12AU7 Gain Question


I recently purchased (3) 12AU7LG tubes. Two for my PS Audio BHK pre amp, and one as the input for my tube amp. The tubes were selected by the seller based on my equipment, and have different gain.

2 pre amp tubes are Mu A=16 and B=16

1 power amp tube is MU A=2016 and B=16

Ok, so I'm a dummy, and although the boxes were marked as such, I mixed the 3 tubes up, and they all look exactly the same.

Question is, can I use a multi meter on the tube pins to test the gain difference? How can I identify the odd power tube?

Do I need a tube tester?

Can I just keep rotating the tubes to try to find a gain in DBs in the room, or channel?

Thanks Mark O

 

 

128x128marktheshark

12AU7 has a mu (usually indicated by the Greek letter for “m”) of 16. If your tubes are all 12AU7s, then mu = 16. Mu is linearly related to gain, usually indicated as “A”. But mu is a product of transconductance (Gm) multiplied by plate resistance (Rp). These parameters vary somewhat according to plate voltage, grid bias, and plate current. 12AU7 has a Gm of about 6700 micromhos, when new. But if you measure Gm of several, they’ll vary a fair amount around that value. So all these numbers are “soft”, just a rough estimate. Your vendor may have sorted those tubes based on noise, but you can use any of them in any socket fit for a 12AU7.

The 2016 is clearly an error or "misprint". That’s more like a transconductance reading than mu (not possible to have mu that high). I wonder if the doofus seller mixed those readings up (don't do drugs).

Don’t use a multimeter. I’d just use them as is. Whether that mu is actually 16 (good!) or 20 (ok) it’s probably fine. 20 to 16 is still within 20%, which is close enough for most applications.

The right tube tester will give you the gain for each triode, but I don’t think that a 12AU7 can have a gain of 2016, that number may just be to mark the box of the tube intended for your power amp. I think you have 3 tubes with a gain of 16 for each triode.

The seller may have selected one of the tubes for the amp for another reason, though. I don’t see any harm in swapping the tubes around to see if one combination sounds better than another. Call the tube seller and ask him how the tube for the power amp is supposed to be different. There may be no difference between the tubes.  If you need to get them tested for some reason, I can do that.