Grid current vs rf drive vs plate voltage….

[K8DI]

I was testing a bunch of 4-400 type tubes the other day.  My basic test was to stick a pair into my RCA BTA-1R1, turn on the filaments for a few minutes, then apply rf drive and adjust for rated grid current. If it needed more than 25% over what known good tubes needed, they failed.  Then I applied plates at the low power setting, 1500v. Tweaked the tune, set the load to the rated current, etc.  if everything looked ok, bump the plates to 3kV. My theory is if it makes full carrier at 3kV, it will modulate 100% at 1500, which is where I run the rig. If it can’t make full carrier at 3kV, it’ll never hit 100% on low, and those tubes were rejected. Of course the individual currents were also checked for match and tubes were selected to achieve that.

I noticed that with some of the weaker tubes, applying HV would drop the grid current to half or less of what it was without HV. Even with good tubes, it drops a little (a couple mA). My question is this: what is the mechanism at work here? Where is the drive going? Why does it drop?

One note, I’ve added about 175v safety bias to the original grid circuit. Even if the drive is off, the plate current is limited, rather than pegs meters and trips overloads…


Ed

[KD6VXI]

Feed through power?


--Shane
WP2ASS / ex KD6VXI