Very Basic 12AX7 Heater Question

[cfitzger]

I’ve been researching for a new (only my 2nd) project that uses 12AX7 tubes and I am confused by the heater wiring possibilities. It seems, based on the posts I’ve read, that given one secondary filament supply of 6.3VAC (ie, from a Hammond 269AX) that if I connect the + side to pin 4 and the – side to pin 5 I would get 12.6V operation. But, if I tie pins 4 and 5 together and connect the + side to that and the – to pin 9 I get 6.3V. My question is, how can the same 6.3VAC supply give double the voltage just by connecting the heaters in series? Or, am I misreading the posts and it actually requires a different 12.6VAC supply for 12.6V operation.

Example thread: (http://www.diyaudio.com/forums/showthread.php?s=&threadid=74853&highlight=12ax7+heater )

I’m sure I am missing something very basic here but I am relatively new to this and would appreciate your patience.

Thanks

 

[Sherman]

How you connect the heaters depends on the voltage supply you have available. If you have a 12.6V supply you connect the heaters for 12.6V by connecting one wire of the supply to pin 4 and the other to pin 5. If you are going to use a 6.3V supply then connect the heaters for 6.3V operation by connecting pins 4 and 5 together and connecting one leg of the heater supply there and connecting the other leg to pin 9.

One other point, there is no + or - with AC operation.

Also be aware that at 6.3V the heaters will draw twice the current, they would at 12.6V so make sure your PTX can supply the required current to the number of tubes you will be using.

 

[cfitzger]

Thanks! That is what I thought - you do need a different supply to operate at 12.6V.

 

[Nigel Goodwin]

Incidently, although I've not used them for many years, I can still remember the pin connections.

1 Anode
2 Grid
3 Cathode
4 Heater
5 Heater
6 Anode
7 Grid
8 Cathode
9 Heater centre tap.

Notice pin 9 is called 'heater centre tap', so you can use just pins 4 and 5 as 12.6V (heaters in series), or use 4 & 5 joined together with 9 for 6.3V (heaters in parallel). Presumably one half of the heater is for each triode? - has anyone ever tried using just half the heater?, and seeing which one works?.

 

[lndm]

Yes, they work independently, can't remember which though.

 

[ashok]

......has anyone ever tried using just half the heater?, and seeing which one works?......

If you look inside the tube you can clearly see which heater terminal goes to which triode . But how does that help ?

 

[DF96]

Which heater does which triode may be defined in the datasheet, and presumably should be constant across different manufacturers - however, this is one of those details which might vary especially with 'foreign' valves which are not really 12AX7/ECC83 but near equivalents with new paint on the outside.