In the commercial RF amplifiers that I have designed, put into
production or installed and operated, filament voltage is measured all
the time. For pentodes, tetrodes or triodes with common cathode
arrangement, it is simple to have two wires going to the socket,
suitably bypassed for common mode as well differential mode RF noise. A
cheap DMM won't be accurate enough, depending on the transformer or
power supply - use true RMS metering. For years this meant taut band
analog meter movements. All the Broadcast Electronics FM transmitters
with tubes had these as well as their quality competitors such as
Collins and Harris. These days one can find a decent digital meter that
has RMS calibration in case of non-perfect sinewave waveform. I have
used Newport meters for this. The point is to measure at the socket, not
the transformer winding.
For common grid circuits where the cathode is carrying common mode RF
voltage with respect to grid and chassis, it's not so easy. If it is a
cavity circuit where the structure itself is used to ground the bottom
end of the resonator (quarter wave cavity for example) then the meter
circuit is applied there at the ground end of the structure. This is how
i do it for 2 MW amplifiers at 200 MHz that are grounded grid/screen
grid configuration. You can tell if there is RF interference, as the
meter will rapidly change with the RF power comes on. If there is
appreciable backheating inside the tube (RF and infrared affecting the
cathode temperature) then it is more complicated and I will leave that
out of this. Assume that the designer did a good job of bypassing the
heater carefully for RF.
For HF amplifiers that often use common grid circuit with triodes, it is
again more difficult to measure at the socket since RF voltage is
applied to the cathode with respect to grid and to chassis. About the
best you can do is measure on the transformer secondary (for a center
tapped filament transformer) just before the bifilar RF choke. You can
measure with RF off on both sides and create a calibration factor,
knowing what it is on the cold side of the chokes to estimate what is at
the tube/socket. Then you know what it is with RF on or off.
I have one amplifier system that is a cathode follower connected
triodes. The RF voltage is as high as 18 kV peak at 2.8 MHz there. It is
very difficult to physically measure the filament voltage. The filament
transformer has low capacitance and RF isolation between windings. In
that case, can only measure the primary AC voltage, 440 VAC in this
case. And then create a conversion coefficient for the output voltage,
measured with a good RMS meter when the RF/HF is locked out.
73
John
K5PRO
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