I built an HF amp back in 1970 with four 4CX250B's in parallel. The tank
circuit was a B&W 851 (I think) bandswitching assembly and vacuum variables
for tuning and loading. The input circuit was passive grid with drive
voltage developed across the non-inductive 50 ohm resistor from a Heathkit
Cantenna placed in the forced air stream under the tube socket subchassis.
As I remember calculating, this provided almost exactly the correct driving
voltage at the 100 watt level - no tuned input to worry about!
Performance and efficiency were great - I remember seeing well over 1000
watts out on 10 meters.
The whole RF deck, with centrifugal blower, was built inside a Heathkit
SB-series cabinet and the power supply with plate, screen and control grid
power supplies in an outboard cabinet. I was able to get Heathkit knobs and
meter for an SB-200 from Heathkit Parts department and laid out the front
panel so it resembled the SB-200 closely enough to fool people at first
glance - until they saw the Bird wattmeter readings!
Sure brings back fond memories - I wish I still had it. I had the same
questions then about why it wasn't being done commercially.
Al - K8EUR
----- Original Message -----
From: "jerry" <jlee@oceanwide.co.nz>
To: <amps@contesting.com>
Sent: Wednesday, July 14, 2004 2:22 AM
Subject: [Amps] HF and 4cx250b's
> Hi all,
>
> Is there some reason why there seems to be very few HF amps built around
> the 4cx250b's? They seem to be cheep and very plentiful.
>
> I am thinking of building an HF amp using 3 or 4 4cx250b's. Is there
>
> anything wrong with doing this?
>
>
> Cheers
>
> Jerry
>
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>
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