I dont believe there is any way to run a 4CX250 style tube in GG without
destruction. Very few tetrodes have the necessary grid dissipation required.
A relay driven bandswitching input network is one way to improve the THP
circuit. Another is a regulated screen supply.
Using 4 4CX350's or 8930's should give an easy 1500W. Or water cooled
8072's since the basic tube is dirt cheap.
Carl
KM1H
----- Original Message -----
From: "Colin Lamb" <k7fm@teleport.com>
To: "Carl" <km1h@jeremy.mv.com>; "robert briggs" <vk3zl@bigpond.com>;
<amps@contesting.com>
Sent: Tuesday, July 21, 2009 1:37 PM
Subject: Re: [Amps] Grounded Grid (Screen) 4CX250B power gain
> "Tokyo High Power marketed a cathode driven 2 X 4X150/4CX250 amp that had
> IMD
> on par with grid driven. The price was high for the power and didnt sell
> well in the US. The few I worked on were well built and looked acceptable
> on
> the SA for IMD......on par with the exciters.
>
> http://bama.edebris.com/manuals/tokyohpl/hl1ka";
>
> I downloaded the manual and it has a nice schematic. As Carl said, it is
> cathode driven. It is not grounded grid. The screen voltage would be
> about 380 volts (regulated only by a heavy bleeder resistor from a
> capacitor only filtered power supply). The grid is bypassed to ground and
> fed from an adjustable bias supply. The cathode is fed through an
> attenuator. By operating the screen at normal voltage, the gain of the
> tube will remain relatively high.
>
> Grounded grid configuration should have some improvement in imd over
> grounded cathode because of the negative feedback inherent in that
> circuit. I have seen quotes of approximately 3-5 db or so. Additionally,
> adding a tuned input would pick up a few additional db. The measurement
> of this improvement, however, would be difficult, because the tuned input
> would improve the imd of the transmitter driving the amp (or, to be more
> specific, the lack of a tuned input could decrease the imd of the
> transmitter).
>
> All this is speculation without adequate tests and measurments to confirm
> it. I wish I had the time, energy and workspace to make such tests.
>
> 73, Colin K7FM
>
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