811 Cold Filament Resistance?

[Justin Thomas]

Hi All,

So I've got a question about 811 tube filaments, Wondering what a cold resistance reading would be. I see they draw 4A @ 6.3V, so I'm thinking the hot resistance would be ~1.5 ohms. Is my math close?I don't have a way to test them besides using my meter to check for shorts between pins. Maybe I could rig up a test jig or something but I don't actually think I have a PT with 4A capacity @ 6.3V.

I'm getting around 0.7-1 ohm resistance on the filaments, cold. I know cold filaments should be lower resistance than hot, but that's even low for my meter. Is there any reason to suspect these are an issue? Someone would like to buy them & I don't want to sell duds.

Or should I just worry about them being open? There are no shorts to any other terminals - all read "OL" on my Fluke 112 resistance scale.

Thanks,
Jusrin

[Mick Bailey]

I only ever check for continuity when checking heaters and don't bother with the actual value so long as it reads low ohms. Due to the temperature/resistance slope you'd expect a low figure from cold. The only real way to check any tube is under operating conditions though, not only just powering the heater, but applying full plate and screen voltage to reveal any problems. Then there's the possibility of a tube being too microphonic and you can only really test for that if it's configured as an amplifier and vibrated.

[Justin Thomas]

Thanks Mick. I'll send em on then. I was only gonna ask about 10% of NOS price for them since I can't actually test them any further.

Jusrin