[Amps] Grounded Grid (Screen) 4CX250B power gain

[Gary Schafer]

Definitions:

The Collins 30S1 is called a "cathode driven" amplifier not a "grounded
grid" amplifier. Even though the grid and screen are at RF ground the DC
grid and screen voltages are not at ground. They have normal grid and screen
voltages applied so the tube can operate in AB1, no grid current.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of DF3KV
> Sent: Tuesday, July 21, 2009 2:13 PM
> To: amps at contesting.com
> Subject: Re: [Amps] Grounded Grid (Screen) 4CX250B power gain
> 
> The Collins 30S1 is a good example were it is done with a 4CX250B style
> tube
> 
> 73
> Peter
> 
> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Carl
> Sent: Dienstag, 21. Juli 2009 20:59
> To: Colin Lamb; robert briggs; amps at contesting.com
> Subject: Re: [Amps] Grounded Grid (Screen) 4CX250B power gain
> 
> 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 at teleport.com>
> To: "Carl" <km1h at jeremy.mv.com>; "robert briggs" <vk3zl at bigpond.com>;
> <amps at 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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