More Power Supply Ignorance!

[david.rogers]

I think I have solved my transformer problem (see Power Supply Headache thread). However, two valves in my power supply - 12B4A and 12AT7 - have their heaters fed by 12.6VAC in parallel. I dont understand, as I thought to give 6.3VAC at each it would be necessary to run them in sequence...or is there some arcane process of running these valves at 12.6VAC? Wouldn't it be better to run them off DC?

[John Caswell]

Hi David,
The answer lies in what the individual valve heaters are rated at, for the 12AT7 because the heater is actually across pins 4&5 with a centre tap on pin 9 if you connect pins 4&5 together then the heater supply is fed to pins 4/5 and pin 9 at 6.3Vac/dc. If you do not connect to pin 9 then you can feed 12.6vac/dc to pin 4 and pin 5.
The 12B4 is similar but the heater pins are 4&5 with centre tap 3
So with 12.6 Vac/dc one heater wire will go to pin 4 of the 12AT7 and pin 4 of the 12B4 and the other wire will go to pin 5 of the 12AT7 and pin 5 of the 12B4.
Therefore the valves are wired in parallel across the heater supply.
If you want to wire them in series across a 12.6V supply you will have problems as the heater currents are different.
The rule is SERIES heaters - CURRENTS the same; PARALLEL heaters VOLTAGES the same.
It does not matter to the valve whether it is ac or dc it is the 12.6 Volts in this case that is important.

John aka Dr John

[david.rogers]

OK Dr John - thanks! I think I have just realised how I got this wrong, but I would be grateful if you would confirm I now understand! I actually have a 6.3VAC supply available from my transformer although the circuit is laid out for12.6. So I guess I have to feed the 6.3VAC heaters to pins 4 & 9 and again to pins 5 and 9. For the 12B4 it will be 4+3 and 5+3. Have I got this right?

Thanks
David