Heater connection/voltage.

[C Moore]

I have always had a hard time understanding how/why heaters are connected. I am having a hard time with this Epi BC 30. By using KVL, Ohms Law, etc., can one of you guys explain how these heaters are wired and what voltage they are running at.
I am referring to the DC heat, of the pre tubes, in the schematic link below.
Thank You Very Much

[passfan]

they're 12ax7 heaters wired in series and run at 12 volts. we wire them in parallel to run them at 6.3 volts. The tube has to be warm inside for the atoms to release so voltage can be transfered between the cathode , grid and plate.

[C Moore]

So each heater, in each triode sees 12 volts?
How do we get away with running them at 6 volts?
Thank You

[martin manning]

So each heater, in each triode sees 12 volts?
How do we get away with running them at 6 volts?

Each triode in a 12A_7 envelope has it's own 6V filament. Both filaments have one end connected to pin 9, and the other ends are connected one to pin 4 and one to pin 5. If you connect the filament supply to pins 4 and 5, the two filaments are in series and require 12V. If you connect pins 4 and 5 together, and apply the filament voltage across pins 4/5 and 9 (typical in guitar amps), then they are wired in parallel and require 6V. I believe that this arrangement was originally designed so that these tubes could function in automobiles with either 6 or 12V electrical systems.

[C Moore]

I am embarrassingly dense sometimes.
It is all Crystal now.
Thanks Martin.
Interesting about the automotive theory btw..........

[passfan]

So each heater, in each triode sees 12 volts?
How do we get away with running them at 6 volts?

Each triode in a 12A_7 envelope has it's own 6V filament. Both filaments have one end connected to pin 9, and the other ends are connected one to pin 4 and one to pin 5. If you connect the filament supply to pins 4 and 5, the two filaments are in series and require 12V. If you connect pins 4 and 5 together, and apply the filament voltage across pins 4/5 and 9 (typical in guitar amps), then they are wired in parallel and require 6V. I believe that this arrangement was originally designed so that these tubes could function in automobiles with either 6 or 12V electrical systems.

The tube that the 12ax7 originally replaced was the 6SL7, which has a 6 volt heater. I don't know if it was their intent, but having the ability to work with either voltage would have made for an easier sell putting out the new tube. I didn't know they used tubes in automobiles, that's pretty wild.

[martin manning]

Yes, automobile radios were all tube from the 1930's through ~1957. Hybrids with transistor output stages followed and finally, by about 1962, they were all transistor. Size-wise, 9-pin would be an advantage over octal.

[tubeswell]

I didn't know they used tubes in automobiles, that's pretty wild.

They used to use them in everything from submarines to jet planes. (hence the acronym "JAN") - or were you just pokin' me with a stick?

[passfan]

I didn't know they used tubes in automobiles, that's pretty wild.

They used to use them in everything from submarines to jet planes. (hence the acronym "JAN") - or were you just pokin' me with a stick?

No I realized their use by the military and in earlier audio things and such ; I've just never seen an early car radio.

[M Fowler]

I get a kick out of some of the guys at my local rod run or car shows. Some of them are running those stock tube radios and others are putting high power tube amps in their trunks. Those large tubes glowing looks sweet.


Mark

[Structo]

That would be cool to see some tube amps in the trunk of a car pumping the subwoofers!

Man, I feel so out of touch and old with the new generation car Fi buffs.