Carl Blare's picture

The expression "exclusive of filament or heater power" as stated in 15.219 has never become clear to my understanding. Obviously it refers to electron tubes, which contain filaments. "Heater" is another word for filament, owing to their contribution to heat within the tube. But in determining the input to a final electron tube stage the power drawn by the filament is clearly separate from input or output power from that tube, so there seems no reason to mention filament power at all. Why are filaments mentioned in the rule?

Hi Car!!

That's a good question. I'm stumped. My
brain wants to think of an answer, but I
don't have one.

Bruce, The Dog Radio Group, makers of fine
Part 15 stations in the past, such as

FLEA 1590



The rule was so written because power into the other elements (grid, plate, screen grid, cathode, suppressor grid) can add to the output power but power into the filament does not. Since the filament power can vary from tube type to type excluding this places the limit only on the things which contribute to the power output while allowing latitude for the type of tube used.


Hi Neil!

Thank you for the explanation. I actually get a kick out
of the rule, because it mentions vacuum tubes, and I
like them.

Best Wishes,
Bruce, the Dog Radio Group,
Makers of great Part 15 stations,
such as:
WBVR, AM 1600 kHz, FM 98.1 MHz,
West Hartford, CT - 1970 AD


How Interesting

Thank you Radio8Z for the lesson in filament relationship to tube power evaluation. What an interesting way of sorting things out.

Like MICRO1700 and probably all old-timers, I also have a romance for tubes and dream about making a tube transmitter for modern day part 15 which resembles large licensed transmitters, with big knobs and meters.

Electron tubes needed to be "warmed up", which was a service provided by the "heaters", whereas transistors operate at ambient temperatures, except that they may drift somewhat owing to surrounding temperature. In fact, heat is the enemy of transistors, whence cooling is often needed.

It's a shame that the plain everyday person fails to realize how their lives would be enriched by participation in the part 15 lifestyle.

Carl Blare


Vacuum tube r-f amplifiers typically use some number of internal elements (e.g. screen and suppressor grids) to enhance the output power (gain) possible from that tube, for a given input power.

However that does not mean that the power those elements consume directly is converted to r-f output power from that tube amplifier. Likewise for filament power.

For example, the screen grid of a tetrode r-f amplifier typically requires a d-c voltage of some value less than the plate voltage, and for that voltage some value of current will be furnished by the power supply connected to the screen terminal of that tube.

That current flows between the screen of the tube and the power supply providing it. IOW the power produced by the screen power supply does not appear directly as a component of the r-f output power of that tube -- it just enables/enhances the power output that the tube can produce.

Directly heated cathode?

Directly heated cathode?

Druid Hills Radio AM-1710- Dade City, FL. Unlicensed operation authorized by the Part 15 Department of the FCC and our Resident Hobby Agent.  

Directly Heated Cathode

Just to note that the a-c heating power used by the filament/cathode of a vacuum tube does not appear in its output circuit.

For example, a grounded-grid triode r-f amplifier with plate voltage and without drive (excitation) has no r-f power in its output, even if the filament/cathode is hot.

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