Manfred,
great minds think alike!
73
Peter G3RZP
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Message Received: Aug 09 2012, 04:55 PM
From: "Manfred Mornhinweg"
To: amps@contesting.com
Cc:
Subject: Re: [Amps] 4CX1500B project
Vic, Bill, and all,
> One thing to be aware of when using swamped grid design: This tube
> has a fairly high input capacitance in grounded cathode mode, aprox
> 80 pF. You will no doubt have an SWR problem on the higher bands so
> you may have to switch in a parallel resonant circuit to compensate.
Instead of a simple plain parallel resonant circuit, which would need to
be bandswitched, a simpler solution is to include a basic low pass
filter section in PI configuration, that cuts above the highest band,
and absorb the tube's capacitance in it. Such a basic low pass section,
designed to cut off above the 10 meter band, will require about 100pF on
each side. So it can be built with the 100pF on the input side but only
20pF on the output, and the tube's capacitance will complete it. The
result will be low SWR on all bands, without needing a band switch
section at the input.
The load resistor would of course be at the tube's grid, after the low
pass filter!
I have two of 4CX1500B tubes in my junk box, but no sockets. These are
pulls from a broadcast station, and were given to me as being in good
condition - but I have no easy way to test them. Maybe some day I bite
the hook and build an amp for them, so I'm interested in this project
too. But for this time, I'm more interested in eventually finishing and
publishing my low cost solid state amp!
Manfred
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Visit my hobby homepage!
http://ludens.cl
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