DC checks of filament voltages
 

After a rather long and arduous thread on a set owned by a fellow VK member, I came across an issue that can effect all of us and I think warrants some checking.

The issue presented as a rapid failure of a shunt tube that was traced down to an incorrect DC potential on the heater of that tube. After thinking about it and noting that the CRT used the same heater source I surmised it would be a good thing to check to avoid excessive stress on our hard to get CRT's.

anyway I have a very short video on checking the shunt, I could have done the same with a CRT test socket and will do so in the future.

https://www.youtube.com/watch?v=ALleVRf3gBE

I don't recall seeing this covered here on VK maybe it has I just missed it or maybe I am all wrong about the issue (if I am feel free to correct me).

Good catch, I don't think I've ever read for heater bias on my sets- checking after work today!

I check the RCA manual on my CTC-5, it has the filament up to 300v which puts it practically right at the cathode level on my 21AXP22.

it was not until the CTC-12 did they drop it down to about 180v using the divider (before that there was just a 10k resistor to the power supply lower B+ supplies.

Not sure why the change unless it was just a compromised voltage to better suit that IF tube (cathode voltage on in was in the 160v range) I did check and the CTC-11 also used the IF the same way with the 160v cathode but still had the filament at 275v, maybe they started having a lot of HK shorts on that IF tube?

Funny, I was considering HK breakdown voltage in an amp I designed, but would not normally think of it as a cause of failure in mass produced gear...What a fascinating reminder that one has to consider EVERYTHING in tracking a difficult fault.