Modulators working different RF power levels

[W4RFM]

In my broadcast engineering days, it was common to reduce power at sundown at several places.  We had a Gates BC-5P that ran 5 KW during the day and only 500 watts at night.  This probably answers my question in itself, but I will ask the forum: when running (say) a 300 watts of audio "capable" modulator to modulate a 250 watt transmitter, you just don't push it as hard - correct? I would like to avoid a complicated power reduction input scheme like the broadcast rig had.

[N1BCG]

Hi Bob,

Our Gates BC1-Gs reduced supply voltage to both the RF and AF 833s for 250W operation. A "Low Power" trimmer pot allowed precise adjustment of the mod level.

With a separate modulator, utilizing a resistive pad to reduce input audio level would seem to be the easiest solution. With that kind of power available, you can employ all kinds of amplification reducing negative feedback to obtain magnificent specs.

Clark

[N2DTS]

On my 813 rig I have a variac on the rf deck and one on the modulator.
Both decks run 2000 volts at full power so it could all be on one power supply.
The 4x150 modulator needs no adjustment of anything else (bias or screen voltage) from 1000 to 2000 volts.

When the plate voltage on the modulator gets turned down to power output goes down so no need to adjust the audio.

[Opcom]

A simple answer might be to reduce RF stage input (plate volts) and back off the drive to the modulator to avoid overmodulating. I do this with a TX having a 4-1000 RF stage and a pair of 3-500Z modulators.

speculation - but using zero bias triode tubes with heavy grids for the audio and RF stages so the drive won't hurt them should allow lots of leeway in plate voltage adjustment.

In the same idea, if tetrodes are used for RF, you got to watch the screen current so maybe running the screen supply from the plate voltage variac would help over a range. Using tetrodes for audio and doing this would also mean readjusting the grid bias.

looks like you get to experiment a bit!

[N2DTS]

If you want to be fussy about it, the impedance ratio of the mod transformer should stay the same, and adjusting the plate voltage to the rf and mod decks should maintain it I think.
Most modulators would require adjustments over much of a range, with only the 4x140/4cx250b tubes not needing any adjustment.
Zero bias triodes would not work over much of a range without needing bias.

As far as the RF deck goes, push pull triodes would work over a wide range of plate voltage, and a tetrode with a separate adjustable screen supply and a large screen dropping resistor will work over a somewhat wide range. As more screen current is drawn, more voltage is dropped in the resistor.
I do that with the 813 rig, 200 volts is dropped in the resistor and that allows for a wideish range of plate voltage adjustment.


Then there is screen modulation.
Dial a power output, you can adjust for any power out you want with a simple pot, and just adjust the audio level to give 100% mod.
Or, include the negative cycle limiter and do the 200% positive modulation at lower carrier powers.
My pair of 4X150's does 40 to 200 watts carrier with only the power output knob adjustment, the 4-400 might do 100 watts to 500 watts carrier.
The only real limitation is the range of the pot used, 500K would likely give full range adjustment, from about 10 watts to 500 watts carrier with modulation up to 2000 watts pep.