Hi folks,

I am responsible for the care and feeding of our GE pager transmitter
on 150mhz vhf frequency range. I don't have the model number here as
I am at home at the moment. It uses a single 4CX250 in the cavity final.

The problem is this, 10 watts output with 10 watts input, Plate
voltage is 2100, plate current is very low at 100ma (normal is 270)
and the screen voltage is around 60 to 80 volts.

The screen is supplied through an 80k resistor from the 2100 volt
supply and has variable resistors to ground to set the actual screen
operating point. I have placed as much as 500 volts on the screen
circuit externally with the tube removed to check the bypass caps and
they seem fine. They withstand the 500vdc without breaking down.
The tube is good, we tried it in another cavity (our spare amplifier).
The plate circuit is good also. At first I thought I had lost a plate
choke or connection causing the screen to draw excess current.

I know the screen is drawing excess current causing the screen
voltage to fall off. The control grid current looks normal and
behaves normally with tuning controls. I don't get it?? How can
I have excess screen current and yet have the low plate current?
The tank circuit seems normal, the tuning controls appear to work
properly, in fact every once in a while the unit will work perfect
for a minute or two making 250 watts output, then it goes back to
acting weird again.

Anyone ever have a GE vhf amplifier do this? Thanks!!

Hi,

If you connect the 80k Screen resistor to ground via an ammeter, how
much current do you see ? I would expect 26mA.

Are you sure that there arn't any shunt regulators on the screen supply
? Zeners, or an active regulator ? I have never heard of screen to HT
without a shunt regulator before...

Regards,

Mark

Gudmundur wrote:
Hi folks,

I am responsible for the care and feeding of our GE pager transmitter
on 150mhz vhf frequency range. I don't have the model number here as
I am at home at the moment. It uses a single 4CX250 in the cavity final.

The problem is this, 10 watts output with 10 watts input, Plate
voltage is 2100, plate current is very low at 100ma (normal is 270)
and the screen voltage is around 60 to 80 volts.

The screen is supplied through an 80k resistor from the 2100 volt
supply and has variable resistors to ground to set the actual screen
operating point. I have placed as much as 500 volts on the screen
circuit externally with the tube removed to check the bypass caps and
they seem fine. They withstand the 500vdc without breaking down.
The tube is good, we tried it in another cavity (our spare amplifier).
The plate circuit is good also. At first I thought I had lost a plate
choke or connection causing the screen to draw excess current.

I know the screen is drawing excess current causing the screen
voltage to fall off. The control grid current looks normal and
behaves normally with tuning controls. I don't get it?? How can
I have excess screen current and yet have the low plate current?
The tank circuit seems normal, the tuning controls appear to work
properly, in fact every once in a while the unit will work perfect
for a minute or two making 250 watts output, then it goes back to
acting weird again.

Anyone ever have a GE vhf amplifier do this? Thanks!!

Hi,

I know the screen is drawing excess current causing the screen
voltage to fall off. The control grid current looks normal and
behaves normally with tuning controls. I don't get it?? How can
I have excess screen current and yet have the low plate current?
The tank circuit seems normal, the tuning controls appear to work
properly, in fact every once in a while the unit will work perfect
for a minute or two making 250 watts output, then it goes back to
acting weird again.

I would say that the screen current is high because the plate
voltage is swinging down too far (i.e. there is little or no
loading on the tank circuit or it has become mis-tuned). One
possibility, if it has one, is an open plate blocking capacitor.

BTW, the screen current on that series of tubes is often used
as a tuning aid; some metering circuits even having a negative
screen current position for when the grid gets too hot and emits
electrons.


Cheers - Joe, G3LLV