Bill Orr specifically says in his notes that the "supper cathode driven
mode" is NOT recommended for the 4CX250 family of tubes.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of robert briggs
> Sent: Tuesday, July 21, 2009 1:42 AM
> To: amps@contesting.com
> Subject: [Amps] Grounded Grid (Screen) 4CX250B power gain
>
> I have built 4 amplifiers using the 4CX-250B-350R tubes in the so called
> "Super Cathode Driven" mode, tying the control grid to the cathode and
> grounding the screen grid....This design was in some old Bill Orr notes
> but I am now unable to find the reference, however I think this has been
> covered in this reflector some years back..
>
> Off the top of my head I remember the idle current for two tubes is
> around 30ma..I used 2500 volts on the anodes and with about 120 watts of
> drive I could get around 650-750 watts output....Not a lot if you
> consider feed through power...I also remember the tuned Pi input circuit
> had to be spot on to get maximum efficiency from the amplifiers..One
> other thing was when fully loaded the Grid current (Screen) was only 2-
> 3ma..
> I don't remember the input circuit parameters but it was fairly
> critical...
>
> You know while typing this it brings back quite a lot of memories..
>
> The amplifiers were very stable and reliable and as far as I know are
> all still in use today...Considering that the 4CX250/350 tube pulls are
> a dime a dozen it is still a cheap amplifier and the tubes seem to last
> forever...
>
> For sockets I used 9 pin loctal types or similar and made up a mounting
> plate with a good air flow around the sockets through the tube
> anodes...I coupled the cathode and grid pins all together using thin
> copper strap...
>
> These days I prefer to play with the readily available Russian triodes
> also very cheap tough and reliable and I wouldn't bother with the 250B
> type tubes again but at the time when that was all I had to play with
> they were good solid workers..
>
> 73.
> Bob VK3ZL..
>
>
>
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