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

To: "" <>
Subject: Re: [Amps] Question about grid current in 4CX or 3CX
From: David G4FTC <>
Date: Sat, 16 Mar 2019 14:59:51 +0000
List-post: <>
As you mention screen voltage, I assume you are talking of tetrode amplifiers.

In a conventional grounded cathode/filament 4CX tetrode linear amplifier biased 
for class AB1 operation, the control grid shouldn't go positive and therefore 
grid current shouldn't flow. Some tetrodes, the 4CX350 if I recall correctly, 
have a zero control grid dissipation - any grid current will destroy it.

In the event of a failure of anode voltage or screen bias and assuming the 
control grid bias is OK, the drive to the control grid still shouldn't take the 
grid the positive (for Class AB1) and therefore the grid should not dissipate 
any power.

The real danger in a tetrode amplifier is if the anode voltage fails with the 
screen supply still applied - this will cause the screen to dissipate lots of 
power with the real possibility of destroying the valve.

Hope this helps

David G4FTC

From: Amps <> on behalf of 
Sent: 15 March 2019 15:37
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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