Modulators And Drivers

[Nowhere-Man]

I've been on this a while now. I am not likely to be building a plate modulation AM transmitter any time soon but it's new terrain. I have before me a typical circuit which made the old brainbox labour somewhat. Roughly it consists of a microphone, a speech amplifier stage and a push-pull modulator. What I did find useful was a very clever grid bias system that places a selenium rectifier in the mains supply. If I understand correctly this passes negative cycles only to charge a capacitor with resistances used to set bias. I once restored a selenium rectifier so that was interesting. As to the main power feed a 5U4G rectifier is used to supply + DC. I'm led to believe a 6 valve, high voltage device like this would cost a fair bit to make.
According to this material, modulator output should be roughly 50 per cent of the voice amplifier power output. In this example 200 watts was quoted as typical.
Another point is the transformer coupling resembled the transformer circuitry (output) on that Saalburg I have. Centre tap is at HT and not grounded. Opposite ends supply anodes. Transformers back then had an adjustable tap.
People in the sixties commonly built these AM modulators.

[pwdrive]

Is it similar to this one?:

http://www.arizona-am.net/PHOENIX/NA7RH/4x1625_sch.gif

Lawrence.

[radioman]

NWM - a reference to the circuit diagram would be helpful.
Can you show us the circuit being discussed or provide a link to where you found it, if online ?

[Ed_Dinning]

Hi, plenty of detail on modulators in the ARRL and RSGB handbooks from 50's and 60's.
The transformers can be difficult to source but are simple enough to wind (mod transformers follow PA amp (not Hi Fi) amp practice).

Ed

[Murphyv310]

Hi.
My mate used to run WMR. He used two output transformers back to back on his quad 807 transmitter. Went round the world with his set up.

[Nowhere-Man]

Is it similar to this one?:

http://www.arizona-am.net/PHOENIX/NA7RH/4x1625_sch.gif

Lawrence.

It's on page 265. Headed as speech amplifier and driver. Uses two 12AX7's as speech amplifier, a 6C4 triode and two 2A3's in push pull. I noticed these latter have a filament winding of 2.5 volts only. Rectifier has 5 volts and the other valves 6.3.
Thus, rectifier supplies HT and microphone allows amplification. The push pull is listed as the driver for the final modulator stage. Yes, this is a bit new for me. I was swatting up generally on AM modulation but was forced to look at the process. I am doing plate modulation first. I had a look at valve prices online and the push pull were around 70 quid. Personally I think you can drop on valves dirt cheap sometimes. My X78'4 are worth a lot but I got them for a tenner.

[Nowhere-Man]

I try to force myself to do something every night after workinng by day. Sometimes I don't feel at all eager but once I start I warm up. I didn't know till recently the sidebands in AM increase power in the carrier. As Chas Miller once stated AM is a very old method but works as well today as it did decades ago.

[Murphyv310]

The accepted way is Anode modulation. It's basically addition of the audio to the RF carrier that is produced by the RF oscillator and PA stage.
All of My mates transmitters were home designed and built. In the early days of WMR (weekend music radio) the transmitter was only 50 watts and being a pirate the equipment was hidden but successfully ran on a number of car batteries. The transmitter was moved around the UK on many occasions.
There were also many years of nothing as it went off for sometime.
Transmitters got bigger overtime with one incorporating 12 valves.
AM though can be done on pantry transmitters with dual control heptodes, dual control Russian rod pentodes, screen grid modulation and many other ways to allow the use of low level modulation. I've made many over the years using my own designs.
Any reference to WMR is for information only and I'm not advocating radio piracy.

[ppppenguin]

I didn't know till recently the sidebands in AM increase power in the carrier.

Really? In simple AM the carrier power is constant. The sidebands represent additional power. In practice AM broadcasters do everything they can to dynamically reduce the carrier power because it costs a lot of electricity (=money) to generate it. The limits to carrier power reduction are set by the need for ordinary AM receivers to be able to demodulate the signal without too much distortion.

If you can do without a carrier you've got double sideband suppressed carrier. Eliminate one of the sidebands too and you have SSB. Both need more complex receivers but make much better use of the transmitter power.

[Terry]

Yes, from memory, the power limit on most of the amateur bands increases from 150W DC input for AM to 600W Peak Effective Power for SSB - at least, they were the limits the best part of 50 years ago!