Mullard 12AU7 tests bad when running grid leakage test?

[sbelyo]

Hello..

I have a Mullard 12AU7 that tests good for emissions but when I run a grid leakage test one side shows questionable, and the other side shows bad.

What will this tube do when I use it?

[Miles Prower]

That excessive grid current will probably shift the Q-Point to a less linear portion of the plate characteristic and cause more distortion than you'd expect. It might also lead to poofage, depending on what the plate load is, and if it can allow that much current to flow.

[DF96]

May be noisy, although whether this is noticeable depends on where it is used. Grid current may cause distortion if fed from a high impedance - this is a separate effect from the distortion caused by wrong bias.

Unless this is being used in a non-critical position (e.g. cathode follower) I would bin it. Alternatively, if this valve has been sitting in someone's cupboard unused for 40 years then it may improve with use so use it, then test it again in a few weeks time.

[ruffrecords]

Heater cathode leakage most often leads to higher levels of hum unless of course dc heater supplies are used.

Cheers

Ian

[BZed]

Its a 12au7 that tests bad. Junk it and move on. All kinds of replacements are availble so I wouldn't risk the amp when it shorts or listen to the distortion it causes.

[sbelyo]

thanks... I got it on ebay with a telefunken 12au7. paid 47 for the pair. The telefunken tests fine. I'll junk it and move on

[DF96]

Originally Posted by ruffrecords

Heater cathode leakage

I think it was grid leakage. Usual cause is gas, but an emissive coating on the grid can do it too when the valve gets hot.