The AM Forum

I have been thinking about how 500W-kW (plate-modulated tube) transmitters are laid out/designed (just an intellectual exercise right now).

It is my basic understanding that what is needed (excluding power supplies) is the following (typical 3 components of a Globe King plus a VFO, or so it seems):

o  VFO (or Crystal Oscillator), which may or may not be part of the
o  Exciter,
o  Modulator
o  RF Amp

What I have been stumbling over is whether the RF Amp stage is typically just a linear amp, in which case the Exciter signal would be modulated by the Modulator prior to it being amplified by the RF Amp.  This seems like it would require a smaller Modulator and may be seen as a benefit.

Or is it instead the case that the signal coming from the Exciter is first Amplified by the RF Amp and then that signal is modulated.  This seems like it would require a much larger Modulator, but perhaps there are benefits to this design that I am unaware of.

So which is it?

You usually modulate the final RF stage

THere are many methods of transmitting an AM signal.  Two that you touched on are Class C plate modulation and linear amplification of a low level AM signal.

Linear amplification, for reasonable distortion levels, is highly inefficient.  A lot of heat is generated, and you generally need a tube with plate dissipation WAY above the power output, due to the PEP issue.  You need plenty of headroom so the power supply, cooling, etc designs have to have that taken into account.

Class C plate modulation of the final RF tubes on the other hand is very efficient in the RF realm but still needs a class AB or class B audio stage which also should have plenty of headroom for voice peaks.  This is the way it's done in the Glob Queens, Valiant, etc.

You *can* however have a low power plate modulated exciter driving a higher power linear amp.

This is exactly how most of the higher powered AM signals were generated in the late 20's and early 30's. These rigs used a plate modulated stage (Heising) or sometime grid modulated stage that drove one or two stages of linear amps. Worked then. Works now.

Talk to Bob, W2ZM for all the low down. I think he even has such a transmitter that he puts on the air from time to time.

A class c plate modulated stage is acting like a mixer. RF and audio inputs with modulated RF coming out.

 ;D

I don’t think there is any best or preferred method of generating AM for amateur radio use. More efficient methods make sense for broadcasters since they run higher power and transmit continuously. Amateurs do not have such requirements, so less efficient methods of generating AM are just as good (maybe even better in some cases) as any other.

I find plate modulated rigs rather boring since there are so many of them on the air. Running something new or different is more interesting. But ultimately, do what’s best for you. Don’t let anyone put you in a box or tell you there’s only one way to make REAL AM. BS.

After all, are we on the air to communicate or just show off rigs? Either the operator is interesting and has something worthwhile to say or he/she doesn't. If they don't, they'll still be boring, no matter what rig they are using.

Hi !!

Well, it's a good question with a not-so-simple answer  ;)

The term "plate" modulation is an interesting term - misused, sometimes misunderstood, but nevertheless interesting :-)

"Plate" modulation really means "High Level Modulation" of the RF amplifier stage - the voltage between the Plate and Cathode (in the case of a tube - or the voltage between the Drain and Source for a MOSFET, etc.) is varied at an audio rate.  The "plate" voltage is the voltage between the plate and the cathode, regardless of what method is used to change it if that's what is being moduated [the difference between plate and cathode] it's "plate" modulation. 

You could tie the plate circuit to a fixed voltage supply (with respect to ground), and apply modulation to the cathode circuit (assuming you are not varying the grid-cathode voltage), and you have the same thing - high level modulation, or in the case of a tube "plate" modulation.  This type of high level "plate" modulation is/was very common in tube broadcast transmitters using high efficiency series modulations.  Note: cathode modulation, where the grid-cathode voltage is varied is NOT high level modulation - it is grid modulation, or a combination of both depending on the amount of grid modulation [rat hole alert!].  8)

This High level modulation can be accomplished in a number of ways.  A transformer coupled, push-pull (or single ended) modulator is one of many ways to get the job done.

A series modulator, varying the voltage between the plate and cathode is a high level modulator.  If this series modulator happens to use pulse width modulation *internally* as a control function it is STILL a series modulator as far as the RF amplifier is concerned - and a high level modulator at that.  Sometimes, one will hear folks talk about a "pulse width modulator", but REALLY, since the pulses are all filtered out and the modulator is generally connected in series with the RF amp, it's just a high efficiency series modulator.

There is no "best" modulator or modulation scheme.  It depends on what you want and how you like to run your station.  For me, I am very sensitive to wasting electricity - it is very expensive here, and, in any event, I don't like wasting power anyway... so I do everything to avoid doing so, to the point of eliminating all filaments [ so there is no stand-by power loss ] in all equipment (everything is solid state, home brew).

If you're staying in the tube realm, and you want high level modulation, and superior audio quality is what you're after, don't use transformers.  You will absolutely be able to produce better audio [all other things being equal] with a direct-coupled, high level modulator.

Of course, low level modulation is also a nice possibility if you don't mind the inefficiency (30 % efficient typical).  I used grid modulation on a number of fairly high power transmitters.  Quality was outstanding... efficiency was poor.... system was dirt simple to get up and running.  That's why I used it.  Used 3 4-400A tubes in parallel - grid modulated.  Made a nifty heater, too :-)  Also had a pair of 813s, grid modulated.  Something to think about.  You don't hear a lot of big grid modulated transmitters on the air.  You hear linears, but not many grid modulators at high power.

Anyway, more info to digest  8)

Talk later and Regards,

Steve

<joke>
Damn You Steve!
Damn You and your Class E Siren Song!
</joke>

:D

Tom, K1JJ (and his Big Tube Siren Song) had me all sold on the idea of making a (very nice) big tube transmitter.  But your post has snapped me back to sense.  I really don't have the space or power to spare here right now.  The days of racks and tubes transmitters are hopefully in my future, in a place where I have mroe space and power.   But for now all my tubes are destined to be in receivers and guitar amplifiers.  I really can't afford anything other than the amount of power that Class E efficiency  can afford me.  Back to looking for parts for my push-pull VFO.  Makes me sad in some ways, as I was really excited about the 813's rig, but glad in other ways, since at least I have a project to work toward, even if it's only a piece at a time.

Oh, that's OK, Dave...  :'(   ;)

You really need several different AM rigs to keep it interesting anyway. A solid state class E rig, an 813 X 813's rig and a lil PW tube rig are three good choices. Mix it up, baby!

I'm just finishing up an all new 813 X 813's rig under plexiglass. It's just so sexy to look at I can't control myself... :D  You'll be bewitched later too.

You have plenty of time to build whatever your heart desires over time, OM.

73,
T