PIN 9 OF 12AX7...[?]

[petemoore]

What's pin 9 for???
 I've assembled a Shaka Tube, and attempring to get it functioning, I notice this open lug...hence the question...
 What is pin 9 on a 12AX7 tube for?

[Peter Snowberg]

If you're using 6.3 volts to power the heaters then you would use pin 9 as one of the power connections with the other being pins 4 & 5 jumpered together. If you're using 12.6 volts, just leave it unconnected. They put that option in there since 6.3 was a very common heater voltage for power tubes.

Take care,
-Peter

[Doug H]

If you're using 6.3 volts to power the heaters then you would use pin 9 as one of the power connections with the other being pins 4 & 5 jumpered together. If you're using 12.6 volts, just leave it unconnected. They put that option in there since 6.3 was a very common heater voltage for power tubes.

Take care,
-Peter

One of the advantages of running with 6.3v AC instead of 12v AC is that by using the heater center tap, the heaters are wired out of phase and provide some humbucking for the AC heater circuit.

Doug

[Paul Marossy]

Yeah, there is a noticeable difference between 6.3 and 12.6 volts on the filaments in terms of audible hum. I found that out on my Firefly build...