Phono stage 12AX7 tubes don't show much glow?

muncybob

Pulled the lid to roll some tubes since I'm noticing some noise.
The other 6DJ8 tubes in my pre have much brighter glow than these. Is this normal for the 12AX7? I can barely make out any glow even in the dark.

mhardy6647

Probably normal (assuming the circuit is designed for 12AX7s in the positions where it has 12AX7s installed?!), but it's my pleasure to make you worry, perhaps needlessly

Well, a 12AX7 is not a 6DJ8 -- the 12AX7 is wired with two 6 volt filament segments in series, with a center tap.

6DJ8 pinout (from TDSL)


6.3V filament connected across pins 4 and 5

12AX7 pinout (from TDSL)



Center-tapped 12.6 V filament connected to pins 4 and 5 (center tap at pin 9)

All the other connections are the same.

If you plug a 12AX7 into a "normal" 6DJ8 socket, it might work, but it will be getting only 6.3V to heat the filament. In that case, it might glow dimmer than it is supposed to. It would also be expected to have lower than normal electron emission. It may or may not work anything like "properly" in that case -- but it's not the proper way to operate a 12AX7.

Hope this is helpful and not too confusing!

muncybob

Good explanation but no worries if the proper tubes are in the proper places.

mhardy6647

That would be my guess, yes
Now, of course, if you can, you might want to check the filament (heater) voltage on the 12AX7 pins... but there's no reason a priori to expect there's a problem. Now, having said that it's pretty common to use DC to heat the preamp tube filaments (to decrease noise and hum) and it is possible that there's a problem with the DC heater supply... again, checking the heater voltage is the best diagnostic.

... but different tubes "glow" to different extents. Tubes with thoriated tungsten filaments (like a 211, for example) glow like light bulbs! A vintage 2A3's filament barely glows at all.

muncybob

I'll pop in some other tubes tonight for comparison. Probably nothing to worry about though, I spun Steely Dan "Aja" last night and it was sublime. Noticing the low glow of those tubes just got me wondering if I'm not getting the full experience.

mhardy6647

If the emission were unacceptably low, it'd sound terrible.
I can probably scrounge a couple of bad 12AX7s if you'd want to hear what they sound like

heiney9

Some glow a bit brighter than others. It depends on how the flashing is and how high the grid posts stick up through the support mica.

The lack of glow vs. another like tube is no reason for concern in and of itself. A bad tube will exhibit other characteristics if it's starting to go bad. I'm talking about signal tubes. Power tubes can red plate (glow very bright) if they are having issues. Then you should shut her down. But a signal tube, glow is almost never an indication of health.

Comparing a 6DJ8 and 12A*7 tube isn't apples to apples, since the construction is a bit different.

Hope this helps

H9