Amplitude Modulation on the Amateur Bands.

[Ian - G4JQT]

The Heathkit DX40 and DX60 used screen grid mod. It could be made to work really well and used much less modulating power than anode modulation.

Unfortunately, at least with the Heathkit designs, mod level and linearity were also affected by the antenna load and it was a bit tricky to set up so the mod sounded really good, especially if you wanted to change frequency.

Yes, the carrier level varied with the mod level (but more than conventional AM), so when you watched a field-strength meter, the needle swings up and down much more than conventional AM.

A dig about on the web should turn up the DX40/DX60 circuits.

Regards,

Ian

[Hermit6345]

Originally Posted by milairuk

Does anyone remember a simpler method of transmitting AM using screen grid modulation ?
G3VUO

Wasn't this called Series Gate Modulation or was that yet another way of getting round not having a heavy modulation transformer and AFPA?


Ian

[Mike-repairman]

Hi John, Post #20.

I do remember screen grid modulation, but it was always considered a poor way of doing it. Never actually tried it myself.
I suppose in theory you could apply AM to any of the driver stages but it wouldn't sound as nice as a proper anode modulation.
Mike.

[Hermit6345]

Originally Posted by Mike-repairman

Hi John, Post #20.


I suppose in theory you could apply AM to any of the driver stages but it wouldn't sound as nice as a proper anode modulation.
Mike.

You could Mike, but then every stage after the modulator would have to be linear and not class C as most driver chains were configured. Modern rigs modulate in the early stages and everything after is linear which is no problem in an SSB rig as the stages are already linear.

Ian

[Mike-repairman]

Originally Posted by Hermit6345

You could Mike, but then every stage after the modulator would have to be linear and not class C as most driver chains were configured. Modern rigs modulate in the early stages and everything after is linear which is no problem in an SSB rig as the stages are already linear.

Ian

Yes Ian I did realise that the drivers would have to be linear, but as you say in the old days everything was class C.

Would still be interested to know if anyone has tried AM on 2 Metres recently and if they caused any TVI?

Latest spec for TV's is a lot tighter than it used to be and should in theory reject most other frequencies other than the TV band.
But just how good is it?

Mike.

[Skywave]

Just a quick 'input' on screen grid modulation . . .

Older versions of the ARRL handbook cover this topic. Mine is dated 1964 and provides a lot of useful info. - too much to reproduce here!

Old tech. books like this are really worth getting hold of - if you're going in for this sort of vintage stuff. You know the usual places to look for them . . .

Al / Skywave

[Herald1360]

We used to use an Emsac TX2 for 2m AM at G8EYC (Imperial College VHF call) in the early 70's- xtal control, series gate modulated 3-10 in the PA. The modulation system was affectionately referred to as "series grate"- presumably from what it sounded like!

Chris