[Amps] Measuring filament voltage

[Jim W7RY]

Exactly John. Great post!

73, Jim W7RY


On 6/5/2022 12:09 PM, John Lyles wrote:
> 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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