wrote:

On Feb 21, 8:42?pm, Tim Wescott wrote:

If I have my numbers right, a peak envelope power of 1500 watts equates
to a carrier power of 375 watts, which requires an audio amplifier to
put out 200 watts (to round up a bit).


Not exactly! You're mixing input and output powers.

Current FCC rules limit us to 1500 watts peak output.
On AM, that typically works out to 375 watts carrier output.

Yes, you're right. Thank you. Hopefully if I were doing this for real
I would have thought of that before it's too late (in fact I have, while
designing fantasy rigs).

To make the math simple, let's assume our Class C
plate modulated final stage has 75% efficiency.
Then we will need to run 500 W DC plate input to get
375 watts carrier output, and 1500 watts peak output.

The audio power needed to plate-modulate that 500 watt
DC input stage is 250 watts - exactly half the DC input.
More modulator power is OK - it's a good idea to
figure on 300 watts or so, to allow for various losses,
and so the modulator tubes aren't running flat out to
give you 100% modulation.

Right. And that's still modest for a pair of 813s, at least used as
pentodes in class AB-2.

You can easily see how the design could get complicated
in a hurry. Suppose your Class C modulated stage is
only 50% efficient, instead of 75%. Then you would need
750 watts DC input to the stage to get 375 watts carrier
output. And you'd need at least 375 watts of audio!

I think at that point I'd decide I needed a better modulated stage,
unless I were operating at UHF.

--

When reading old Handbooks and other amateur
radio publications, it's important to remember that in
the past hams rated transmitters by DC input, not output.
If you build a "100 watt" transmitter from those days and
expect to get 100 watts of RF from it, you will either be sadly
disappointed, or will run it way over design ratings
in order to get those 100 watts.

In the bad old days before 1984, Part 97 simply limited
US hams to 1000 watts DC plate input to the final. Which
meant you simply needed 500 or so watts of audio to plate
modulate a legal-limit AM rig, regardless of efficiency.'

The old rules are one big reason plate modulation was
the AM method of choice for the Ancient Ones. The
carrier efficiency of most other AM techniques (grid
modulation, screen modulation, suppressor modulation,
cathode modulation, conventional AM linear, etc.) is
only 30-35%. So for a kilowatt DC input, you could only
get about 300 watts of carrier output, and you'd need a
final that could dissipate 700 W!

Under today's rules, the DC plate input doesn't matter -
only the peak RF output matters.

btw, there's all sorts of tube data info online. No need to
buy tube manuals anymore unless you are really into paper!

The tube manual I quoted is good for two things that you don't get as
easily from the web -- the front pages are a very good text on tube
amplifier design, and you can read it while you're eating your cereal in
the morning. I'm just old enough that I need things on paper -- were I
ten years younger I'd probably not need it at all.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html