I am building a regen receiver using a 1T4 tube as the detector.
I've noticed that the circuit is very microphonic. In fact, if I switch
off the filament supply, before I cut the plate voltage, I can hear a "tinkling" sound
in the headphones...sounds like the filament cooling and shrinking.
Have also noted that I can run with as little as 12 V DC on
the plate and still get good performance in the AM broadcast band.
Do all directly-heated low voltage tubes have a tendency to be
microphonic?

Rich

Hi Rich

Filament type tubes are microphonic. It's especially noticeable when several are used as audio amplifiers. The filament is a thin wire running down center of the tube. Any vibration and you will hear pinging.

Norm

:I am building a regen receiver using a 1T4 tube as the detector.
:I've noticed that the circuit is very microphonic. In fact, if I switch
:off the filament supply, before I cut the plate voltage, I can hear a "tinkling" sound
:in the headphones...sounds like the filament cooling and shrinking.
:Have also noted that I can run with as little as 12 V DC on
:the plate and still get good performance in the AM broadcast band.
:Do all directly-heated low voltage tubes have a tendency to be
:microphonic?
:
:Rich