Heater to cathode insulation test jig

[catalin gramada]

Hello. Get a batch of Russian 6H8C (6SN7 equivalent)I intend to use as cathode followers. I want to run a test to determine how much potential between heater to cathode this tubes suport to may be used in application. I am able to sacrifice random couple of tubes to stress or broke the insulation but I need a help how to run a test to get the right conclusions. I have no ideea if have to follow a DC strike or a leak or AC+DC to establish the limits. May someone give me an ideea how to build a jig test ? Any thoughts please ? Thanks.

[Helmholtz]

Have you ever seen heater- cathode insulation breakdown in your application?
How does it show?

[catalin gramada]

Never happen. But still never used Russian octals who are suspect from previous experiences. When you was burned with soup you blow even in yogurt.

[Helmholtz]

In principle you'll need a circuit that operates the tube (say a simple gain circuit) having a floating heater supply.
Then a variable high voltage DC supply for heater elevation with a series resistor of maybe 1M for current limiting and current measuring.
Increase the elevation voltage while monitoring the voltage drop across the resistor.
Breakdown should show as a significant increase of voltage drop across the resistor.

[trobbins]

I'd suggest the simplest test would be if you could buy a used digital insulation resistance tester with common test voltages of 100V, 250V and 500Vdc (and even 1kVdc). Such testers are often handheld instruments and often on ebay for not too much. They typically display the resistance in meg or gigohm at the preset test voltage, and are a very useful instrument for those who restore or make new gear, as they confirm that mains primary circuits and power transformer and output transformer windings are ok at say 1kVdc to ground or between primary and secondary windings, and also for testing capacitor leakage.

As per Helmholtz's suggestion, I've also used a HV dc supply with sufficient series resistance and a current sense resistor like 1k to 100k, or even a voltmeter if you know its internal resistance.

I've measured Rhk for a batch of 12AX7 (see link) and also for 5U4G (as as a PIV measurement).

Your proposed test is well worth the effort for checking that sample valves are ok for cathode follower use - noting that there is reasonable evidence that tubes found in that location often test with lowish Rhk as there is a known slow degradation mechanism of the heater insulation with large dc bias - so something to add to the maintenance list after 5-10 years of operation.

Link: https://dalmura.com.au/static/Hum%20article.pdf

[Tom Phillips]

...Link: https://dalmura.com.au/static/Hum%20article.pdf

Tim,
Thanks for the link to your updated Valve Amplifier Hum article. I have previously read the 28 July 2015 version and I see that your 9 June 2023 update includes 10 additional pages of material. I think that the fault finding appendix will be especially useful to readers.
Cheers,
Tom