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Title: 4-400 screen modulation Post by: KA4RFA 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 Title: Re: 4-400 screen modulation Post by: KC4VWU 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 Title: Re: 4-400 screen modulation Post by: KM1H 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 KM1H Title: Re: 4-400 screen modulation Post by: WQ9E 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 Title: Re: 4-400 screen modulation Post by: Jeff W9GY 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'.
Title: Re: 4-400 screen modulation Post by: The Slab Bacon 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. |