[AK-VHF] RF amp troubleshooting

[tom at telmore.com]

 Hello, I would like to pose a troubleshooting question to the group in regards to an RF amplifier I am working on. I have an FAA AM-6154 amp I use for two meter SSB that uses a DX-393A tube. I have been getting close to 400 watts out since I started using this amp over a month ago now. This past Saturday I was adjusting the final by way of the tune and coupling control to make sure it was still peaked for max power output and the power output started jumping up and down and went down to about 3 to 5 watts. Since then I have not been able to get any power out of the amp say more than a couple of watts. The interesting thing is I still have plate voltage about 2500 volts, screen voltage is 390 and the bias set around -80. When I drive the amp with about 5 watts or so I get lots of plate current easily over 300 mils but no output. I can peak the input tune and coupling by looking at the plate current going up and down. It acts like the output connection has malfunctioned but I get continuity form the output connector back to the metal disc that acts as a coupling capacitor up near the tube anode connector. Those not familar with this amp it uses a variable disc driven by a front panel knob to control the output coupling and another knob controls a sliding inductor with finger stock type arrangment for the tune control. I checked the output tune cavity for shorts and finger stock not making contact with a ohm meter and I don't show any opens. Has anyone ever see a tetrode fail where it still draws plate current but won't produce any output power? I can't believe the tube is bad and the next place I need to look this evening is at a set of bypass caps at the tube socket on the screen pins to ground.







Thank You 
Tom Elmore KA1NVZ
Anchorage, Ak