27-10-2016, 10:29 AM
(This post was last modified: 27-10-2016, 10:32 AM by Mike Watterson.)
Unless you are using it to derive curves, a valve tester really only verifies if the valve is similar to a new one or not.
You'll find that resistors in the old radios can be off by over +/- 30% or worse and the radio is fine, they were often only +/- 20%, whereas now if I buy 5% resistors they are often +/- 1%.
Similarly capacitors other than for tuning (which tend to be stable mica) can be -50% and +300% or more and the radio will work.
So similarly the radio may seem fine even if the valve has fallen to 30% emission.
The VCM163 tester I borrowed was very useful as it tested inter-electrode leakage. It's worth adding a 300V to 600V DC power source* limited by two 1 M Ohms in series and using a uA meter to measure leakage when heater is running. Leakage from G2 to G1 is a particularly bad issue in tetrodes, pentodes, Hexodes, etc and seems common on some rimlock power out types.
(Either a voltage doubler using 2 x 1N4007 or the electronics of a flash gun of disposable camera with 300uF replaced by 1uF plastic foil)
You'll find that resistors in the old radios can be off by over +/- 30% or worse and the radio is fine, they were often only +/- 20%, whereas now if I buy 5% resistors they are often +/- 1%.
Similarly capacitors other than for tuning (which tend to be stable mica) can be -50% and +300% or more and the radio will work.
So similarly the radio may seem fine even if the valve has fallen to 30% emission.
The VCM163 tester I borrowed was very useful as it tested inter-electrode leakage. It's worth adding a 300V to 600V DC power source* limited by two 1 M Ohms in series and using a uA meter to measure leakage when heater is running. Leakage from G2 to G1 is a particularly bad issue in tetrodes, pentodes, Hexodes, etc and seems common on some rimlock power out types.
(Either a voltage doubler using 2 x 1N4007 or the electronics of a flash gun of disposable camera with 300uF replaced by 1uF plastic foil)
