How important is the Ia measurement?

[Anyoldears]

I've seen some tubes up for sale that have a low Ia (presumably anode current) result in testing. The spec is 13+-3mA and the tube measures 8mA. How big a deal is this and what impact will it have?

Thanks

 

[sparkie]

It just means the cathode is not conducting as much. Ia is relative to the plate resistance is higher because the cathode to plate current is low. Which effects the transconductance & gain.

 

[Argonaut]

I've seen some tubes up for sale that have a low Ia (presumably anode current) result in testing. The spec is 13+-3mA and the tube measures 8mA. How big a deal is this and what impact will it have?

Thanks

To a degree it depends on the type of tubes you are talking about and the job that they are doing , for output tubes that measurement is beginning to head towards a little tired , power rectification / regulation perhaps arguably a little less of a concern depending on the quality of rest of the power supply , however for my part I would look for stronger measuring examples, if not even NOS .

 

[DasguteOhr]

I've seen some tubes up for sale that have a low Ia (presumably anode current) result in testing. The spec is 13+-3mA and the tube measures 8mA. How big a deal is this and what impact will it have?

Thanks

If the tube is specified as 13mA, this corresponds to 100% emission, new as if from the factory. These values result from the test report of the tube measuring device. which is operated at a certain voltage to test the tubes. Please note that different manufacturers of testing devices may produce different values. The device is only good the way the person uses it. If the tube is only 8mA, it doesn't mean it isn't working correctly. The gain factor may be lower and the stated lifespan may be shortened.

P.S
If nos tubes have been unused for a long time, it is possible that the tube will recover and show better values after a few hours of operation. She first has to be brought out of hibernation again

 

[Anyoldears]

Thx all for the replies. If theyre cheap I might risk it then

 

[sparkie]

Thx all for the replies. If theyre cheap I might risk it then

If they been sitting with a private collector, you probably can bring it all back by running the heater (AC voltage on heater, without plate supply) on the tube for a week and burn off the oxidation from storage.