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Author Topic: 4-400 screen modulation  (Read 4429 times)
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KA4RFA
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« on: September 28, 2009, 08:45:47 PM »

I'm just getting back into ham and I'm new to the forum.

I'm amazed at wealth knowledge of the forum members.  I have some questions.

I'm building a 50.4 AM transmitter using pair of 4-400's.  I do not have iron to plate modulate, so I want to screen modulate instead.  Has anyone done this?

Thanks!

Jim
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KC4VWU
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« Reply #1 on: September 28, 2009, 09:27:13 PM »

I know I'll probably get beat up over this, but let me make a suggestion; Cathode modulation. It's simpler, and you'll probably not notice much of a difference in the results.
73, Phil
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KM1H
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« Reply #2 on: September 28, 2009, 09:51:32 PM »

Run it as a linear, less hassles and you will still have a respectable signal if your exciter has decent audio.

In the 60's I built up a 4-400A for mountain top contesting with a club and used it for AM, CW and SSB driving with a 10A and TX converter.

Carl
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WQ9E
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« Reply #3 on: September 28, 2009, 10:07:53 PM »

Carl makes a good point concerning simplicity and the ability to use it for other modes is definitely a plus.  I am pretty sure one of the ARRL handbooks carried a design for one.

Rodger
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Rodger WQ9E
Jeff W9GY
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« Reply #4 on: September 29, 2009, 03:51:36 PM »

Built a HB pair of screen modulated 4-400's several years back.  Modulation is introuced using a hi fidelity output transformer, wired hi-Z in series with adjustible screen supply. Low Z side of transformer driven by a "stick" solid state 10W audio amplifier. A little "kink"  in the envelope, but feedback not necessary. System works great, good fidelity...of course, not efficient, but for amateur use efficiency not a big deal unless you're into 'green'.
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Jeff  W9GY Calumet, Michigan
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« Reply #5 on: September 30, 2009, 08:07:52 AM »

Built a HB pair of screen modulated 4-400's several years back.  Modulation is introuced using a hi fidelity output transformer, wired hi-Z in series with adjustible screen supply. Low Z side of transformer driven by a "stick" solid state 10W audio amplifier. A little "kink"  in the envelope, but feedback not necessary. System works great, good fidelity...of course, not efficient, but for amateur use efficiency not a big deal unless you're into 'green'.

Applying a little of the modulating voltage in phase to the control grid biass will get rid of the "kink" and make the modulated envelope waveform look like a plate mod rig. Been there, done that.
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