[Amps] What causes arc over on the BeO2 ceramic insulator of an 8877?

[John Farber]

What causes these to occur, how can they be prevented, and is it possible to safely remove the carbon track made by the arc and return the tube to normal operation at the same typical bias voltages and plate current?  My amp runs at 4 kV on the plate. A 6 v Zener is used to cut off the idle current and the heater runs at 5 v. It performed well > 2 k out all day long for serious contesting and dxing for 4 years. Then one fatefull day it arced across the outside of the 8877s ceramic insulator for abt 6 secs then the hv caps in the ps arced and my quick response (scared the ;^5"$ out of me,)...I flipped the power off. FrIed all 20 diodes in the FW bridge and about 1/2 hv caps looked pregnant so I replaced em all. Installed a proper hv glitch resistor, removed the screaming blower which was way too noisy and too large for the rf deck and replaced it with a surplus squirrel cage blower made by ECC/Rotron for Collins Radio to cool a 4cx1500 used in a commercial FM radio station that was upgraded. Plus I remoted the blower to the shack floor and mounted it in a plywood box with a proper air filter for the blower and feed the airstream to the rf deck via a 6 ft run of 2 1/2 in dia ShopVac flexible air hose. I now have room to install a Triode Board by GM3SEK which Provides the missing multi-level fault protection sadly lacking in the orig amp design. I also shielded the input section from the tank, and cleaned up a lot of scabbed together K-mart construction and fabbed a new al sheet to cover all the UN used holes in the top of the chassis. Any suggestions will be apreciated. I'm willing to learn.  Thanx 73 John


















Sent from my Verizon Wireless 4G LTE Smartphone