12au7 phase splitter/driver advice

[wiseoldtech]

Thank you kindly Andy. As it is the tubes are wearing out, it's just a simple thought seeing if there's some similar tubes that I can replace to improve the sound some what. As it is a commercial product & with no schematics, I wouldn't want to muck around with the internals for now.

Thank you again

How old is this unit?
I ask because if it's less than 20 years old, it must be designed to "wear out tubes" faster.
I've serviced hundreds of tube type equipment over the decades, and even some 1920's to 1940s stuff that had tubes still in good shape with high emission levels.

 

[sumotan]

Coming 9 years Wise. They're all factory stock, noticed that one of the 12au7 has started to glow brighter then the others which never did before.

Thanks you

 

[nigelwright7557]

Im also curious why Primaluna uses 2x 12ax7 in gain stage & 2 x 12au7 in phase splitter/driver stage.

The 12au7 has better drive for phase splitter.

 

[wiseoldtech]

Coming 9 years Wise. They're all factory stock, noticed that one of the 12au7 has started to glow brighter then the others which never did before.

Thanks you

Filament "glow" isn't any indication of a problem, unless it's actually open.
As for power tubes, Plate "glow" is destructive, called "redplating", and once that happens, the tube is damaged.

9 years...... have the tubes tested on a reliable tester for emission, because no one can determine tube condition by "looking" at them.

 

[nigelwright7557]

As for power tubes, Plate "glow" is destructive, called "redplating", and once that happens, the tube is damaged.

I designed and built an four output EL34 valve amplifier. Turned it on and one of the EL34's plate glowed bright red. I quickly turned it off. I did an inspection of the pcb and I had missed an EL34 grid stopper resistor. The valve was fully turned on and passing loads of current. Inserted grid stopper resistor and everything was fine.

 

[wiseoldtech]

I designed and built an four output EL34 valve amplifier. Turned it on and one of the EL34's plate glowed bright red. I quickly turned it off. I did an inspection of the pcb and I had missed an EL34 grid stopper resistor. The valve was fully turned on and passing loads of current. Inserted grid stopper resistor and everything was fine.

A very short duration may have only eaten some small amount of the plate coating. You wouldn't notice it upon listening. But I bet you if you ran a comparison test, that tube would show reduced emission level over the other tubes.