[Alex]

OK,
Thanks all. So, general consensus is that most likely there is a strict rule
as to which heater pin heats which triode in 12A*7 tubes.

"flipper" wrote in message
...
On Sat, 20 Mar 2010 10:42:46 +1100, "Phil Allison"
wrote:


"Alex"

If I need only one triode of 12AX7, and thus apply 6.3Vac on pin 4 with
pins 5 and 9 grounded, can I be assured that for all brands and makes of
12AX7s the *same* triode of the two will always be heated?


** Think that is a safe bet.

The leads from pins 4 and 5 would have to cross over inside the valve for
it
to be otherwise.

I pulled two from my amp, looked, and as for 'crossing' leads, the
internal layout defies all logic. One of them seems almost rational,
with the triode structures oriented parallel to the associated pins,
while, with the other one, they seem to have gone out of their way to
make it as convoluted as possible. Neither triode is physically
arranged to be near the associated pins so you can't suppose that a
pin will be 'near' where it is to go or that things don't 'cross'.

However, on the second one they ran pin 4 all across the bottom of the
base to get the heater on the far side, which looks to me like they
were making sure to grab the 'right' heater end for the triode on pins
1,2 and 3 so, on that basis, I'd say it's a fairly good bet.


..... Phil