Heater/Cathode Leakage testing
 

Been working on a couple old GE radios.

Typical cheap AA5 AM only radios from the mid 1960s.

My EICO 667 has been amazing for tracking down heater/cathode leakage issues.

For the first radio The 12BE6 failed miserably with 1M, and replacing it cured an annoying modulation hum issue I was dealing with. I also replaced the 50C5which had 5M leakage. The other three tubes all passed with flying colours.

What do the vetran techs use as pass/fail values? The 12BE6 made a big difference, the 50C5 much less.

It's a matter of the circuit and how much hum you are willing to tolerate. Each situation is different.

If I find a tube with noticeable leakage I try it and see if a problem arises because of it. If not, I use the tube. If I am working on a set for someone else I might be a bit more strict to avoid problems later.

For leaks H/C sometimes its possible to 'bias' the heater-chain more positive than the cathodes of the tubes, by about 30V--providing that one side of the chain isnt earthed to chassis .

The theory is it reverse biasses the 'diode' created between the leaky Heater and the Cathode.

This Ive done on a set some time ago where I couldn't easily get the valve to repair it properly.

It works fine.