Which tube do you think would be best to have your pre amp wired for? Why, for, how, come? :dunno:

WW,

If you are running smoothed DC to power your preamp tube filaments then it doesn't much matter, but if the filament supply is AC, then the lower the voltage the less radiated electrostatic field around the wiring, socket and inside the tube, and this will lead to less hum. With tubes like the 12AU7, 12AX7 you have a choice of wiring their center tapped filaments for 6 or 12 volts. There are recommendations in the old tube manuals that if you can use a 2.5 volt filament version of the same tube instead of its 6.3 volt counterpart, that this is the route to go for less hum. The 2A5 is a 2.5 volt filament version of the 6.3 volt heated 6Y6. Sure, these are output tubes, but there is succeptibility of hum even here, and that is just a quick example I can think of. With AC, go low.

Rob

ahhh, okie dokie Rob. Thanks for that input!

Typically if you are using a vintage transformer you will have 5 volt taps for the rectifier and 6.3 volts for the rest of the tube filaments and the on/off light(s). I don't know why but it was pretty much an industry standard.

Filament voltages had their origins from being designed to operate off batteries. 6.3 volts was standardised so that the filaments could operate on a 3 cell lead acid storage battery. Ditto for 12.6. The same story applied to the early 2.5 volt filaments (a single wet cell is 2.2 volts).

I don't know why 5 volts was made a standard for many rectifier tubes.

If you want to hear a real conspiracy theory, those goofy series strings to operate off the AC mains, or DC mains, so that the expensive power transformer could be eliminated, led to the proliferation of huge stockpiles of worthless consumer item tubes we find today. I mean what the heck can you do with a box of 33JV6's? :( I wouln't walk a mile for a box of N.O.S. 7AU7's.

Rob

I thought those goofy voltages were for TVs.

Originally posted by Thatch_Ear
I thought those goofy voltages were for TVs.

Thatch,

Yes you are right, but they are for 60's TV's that I would never collect, and very few other collectors do either. The most collectible 'vintage' TV's are mostly sane design with power xfmer and 5 and 6.3 volt filaments. Even the odd series string TV's from the 40's used all 'normal' tubes with 6 and 12 volt filaments, with only the occasional weirdo like a 25L6, and hey, any tube who's part number ends in an "L6" is from good geniology and can't be all that bad. :)

Rob