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12vac heater HUM - do I have this wired correctly?

1/17/2000 10:02 PM
12vac heater HUM - do I have this wired correctly?
I'm goofing around with a little Webcor phono satelite amp that originally hosted a 12AT6 for the preamp, and I converted over to a 12A?7 for the extra triode, and to add a tone control. The whole amp runs on a 12vac heater winding, w/a 12X5 rect & a 12AB5 output tube.  
I can glean from the tube manual that I should probably wire the fil leads to pins 4 & 5 on the 12A?7, but should I leave pin 9 unconnected? (This is how it is currently wired, and it's humming quite loudly whenever I pop in the 12A?7).  
Does the sequence of the tubes matter in this case, as it apparently does (according to the RCA tube manual) in 117vac series filament arrangements?  
What am I missing here? I know it's something simple...  
Thanks in advance for any help,  
- P
1/17/2000 10:34 PM
Rob W.

Pins 4&5 should be tied together on a 12AX7 and should be connected to only one of the heater wires, the other heater wire should be connected to pin 9.  
Make sure that you twist both heater wires together, and keep them away from everything else thats running DC...  
This should help out with the hum...  
Rob W. :-)  
1/17/2000 10:45 PM
"Pins 4&5 should be tied together on a 12AX7 and should be connected to only one of the heater wires, the other heater wire should be connected to pin 9."
Huh? Is that right? I remember pin 9 as being the junction of two 6.3VAC sections. To get it to run on 6.3VAC, you parallel the two sections by tying pins 4&5 together and connecting 6.3VAC across them and pin 9. For 12VAC you tie 12VAC between pin 4 and pin 5 and leave pin 9 unconnected.  
I think you'll kill tubes by overdissipating the heaters if you hook 12VAC across 4 to 9 and 5 to 9 in parallel.
1/17/2000 10:49 PM

R.G., your recommendation is exactly what I came up with after pecking thru the RCA tube manual. I must have another issue here then. I plugged in a guitar cord and did the 'touch your finger' test, and it sure sounded like 60 cycle hum to me (if it was an octave higher I'd have a power supply bug, right?)  
- P
1/17/2000 10:49 PM
Scott Swartz
Rob's Post Above Is For 6.3V Wiring of a 12AX7!
For 12.6V wiring of a 12AX7, one wire to pin 4, one wire to pin 5, no connection to pin 9.  
It doesn't sound like that amp uses a series filament string since all the tubes in a series string must be rated for the same current draw.  
On the hum, maybe the filament isn't ground referenced. Its possible they were doing that on the 12AT6 socket that you changed out. You could use the two 100 ohm resistors ala Fender Blackface, etc. or just ground one of the filament wires ala the Fender Champ.  
1/17/2000 10:55 PM

my error on the 'series' filament reference - good catch Scott. Re: the gnd ref theory, the original circuit had the #47 bulb attached to the top rail of the filament schematic, with the other side of the buld to gnd. The 12vac sec has a CT to gnd which sounded fine before I swapped the 12AT6 out.  
I have been doing my tests so far with the bulb OUT of the circuit - I figured that wouldn't matter, but maybe it does?  
Also, does the order of the respective tubes from sec matter?  
thanks all,  
- P
1/17/2000 11:05 PM
Scott Swartz

The bulb will cause an unbalanced current draw on the 12vac winding, but that probably not the problem as its the original hookup.  
It may be the 12AX7 itself. The 6.3v hookup is normally used because it is "humbucking". Some of the Russian and Chinese tubes are susceptible to filament hum even in the 6.3V hookup, which would be worse in the 12.6V. Maybe try a different 12AX7.

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