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Author Topic: How to Phone Modulate ARRL CW Transmitters?  (Read 1211 times)

Posts: 208


« on: November 13, 2007, 08:12:03 AM »

Hello Elmers!

I’ve been entertaining the idea of home brewing a vintage tube
AM transmitter capable of phone operation.

My old ARRL handbooks show numerous designs for CW
transmitters, linear amplifiers, and a whole section on audio

What I cannot seem to locate is a link between any given transmitter
and any given phone modulator.

There is a section on methods of modulating a carrier, but there does
not seem to be information on how to take a given CW transmitter and
interface a phone modulator to it.

Is it simply assumed that one will understand which modulation
method (grid, plate, etc.) and how much modulation power is needed,
(and what ever other parameters) and then, re-wire one of the given
CW transmitters to accommodate a phone modulator?

This piece of the puzzle seems to be absent.  What am I missing?


--Tom Nickel AKA KC9KEP

Posts: 3227

« Reply #1 on: November 13, 2007, 08:20:34 AM »

Big question, I will try for a simple answer.

First, I assume you mean you're trying to make double-sideband AM.

Your transmitter amplifier stages must be linear for this to work.  If they aren't, you'd have to modify them, too.

Then, you could vary the power available to the transmitter section, with the audio signal desired, and that would create AM.

LOTS of other ways to do this!

If you had a spectrum analyzer and an old CB radio with schematic and parts-placement diagrams, you could learn a great deal on your own about how to do this.  Failing that, are you wanting to build something yourself, or, are you just trying to learn how it works?  The answer to this question will tell what the next steps for you will be.

Posts: 208


« Reply #2 on: November 13, 2007, 08:38:57 AM »

Hello KZ1X,

Yes, your assumption is correct .. at least for now,
I was looking at a "classic/vintage" double sideband
AM transmitter, and am planning to build my own.

I guess that my question is, as much as anything, why
do the ARRL books cover the details of building
CW transmitters, phone modulators, etc. but seem to
disregard the critical interconnecting of these

Thanks again,

--Tom Nickel KC9KEP

Posts: 1

« Reply #3 on: November 13, 2007, 08:50:09 AM »

If you want something to copy you need to get an ARRL handbook from the 50's or 60's.  You are not going to get AM phone transmitter construction articles in a 2007 ARRL handbook.

My 1963 handbook has 5 AM modulators for transmitters from 50 watts to 500 watts using tubes like 6146 and 811.  These old handbooks are always on ebay.


Posts: 4380

« Reply #4 on: November 13, 2007, 08:50:49 AM »

Almost any CW transmitter can be modulated by any of the several modulation systems.  The simplest to understand is high level plate modulation.  The modulation transformer is inserted into the plate (and often screen) circuit so that the plate voltage is increased and decreased at an audio rate.  In effect adding audio power to the RF power the CW transmitter is producing.  The transmitter is basically unchanged.  With other systems, often modification to the CW transmitter is required.
As long as the modulation is applied to the final amplifier stage, there is no necessity that any transmitter stage be linear.  Only low level modulation requires linear amplifier stages (or if an high level modulator is further amplified).
While one can use a spectrum analyser to check the transmitter, adding modulation to a CW transmitter shouldn't require any special instrumentation.
Older handbooks and radio magazines may contain complete schematics for AM transmitters.  
A problem today is the difficulty of finding suitable interstage and modulation transformers making grid or clamp tube modulation easier to accomplish.
Good luck Allen

Posts: 5434

« Reply #5 on: November 13, 2007, 09:15:00 AM »

Remember you will need about 35 watts audio output to modulate a 75 watt transmitter!
The 75 watt transmitter will be class C, but the audio amplifier will be class A.
And, you might want to add a low-pass filter.


Posts: 12667

« Reply #6 on: November 13, 2007, 10:15:53 AM »

If you are applying modulation to the final stage, it does not need to be linear. When a CW transmitter is plate modulated the audio power needs to be half of the RF power (50W of audio for a 100W transmitter for example). The RF power of a CW transmtter will likely need to be reduced in order to prevent clipping of the modulation peaks.

Getting hold of an older Handbook or other book is a good idea. You may also be able to find some info on the Internet.

Posts: 3227

« Reply #7 on: November 13, 2007, 11:30:55 AM »

If you tried to do high-level modulation to the output stage of a CW transmitter, it wouldn't need to be a linear stage ... but the audio power requirements would be high.  And it might not sound all that good, without extra work.  So I would not recommend it, at least at first, for a new guy.

On the other hand, I guess audio power is easy to generate these days, with way cheaper hardware than was available 30 years ago, when I first (and last) played with such stuff.  And, it's fun to use a big-iron modulation transformer.

AA4PB correctly points out that you can get some clipping if you don't regulate the output power ... of course, if the output stage WAS linear ... never mind.

Take a look at the schematic for a DX-60.  If memory serves, and lately it hasn't, that should give a good idea about some other AM options.

Posts: 3585

« Reply #8 on: November 13, 2007, 01:46:18 PM »

Back when decent modulation transformers were as close as the nearest surplus store, many hams found obsolete or obsolescent PA amplifiers made fine modulators. All that is required is clean audio with at least 50 percent of the RF stage's DC input and a modulation transformer with a good match to the PA tubes and the transmitter.

(SHHH, 50 percent of DC input is heresy - but it works a whole lot better than 50 percent of the Xmittr's RF output.)

A couple of 6K6's - 6V6's - or 6L6's work very well with a 5K ohm CT primary to whatever the TX needs on the secondary, and a 2 X 6L6 or 2X 5881 audio amp with 400 V on the plates is adequate for a transmitter with up to 75 watts DC input. A single 6146, for example.

Hope that helps

73  Pete Allen  AC5E

Posts: 9304


« Reply #9 on: November 14, 2007, 04:26:03 AM »

To modulate a stage the stage being modulated has to be very NON-linear. It has to be well in class C and switching hard. This is true no matter what modulation scheme you use on that stage.

You never want the modulated stage to be linear or modulation is impossible.

You can use a variety of method to apply audio, but the method and modulation power all varies with the type of tube and the method.

For example a plate modulated tetrode has to be run in class C, and it HAS to have part of the modulator power applied to the screen. Otherwise it won't make undistorted modulation. It takes a bit less than half the plate input power as audio for 100% modulation with a sine wave.

If that PA is a plate modulated triode, it takes about half the dc plate input as audio power for 100% sinewave modulation.

There are several other schemes. You have to pick one.

73 Tom



Posts: 208


« Reply #10 on: November 14, 2007, 08:45:05 AM »


Thanks for all your replies, suggestions and caveats!  The light bulb
in my brain is beginning to turn on :-)

I have am acquiring quite the collection of ARRL books (my current
studies being focused on the 1940’s era).

It’s funny .. I did found only *one* complete AM/CW transmitter circuit in my
1965 edition (Forty-Second Edition) on page 181 that appears to grid
modulate a 6146B output tube.  (double sideband AM).

I guess that the League assumes that one is to roll-their-own when
it comes to modulating a transmitter’s output.

Thanks again!


Posts: 9304


« Reply #11 on: November 14, 2007, 07:39:14 PM »

The best book I have ever seen for technical explanations of modulators and how modulation works is:

Essentials of Radio
Slurzberg and Osterheld

It has many pages on AM modulation and how and why it works.

73 Tom

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