Sometimes that seems to be the case.
Figure a transmitter using a 1000 watt Plate input triode class C push-pull
amplifier. At 75% efficiency you would have 750 watts output power. But
remember your class AB audio amplifier is really running about 60% to 65%
efficiency so the same tubes would produce 650 or so watts output which is
not much more than what you really need 500 watts. Also, if it is capable
of 600 watts the modulator would produce less distortion at the needed 500
watts. And besides if you have a pair of 810s in the RF amplifier why not
use 810s in the modulator. That way you would only have to stock 1 power
tube type and not 2.
73
Bill wa4lav
At 04:27 PM 8/27/2004 +0000, Rob Atkinson, K5UJ wrote:
Okay Bill thanks. It seems like I have always seen broadcast tube AM
transmitters with modulator tubes that are the equivalent of the rf
lineup, i.e. 1 kw out from pair of 3-500s and pair of 3-500s in modulator
to provide audio pwr equal to rf carrier and so I thought the audio power
had to match the carrier limit for 100% high level plate modulation.
73,
Rob/K5UJ
From: Bill Fuqua <wlfuqu00@uky.edu>
To: "Rob Atkinson, K5UJ" <k5uj@hotmail.com>
Subject: Re: [Amps] Centurion cooling improvement ideas for AM service
Date: Fri, 27 Aug 2004 11:24:56 -0400
At 02:43 PM 8/27/2004 +0000, you wrote:
When he says "the carrier is at half level the output is one-fourth the
modulation peak output", he really should be saying that the carrier
VOLTAGE output is at half level and that the power output is one-fourth
peak envelope power out with modulation.
"the carrier is at half level ( as seen on an oscilloscope), the (carrier
power) output is one-fourth the modulation peak output (PEP output)",
You should see a positive deflection in plate current of a linear
amplifier when modulating. This is not the case when using a plate
modulator. That is because the extra power that is going into the
sidebands is coming from the modulator. That is why the modulator is
chosen to have an output power of 1/2 the plate input power of the class C
amplifier. The amplifier's power supply may provide 1KW which would
translate to about 750 watts of carrier output (assuming 75 percent
efficiency). And the modulator would add 500 watts to that to provide a
total of 375 watts of sideband power on the output or 187.5 watts per
sideband (6db down from the carrier). However, the Peak Envelop Power
output of the class C amplifier would be 3KW (4 times the carrier power).
73
Bill wa4lav
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