"Robert Casey" wrote in message
...
Powell wrote:

I have Penta 12AX7A tubes. While looking for a replacements
I found these variations:
12AX7
12AX7EH
12AX7WB/7025
12AX7LP
12AX7LPS
12AX7-C

What accounts for the difference?

The "w" suffix meant that they met some mil spec, long life
or such. But realize that the military was interested on other
aspects of vacuum tubes than high fidelity audio work.

"W" stands for wolfram, which is anothe name for tungsten.
Tungsten produces a stronger filament than alloy substitutes.
LP -- low power. Filament designed for low power at the expense of reduced
tube life.
EH -- high power variant, possibly with a stronger plate structure.
"C" -- probably modified for a minor characteristic, such as cut-off
behavior.

For Powell, I would recommend the 12AX7UYA.