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Re: [Amps] Question about grid current in 4CX or 3CX

To: Peter Voelpel <>
Subject: Re: [Amps] Question about grid current in 4CX or 3CX
Date: Thu, 21 Mar 2019 11:29:44 -0500
List-post: <>

Thanks for your reply.

I used the 4CX10,000 as an example because not many transmitters
use the 4CX7500 ( Harris HT10-FM ) A FM transmitter that I have installed in 1995 .... almost 25 years now.

I recently added a new exciter that is capable of outputting 1000 watts and it has a forward / reflected meter. ( the old 750 watt solid state driver didn't have a forward meter ), so I didn't know what power level was being put in the grid to meet factory specifications of 90ma to 100ma of grid current.

With the new exciter with the power meter, to get to 100ma of grid current, the rf watts was indicating 250 watts. The 4CX7,500 has a 50 watt grid maximum.

Both Plate and screen current is well below rated dissipations.

These tubes last 24 months or so in 24/7 broadcast service.

the Chinese 4CX7500A lasts 12 to 18 months.

If the grid dissipation is only 50 watts......
and if the manufacturer designs it to run at 100ma grid current
and if it takes 250 watts to make 100ma grid current ...


That is the question I am trying to get everyone to attempt to answer.

I have run these tubes for weeks at 250 watts into the grid with no screen supply due to failed screen supply .... I'm guessing that at 7000 volts plate voltage, some electrons are exciting the screen somewhere above zero, but power output under these conditions is less than 20% or maybe 1,500 watts.

I have yet to destroy a tube  ( ??? )  At least not instantaneously ....

Usually, they just slowly fade away to where I can't make 100% power at the rated filament voltage..... new tubes are run just above the "knee" of filament voltage.


Quoting Peter Voelpel <>:

Hi Don,

The grid dissipation of the 4CX10000D is 75W.
   ( and the grid dissipation of a 4CX7,500 is even less.. 50W )

It takes 150W of drive for 250mA of grid current with that tube in class-c
with anode and screen voltage supplied.

Are you thinking of a different tube?


-----Original Message-----
From: Amps [] On Behalf Of
Sent: Freitag, 15. März 2019 16:38
Subject: [Amps] Question about grid current in 4CX or 3CX

Question :

For the larger tubes with handles  ( commercial service )

Does grid current increase or decrease as plate voltage and screen
voltage is removed. ?

If it takes 250 watts to produce 100ma of grid current in a 4CX10,000.
is the grid just a big dummy load and is it doing the tube harm to
dump 250 watts into it when both plate and screen voltages are removed ?


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