Heater wiring 12AX7 (one triode only)

[brett]

Hi.  I'm not thinking clearly in Australia's scorching summer heat.  35C.  I need a clear thinker to check this.
I've got a 6.3V supply to a 6J1 "power" tube, with one "end" grounded via the pcb and chassis.

The amp hasa 12AX7 (twin triode) preamp, using just one of the triodes (the 'first', pins 1,2,3).  My question concerns the heater. 
The hand-drawn schematic shows the current (pun) set-up (pin numbers have a '/' after their number). 
Pin 4 is grounded and so is 9, so there's no heat on the first triode. 
My first thought is to
1. swap the connections to 4 and 5 on the triode. (so pin 4 is "live", not grounded).

Is it ok if pin 9 stays as it is - directly grounded?  Is it ok if pin 5 (heater for the unused triode) becomes grounded via the connection to pin 4 of the 6J1?
Thanks.

[PRR]

The preamp stages will usually want the heater circuit center-tapped for hum cancellation. Then pin 9 isn't at ground.

[anotherjim]

The drawing suggests there is no centre tap. You probably need an "Artificial Centre Tap".
Read this...
http://www.valvewizard.co.uk/heater.html

[brett]

Thanks. 
Unfortunately the PT doesn't have a centre tap, and it's 6.3 VAC.
The preamp B+ has (theoreticly) very little ripple (0.1-0.3 V), and the 12AX7 (Sovtek) has a hum-minimizing spiral filament (most do now, but you used to have to buy a 12AX7S and 7025).

Tmrw I'll give it a test as-is, and report how it goes.  It's a bit crazy spending 100 hours to build a 12AX7 preamp for a 0.3W $25 6J1 power amp.
Could have just used the 6J1 (or maybe a 12AU7 for both pre and power?).

[amptramp]

Is there any spec on which triode section is powered from which filament pin?

If you are only using a single triode section, you might benefit from switching to a 12AV6 which is a triode - double diode used in radios.  The triode section is identical to that of a 12AX7.  It has a 7-pin socket and the diodes can be used as diodes to ground in a fuzz.  If you want a lower-gain triode, you could switch to a 12AT6, which has the same pinout as a 12AV6.  The 12AV6 and 12AT6 are cheap devices.

One other bit of advice is to not ground the filaments.  If the filament voltage is biased positive, hum will decrease.  The filament itself is an electron emitter and there is always going to be some showing below the cathode and with some tubes, above it as well.  Electrons are emitted as a cloud around the filament - not as much as from the cathode but still some.  The filament emission travels above the upper spacer and below the lower spacer and is in line with the grid and plate.  If the filament is higher in voltage, the grid will repel all of these electrons back to where they came from because it will act like there is a cutoff bias.  If not, the filament electron cloud will modulate the plate current and induce hum.

[PRR]

It is a heater, not a power feed. Obviously two 100r resistors.

> The preamp B+ has (theoreticly) very little ripple (0.1-0.3 V), and the 12AX7 (Sovtek) has a hum-minimizing spiral filament

B+ buzz is not heater-induced hum. Spiral heater is not 100% rejection. Decades of studio and guitar amplifiers suggest a CT heater circuit and probably biased-up to over +20V. (Oddly, negative may be just as good, but rarely convenient. Over 100V risks slow breakdown.)

> Australia's scorching summer heat.  35C.

I'd trade you: we have freezing rain, icicles on the dog. But 95F may not be my thing anymore.

[GibsonGM]

50F is a heat wave.