Variable Heater Voltage

[Loon]

I would like to be able to use either 6sn7/b65 (6.3V heater) or 12sn7/b36 (12.6V heater) in my Crack.

To achieve this I will install a regulated dc filament supply http://www.pmillett.com/dc_filament_supply.htm

I would like to have the ability to change between these two heater voltages, but through a process more complicated than a switch as I don't want to run 12.6V through a 6sn7 heater. 

Would it be possible to achieve this by adjusting the trimpot on the filament regulator?

Next question.  Am I able to use the transformer supplied with the kit to feed the filament regulator, or do a need a separate power source?  Would I need to adjust the power coming from the source every time I adjusted the voltage supplied to the filament?

Please keep your answers simple.  In the words of Schultz - "I know nothing...nothing!"

 

[mcandmar]

As far as i know the crack uses a 6.3v heater supply so i don't think 12v will be an option without adding another transformer.

 

[Loon]

Can I increase voltage by adding a resistor?

 

[mcandmar]

No, that will do the opposite.  As the old analogy goes adding a resistor is the equivalent of reducing the size of a water pipe, hence restricting the flow.

 

[Paul Birkeland]

I would also have some misgivings about that regulator supplying enough current to run the Crack.  Though Pete does say it's good for 5A, the temperature rise at that current will be quite large, and well beyond what we would put into a kit.  Then, you must also consider that the AC current needed to generate that much DC current in the Crack heater circuit will be greater than just the AC current needed to heat the tubes in the first place.

This is better done with an extra power transformer that either has two 6V windings or a tapped 12V winding.

Technically speaking, there is a way to generate enough 12V current from the Crack power transformer to heat a 12SN7, but it would take up a fair amount of extra space.

 

[Loon]

Can you suggest a suitable transformer?

 

[Loon]

Would this transformer be any good?

http://www.mouser.com/ds/2/410/F-105Z-222625.pdf

 

[Paul Birkeland]

I would suggest using a 12V wall transformer, which are generally what a wifi router or cable modem will come with.

 

[Loon]

Thanks

 

[Paul Birkeland]

It is also worth mentioning that if you switch the 6.3V from the PT-3 transformer and 12V from a wall wart, I would ground one end of the wall wart just to be sure that your driver tube doesn't have a floating heater supply.

If you like the wall wart approach, there are jacks that actually have switches built in, so you could wire the amp so that it was 6.3V with nothing plugged in, then it would switch to 12V when you plugged in the wall wart to your Crack.

-PB