[Amps] bias problem

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Tue, 20 Jul 2010 20:16:35 +0300, ???????????? ??????????
<panos2 at otenet.gr> wrote:

> 
>it will be a greate help if you tell me why do i have zero bias
>when i drive a linear amplifier at the G1 and when is not drived the bias is normal minus 125 volts there is a pair of 
>4-400 in paralel the linear was working ok anode is 2800 dc and G2 is 480 vdc there is a pi input and a pi output
>in sort is a clasic desine
>eny ideas why is hapening when i put drive power the bias going down ??

REPLY:

Hi Panos:  It sounds like the bias supply has lost its regulation. It
is still putting out the -125 volts, but when current is drawn it
fails. Does the supply have a load resistor? It may be open. 

You could test the supply by itself. Measure the -125 volts and then
put a resistor across it to simulate a load and see what happens to
the voltage. It should change very little. 

Also, if there is an RF choke in the bias supply line, make sure it is
not open or raised very high in resistance. Good luck.

73, Bill W6WRT