[Amps] Measuring filament voltage

[Jim W7RY]

Indeed!

LOL

73, Jim W7RY


On 6/22/2022 3:09 AM, Alek Petkovic wrote:
> 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 converter.
>>
>>   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 
>> <jtml at losalamos.com> 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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