[rockadanny]

It could be the risk of blowing up tubes. I would imagine the last thing any manufacturer wants to hear is that a customer put some valuable "NOS" 6SN7 into a socket switched for 12V heater...

 

I almost did just that! Actually did the reverse (12SN7 tube into 6v circuit) so no harm, other than less than optimal operation until I realized I forgot to flip the switch from when I was messing with 6SN7 tubes.

[JohnR]

It may be a micro processor doing this difficult job for a lazy audiophile.

No, it's not a microprocessor. I'll find the schematic later and post a link.

[JohnR]

Here's the schematic: http://www.mediafire.com/view/?0vib1rvumiegnc2

(From https://diyaudioheaven.wordpress.com/schematics/amplifier/, it is for the garage1217 Project Ember)

I see how it works now (more or less). It won't work with 6SN7/12SN7 types. It's intended for autoswitching 12AU7/6DJ8 type of tubes.

[Ericus Rex]

The power transformer had to provide 6 and 12V what is a extra cost.

It's far simpler than that.  The switch changes the filament wiring from series to parallel.  No extra windings needed.

JohnR has a great point about end users blowing up there tubes.  But then again, some manufacturers have a switch on the back changing the voltage from 120-240v.  Europeans could certainly blow up their entire amps with that switch!