used a bench supply set at 6.3 Vdc to power the tube fil .... I couldn't get either stage to bias correctly and were loading down the other circuits .... this aint my first rodeo but nothing would yield with multiple tube changes including known 'good' ones .... demonstrated heater to cathode leakage ....huh?


so I googled it and found  some discussion on the subject .... one guru said he had looked in the RCA 'bible' or radiotron designers handbook and found only limited info with no conclusion .... one other guru had found some rare sources that discussed the insulating alumina or aluminum oxide which is a very good room temp insulator but not so good at the red heat temperatures of an operating indirectly heated filiament.... this is getting interesting because it is starting to weakly act as a semiconductor .... it is important to note that this is a weak manifesttion and when cathodes are held at a low impedance to ground it is small potatoes.

operating filaments on ac does not seem to exhibit this unless you consider impressed hum (or 60 Hz modulation) to be a  problem ....like in preamp circuits using cathode followers, srpp stages, etc ... the other thing of concern is is this an age related problem that gets worse ...perhaps so because it seems that indirectly heated rectifiers (like a 6x4) seem to fail with hk shorts more than they used to .... may be application related

I would like to hear from youall to learn ...

In sensitive pre-amps, this was a problem. There is also some electron emission from the filament. When you have a sensitive pre-amp without decoupling of the cathode resistor, this can result in hum. It are micro amps, but noticable. It was solved by putting a positive voltage at the filament via a resistor, so the cathode was always negative wrt the filament. I diminished the hum this way in various amps I did build over the years.

This has been a hot topic, pardon the pun, on 75M recently and I've given this phenomenon a lot of thought as a result.

An alternating current would only be imposed on the cathode if there were a complete electrical circuit and the heaters powered by A.C. In other words, if the heater supply is floating then there's no return path for current between the two elements. Conversely, for heater supplies that have a grounded center tap, a hum-balance potentiometer, or use the chassis as a common return (GASP!) then there could easily be current flow between the heater and cathode.