[Amps] New GS-35B amplifier..

[robert briggs]

Hi all....I have just completed a new single band 160 meter GS-35B 
amplifier...After troubleshooting a minor bias problem I put some RF 
into it this morning....Sadly I seem to have a problem which I have been 
unable to resolve as yet....The HT supply is 3300volts using a new 
AL-80B Power transformer....All this is operating normally....
Variable bias controlled from the front panel gives me a range from 
nearly zero to 250ma.....This is also operating normally and for testing 
purposes I have set idling current at 90ma......

Applying 20 watts of drive the amplifier tunes and loads properly grid 
current loading down to around 60ma anode current to around 200ma...If I 
increase the drive beyond 20 watts the anode current immediately shoots 
to 800ma and the grid current off the scale tripping the protection 
circuit....Power also drops to zero prior to the fault condition..

I have thoroughly checked all the components in the pi network and all 
appears fine.....

I am wondering if I have a bad tube or is something else breaking down 
in the output network somewhere...Maybe someone may have some ideas....

Happy Easter all........Bob VK3ZL....