>
>I'm building a 'retro' transmitter for high quality amplitude
>modulation.
/\ John - I have done AM with plate-modulation and with low level
modulation using an AN-612 modulator IC - followed by a >1499w-PEP
linear amplifier. With traditional plate/anode modulation, some IM
distortion could be heard. With the AN-612 modulator/linear set up, no
distortion could be heard. / note: the Trio-Kenwood TS-440, 930, 940,
et cet. - as well as some other mfg's transceivers use the AN-612 for
generating AM and SSB. Motorola makes a similar modulator-IC.
>The RF part is to be a Johnson 500 (single 4-400A), and
>the power supplies, modulator and control relays are homebrew. To
>pay homage to tradition and have the bouncing violet glow when I say
>1-2-3, I started with 866A mercury vapor rectifiers. But due to space
>limitations on the big chassis and the size of the ancient iron I
>have acquired, now I am using silicon diodes. What was interesting in
>all this is that after reading old handbooks (both ARRL and Editors
>and Engineers - pre Bill Orr and after) you can observe the trend
>away from L input filters and center tapped secondaries with 2
>rectifiers >>>> to Cap. input, 4 diode full wave brides, or doublers
>for SSB amps. With the change to silicon came higher peak current
>ability (than tube rectifiers) and the change in filter
>configurations. I hear some of the folks here using as much as 100 uF
>of capacitance, unheard of 30-40 years ago. Back then it was big
>iron, swinging chokes, smoothing chokes, 4-8 uf of cap, low peak
>currents, low noise,
/\ Mercury-vapor rectifier hash is hardly low-noise.
Ozone is not healthy for humans to breathe.
>and fairly good regulation with load changes (as
>long as a resonance wasn't hit during modulation). Resonant input
>chokes were good here also.
/\ Resonant chokes are undoubtedly the best thing when load current
swings. However, swinging chokes are not - but few realize this unless
they have: a DC o'scope, a HV o'scope probe, and a stong curiousity. I
plead guilty your honor.
>
>I have heard that some of those old CT secondary transformers can't
>run FWB at full tilt without risking punch through near the center of
>the winding.
/\ The UTC S-type were infamous for this gripe.
>
>Another observation:
>I thought that iron was going to be hard to come by. Two local
>surplus junkyards had more than I could hope for, anything from 1500
>to 3500 VDC, single and three phase, 0.2 to 1 Amp DC. To top it off,
>I now have THREE plate modulation transformers, all variable tapped,
>from 300 to 600 W capability. All this for about a hundred dollars. A
>pair of 813 beam power tubes will be modulators, running AB1.
>
>This is the fun stuff in ham radio that I had nearly forgotten about.
>Thanks to this mailing list, I have been inspired to do it.
>
/\ The main problems with running AM these days is that it wastes watts,
and virtually all AMers tend to take longish, "lock to talk"
transmissions that tend to be a sure-cure for insomnia.
/\ cheers, John
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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