[Amps] amp cooling / triodes vs tetrodes

[John Lyles]

The maker of the TH628 tetrode in France tests them without bias 
switching. With 5 Amps of idling Ib, at 28 kV, there's 140 kW kW of
plate dissipation just from the electron beam with no RF drive. In our 
test system for cavity amp using the same tube, we have bi-level grid 
bias voltage. Normally it is cutoff with -600 VDC, and when an RF pulse 
comes along, the bias kicks up to -400, which gives about 5 amps of Ib.
Its all timed from a logic system, so that the bias can change state in 
less than 2 microseconds and RF comes up with a PIN diode. The quiescent 
dissipation is nearly zero in this system.
K5PRO
John