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

On Feb 24, 11:23�pm, Tim Wescott wrote:
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.

You're welcome!

*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).

Works for me!

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.

Agreed - in fact, maybe even in AB1!

A pair of 813s in full-yellow pushpull Class B audio service
can deliver 650 watts of audio, according to my 1948
ARRL Handbook. With just 0.35 watts of driving power.

I think the main reason the Ancient Ones used triodes
in their modulators so much was to avoid the problems
of a screen supply and a bias supply. Since they were
building a separate speech amplifier anyway, building
one that delivered 10 watts or so wasn't much different than
building one that delivered a watt or so.

Two articles of interest:

In QST for November, 1958, is "Medium to High Power Audio from 813s".
A modulator using 813s in pushpull AB is described. It is really quite
simple, too.

The bias supply is just a halfwave rectifier feeding a 0B3/VR90.
Screen supply is obtained from the plate supply with good regulation
and without a humongous power resistor by using a 211 triode as a
series regulator, and a string of NE-2s for reference.

This series regulator setup has a lot of advantages. The screen supply
is regulated, there's no enormous dropping
resistor, and most component failures result in a loss of screen
voltage rather than excessive screen voltage or screen voltage without
plate voltage. But you need a big
triode for the pass tube, with a well-insulated transformer to
light it.

According to the article, a pair of 813s in AB1 will provide 260 watts
at 1500 volts B+, or 335 watts at 2000 volts B+ - with *no* driving
power!

In AB2, it says you can get 455 watts at 2000 volts and 650 watts at
2500 volts, with less than a watt of audio drive.

There is also a "peak extension circuit", which is a form of
supermodulation by limiting the negative modulation percentage. Such a
system was a big deal in 1958, but if
you use one today, the carrier power has to be *decreased* to stay
legal! Hardly seems worth the trouble.

Another interesting article is from QST for March, 1948. "500 Watts of
Audio from AB1" shows a modulator
that ends up with a pair of 4-250As - driven by a pair of
6SJ7s in a phase inverter! Speech amplifier, mod iron, screen
regulator VR string, bias and low voltage supplies are all on the one
17x13x3 chassis...all you need is 2500 volts for the 4-250As.

The only transformer in the audio chain is the modulation transformer
- all other stages are resistance-coupled. The speech amp is just four
tubes - 6SJ7 mike amp, 6J5 2nd audio, pair of 6SJ7s phase inverter.

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.

I chose 50% to make the math simple and to illustrate the point. And
it's easy to get less than 75% efficiency in real life when a rig has
certain compromises, like trying to cover 160 through 10 with tubes
that have lots of output C.

--

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.

As one who is "really into paper", I agree. Google my
call to see part of the library...

73 de Jim, N2EY