Pilot light

Recapped a Philco 46-420 code 125 and the pilot lamp comes on slowly then continues to brighten until burn out after about a minute. All tubes check out and 35Z5 has been replaced several times with no improvement. What is causing the burnout?

Check the filament tap on the 35Z5. Should have continuity between pin 7 to both pins 2 and 3. If the filament tap is open what you have described will happen.

35Z5 replaced with one having continuity between 7, 2 and 3.  After 5 seconds in the radio the tube now has continuity between 2 and 3; 3 and 7 but not between 2 and 7.
So now my question becomes why am I losing continuity between 2 and 7 pins of 35Z5?

Have you tried replacing some of the resistors and capacitors in that part of the circuit? You might have some faulty caps and or resistors causing your rectifier tube to go open.

Thanks to all for your reply. I have solved the problem and am somewhat embarrassed. I have restored 200 to 300 radios and broke the first rule you learn with capacitor replacement. I installed the electrolytic caps backwards.
Surprisingly there was no damage to anything. I switched the caps around and all works as it should.

Ah, cockpit error! Been there, done that. Glad you found the problem.

I figured it had to be something with the power supply circuit wiring (caps and their wiring or what not) because usually a rectifier tube doesn't just go open for no reason, because usually rectifier tubes are fairly durable tubes and can take quite a beating before they go out unless something in the power supply circuitry is messed up which then the rectifier tubes are the least durable tubes in any tube circuit.

Patrick wrote:Thanks to all for your reply.  I have solved the problem and am somewhat embarrassed.  I have restored 200 to 300 radios and broke the first rule you learn with capacitor replacement.  I installed the electrolytic caps backwards.  
Surprisingly there was no damage to anything.  I switched the caps around and all works as it should.

No need to be embarrassed-we all can learn from your mistake --glad to hear you found the problem --Gary