{Collins} Re: AM Modulators



If you run tubes in parallel, their output (plate) impedances appear in
parallel.  I think you're asking about running two sets of parallel
4-125as in push-pull as a modulator.   In this case, the required
plate-to-plate load impedance to match the driving impedance would be
half that of single 4-125as in push pull.  This presumes you have well
matched tubes in parallel and have taken steps to be sure they share the
load equally. A caveat that comes to mind is that the usual method used
to assure load sharing, adding series resistance in the cathode leads,
will reduce the stage gain and increase the output impedance somewhat.
I think it might be easier/cheaper to design around single tubes with a
higher plate dissipation capability, unless you just have a lot of
4-125as and all their peripherals laying around.

Bob
WB4TGG

Clio Gunsmith wrote:

>Good morning to all. I have a technical question for the engineering background members that I am unable to find the answer for. If we examine the Eimac data sheet for the 4-125A tube used as a AM modulator  in Class AB1 we find that two tubes in push pull at 2500 volts on the plates will produce 330 watts output and exhibit a plate to plate load of 20,300 ohms. If we connect four tubes in push pull parallel the output would increase to 660 watts. Will the plate to plate load resistance be cut to 20,300/2 or 10,150 ohms???  Thanks for any help you can furnish. 73 Jerry K9GOZ
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Sponsored by the Collins Collectors Association http://www.collinsradio.org
Nets: Tues: 3.805 Mc-2000 Central / Thur: 3.875 Mc-2000 Central
Fri: 3.895 Mc-2000 Pacific / Sun: 14.263 Mc-2000 UTC
1st Wed (of the month) AM Net 3.880 Mc-2000 local (ET, CT, MT, PT)
Sun AM Net: 29.050 Mc-1200 Central






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