I am going to play "Devils Advocate" as well...
All this dual triode stuff is really great, BUT caution needs to be used in the selection of the tubes used. Rolling most of the 7 pin tubes were no problem, but with the 9 pin tubes you can be exceeding the design specs of the amplifier's transformer. Which is my main concern as I don't want to see any ones amplifier stop working.
The transformer label from my MK IV shows that the driver filament maximum current is 1Amp, this would be split between the two sides.
This transformer is a custom made one either for or by Little Dot themselves. With China being notorious for bad labeling, out of spec / poor tolerances and minimal quality control, the 0.5A rating could be anywhere from 0.4A to 0.6A all depending on who was running the toroid winding machine that day. The LD's are designed to use tubes that draw less that 0.3A and most of the tubes suggested in the manuals to roll are in the 0.175A or 0.2A range which is well below the 0.5A theoretical winding output.
Some of the dual triodes are very close to or are exceeding the maximum current for the winding depending on the tolerances of the transformer. The 6V duals such as E288CC / 8223 draws a filament current of 0.47A and the ECC85 / 6AQ8 / B719 & 6L12 draws 0.44A. If all was good and transformer tolerances were tight, even then these two are pushing the edge of the maximum current. Which will cause the winding to heat up and open one day leaving you with a cold tube and no filament power.
I doubt LD will sell you a replacement transformer, they might... If they don't, you would have to spec one out through one of the transformer manufacturers online. Which might in turn have to be a custom One Off build and that would not be cheap. There are probably lots of ready made ones on the shelf but they will not have the right dimensions to fit in the housing or they will have extra windings etc...
I still have an issue concerning this 6V/12V switch as there are 12V dual's that can draw current on the edge of maximum as well such as the 12AD7 drawing 0.45A.
Someone should really measure the current on a 12V tube to make sure it is not running too high.
Any Multimeter should do, just put it on the AC amps function and insert it between the center tap of the tube filament and the pin that usually connects there.
I am really not here trying to discourage anyone from playing with the dual triodes, and I am not here to spread the doom and gloom, it's just people should be aware of what the consequences might be if they decide to join in on the fun. It is good to see that the bridged filaments were discontinued as that had me concerned, since there are people trying the duals that have very little electronic knowledge.
In my opinion I would stick to the 6V / 0.35A or less tubes to minimize the chance of possible transformer failure and forget about implementing the switch unless you know for a fact what the current draw is and you understand how to wire such a switch...
Edited by TrollDragon - 10/21/13 at 8:30pm