I just convert the AC voltage at the socket to DC with a bridge rectifier
and calibrate the panelmeter for the specified voltage.
From: Amps [mailto:email@example.com] On Behalf Of Alek Petkovic
Sent: Mittwoch, 22. Juni 2022 10:09
Subject: Re: [Amps] Measuring filament voltage
Well, that keeps things wonderfully simple.??????
73, Alek, VK6APK
On 22/06/2022 9:00 am, Radio WC6W via Amps wrote:
> Hi John,
> If you want to measure the voltage at the filament on those follower
triodes may I suggest the following scheme:
> Use an AD536 to convert to RMS right at the tube.
> Use a LM331 to convert the voltage to frequency.
> Send the frequency out over an optical fiber and read it with a
frequency counter or alternately convert it back to voltage with a F-V
> Power it with a tiny hi voltage isolation transformer. Only requires
milliamps to run.
> Or use a power over fiber cable scheme, though those are rather pricey.
> Just a thought,
> Marv WC6W
> On Sunday, June 5, 2022, 10:08:38 AM PDT, John Lyles
> 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.
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