You know, you have a very good point!
Looked it up. Here's your answer!
The filament is also the cathode. The other parts are the plate(anode)and grid. The cathode is the heater that is needed for the tube to work, but is also one of the electrodes. It releases electrons when heated. So in an amplifier, signal can be applied to the cathode for amplification. In the case of the part 15 AM transmitter, you have DC at the amplifier if no signal applied and this is where the 100MW DC at the filament comes from. The emitter, collector, and base of a transistor, replaces the electrodes of a tube and of course needs no heat. just their proper electrical set up(biasing).
So that's why the rule says DC applied to the filament. The filament is actually also one of the electrodes.
With transisters, the filament could be the emitter.
The power needed to heat the filament(s) like in a radio comes right from the A/C line and the filaments voltage needed for heating is the number of the tube, 12BA6...the voltage for the filament is 12 volts. Add up the numbers in a typical 5 or 6 tube radio and you'll get 115 to 120 volts. But the FCC should modernize it's text and just say 100MW DC(no signal applied), at the output amp. Or just change the rule and say 100 MW output at the antenna, for example...much simpler!
Mark
15.219 (a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.
Although "input power" is not defined in the rules, it is commonly accepted to mean "DC input power to the plate circuit" when specifying the power level of an RF amplifier stage. This definition can be found throughout transmitter theory and design literature dating back almost 100 years. You can substitute "collector" or "drain" for "plate" when using BJT and FET devices rather than tubes.
DC input power is calculated using the formula Pdc=Vdc*Idc. Vdc is the DC voltage applied to the plate circuit and Idc is the DC current flowing to the plate circuit with no modulation. These values can be measured with a DC voltmeter and a DC ammeter. The Rangemaster and AMT5000 have meter test points to measure these values in order to set the power control pot for 100mW.
The filament power and grid (base, gate) power are not included because the filament and grid circuits are not part of the plate circuit, so they don't contribute to the plate circuit power.
Confusingly, there is another amplifier circuit parameter sharing the "input power" designation, the input power to the grid (base, gate) of the amplifier. It is usually designated as Pin. Pin is used in the process of analyzing amplifier power gain. Amplifier power gain is not a concern In the context of the Part 15 rules. The power gain can be any value as long as the DC input power to the plate circuit doesn't exceed 100mW.
Thank you PhilB and everyone for addressing my questions. I am formulating another question, but got involved looking at tube circuits for Part 15.
Neil wrote earlier that his dream tube project would replace a 50C5 rectifier with a 6C5 so as to maintain 6.3 VAC as the filament of choice, which I appreciate.
Strangely my RCA Receiving Tube Manual doesn't list the 6C5, but I'll look elsewhere.
And to comment on the tube transmitter noted by WDCX, the MW-250 from ontheair3.com (out of stock), the disappointment for me is that one of the tubes is solid black and not transparent.
Today nature has turned up the filaments here in the midwaste and there is plenty of heater action.
... Today nature has turned up the filaments here in the midwaste and there is plenty of heater action.
Is this another of your alleged "dances," Mr Blare?
Dancing on the ledge?
I spent over $1200 on dance lessons-still can't dance.
By the way, a 6C5 is a triode. Tube data HERE.
The new LPFM being built by MRAM1500 could become a national model as a dance station, with high school prom, fireman's fund ball, square dancing and old hotel fox trot.
Eureka!
Viola!
That's It!
I just got the whole answer to "input to the final" and "filaments" and everything!
See, I thought the rule applied to the final RF stage SIGNAL input.
But the rule doesn't apply to the signal input, which would be (mainly AC) to the grid or the base...
We are talking about DC voltage as input to the entire final stage!
And yes, as Neil tried to tell me, filament voltage is literally input to the final stage, albeit AC voltage, but it doesn't count in determining the OVERALL input power, which should not exceed 100mW.
But couldn't anyone logically and reasonably make the same erroneous assumption I made?
Therefore Rich(F) is correct in suggesting that the rule could be better written for non-techy types.
I may have been wrong, but I was very sensibly wrong with a reading of the rule that would apply if it did, except that it doesn't... not the way I read it.. but it does, the way I misunderstood it and everyone else tried to explain.
Now I am a techy type!
My ALPB experience is paying off!
A 50C5 is a Beam Power Amplifier.