12.6 VCT filament circuit wired in parallel??

[DrGonz78]

I have experience wiring 6.3vct filaments on tube amps but have never tried it with a 12.6vct supply. So I want to wire up a parallel filament circuit using 12.6vct powering 12au6 and 12cu5 tubes. Obviously I could wire it in series and I believe the datasheets for these tubes show the amperage specs running it 12 volts in series. If I run it in parallel should I expect that the current through the filaments will be higher than would be in series? I could not find schematics showing 12.6vct parallel filament examples so it made me unsure if I should proceed. I would be installing a 2A 12.6vct filament transformer in parallel for the 12au6 and 12cu5 tubes only.

[Enzo]

Um... I am not sure what you are talking about. The 12AU6 and 12CU5 are 12v heater tubes. Period. There is no series and parallel. You put 12v across their heaters and they light up. You could put two in series and run them off a 24v supply. You can wire two in parallel across 12v.

There is no way to power these two tube heaters from a 6v winding.

[g1]

12AU6 is 150mA, 12CU5 is 600mA for heaters. So for one of each you would need 750mA current from your supply.

[Helmholtz]

Just to make sure, when wiring in series the tube types cannot be mixed as they have different current demands, which means they have different heater resistance.
When wiring the heaters of a 12AU6 and a 12CU5 in series and connecting to a 25V supply, the12AU6 would overheat, probably blow the filament and the 12CU5 heater would hardly glow.

[DrGonz78]

Okay definitely not wanting to connect them in series as Helmholtz mentioned.

I do want to use a 12.6vct filament transformer to power them in parallel. I would then have 6.3vac on each tube heater connection with respect to ground. Measuring across the heaters should be 12vac correct? I am not trying to power them with a 6.3vct transformer.

[Helmholtz]

I would then have 6.3vac on each tube heater connection with respect to ground.

Depends on how/where you ground the heater winding. With a grounded CT definitely yes.

Measuring across the heaters should be 12vac correct?

Correct.

[DrGonz78]

I figured it was correct but sometimes I overlook something. Even on something so simple. Thanks for the help everyone. I will follow up on the widow maker amp thread (another thread I started) later when I install the filament supply.

[g1]

Grounding via CT is just an option, like Helmholtz suggested. You could have a CT referenced to ground, or to some DC voltage. Or no CT but a virtual one via resistors. Or one side of the winding grounded with no CT or virtual CT (like some old Tweed stuff).
In all cases there must be 12V measured between the 2 heater pins.

[DrGonz78]

Yeah before I cut the length of the CT wire on the transformer I will test it out to see which produces the least amount of hum. Connecting to the power tube cathode could be a good option. Got to order the part and will follow up on it all later. Thanks for the information.