what transformers to use for The Real McTube?

[lightningfingers]

I have a 230V wall voltage here :?

[cd]

Two 230V primary, 12V secondary transformers.  Or 230V primary, 6V secondary.  Either will work fine in that circuit.  You can always use a 230V primary, 12V secondary and 120V primary, 12V secondary if you want the exact circuit equivalent, but in my experience the higher voltage doesn't make a lot of difference.

[lightningfingers]

Maplin have some transformers that are have a 0-240V primary and 12V secondary at 250ma. Will they work?

[Mike Burgundy]

you can use Duncan's power supply sim (www.duncanamps.com) to figure out what B+ voltage you'll get from what circuit.
The first transformer (230 -> 12V) supplies both the heater power, and, through the other transformer, B+. The number two transformer only carries B+.
Have a look at a 12AX7 datasheet (f'rexample the ones available at the link mentioned above); you can make a rough-and-tumble estimate of what works.
That says one of those takes 0.15A at 12.6V (heaters in series),  and 1.2mA (at 250V) anode current (per triode).
It will work if you just heat *one* tube (0.15A is "sufficiently"(?) below 0.25A. But not by too much). Will it support the anode supply? Two triodes in one tube.
So, for one tube, you're using 2*1.2mA*250V= 0.6W, equals 0.6/12V =  0.05A *at the primary(12V, now) side of the second transformer*.
That's NOT taking losses into account (which can be somewhat substantial), etc etc.
This leaves the first transformer to supply 0.15 plus 0.05=0.20A.
This could work, but I don't  feel comfortable with it at all. First off, we're ignoring losses. Next, there's the power-supply filter rush-in (high current while the emptied capacitors are charging up - nicely simulated in the sim mentioned above), and, possibly worst of all, the filament rush. Think lighbulbs: they carry a very high current while the filament is still cold. It then heats up (to the point of glowing), resistnce shoots up and current goes down accordingly.  This is also why lightbulbs always fail when you switch them on: they can't stand the current surge any more.
Long story short: it'll be very, very fine as the second transformer, but I'd go AT LEAST twice as high on the first one. Keep in mind that the output voltage of the first *should not* exceed the rating of the second.
I've used two 10W transformers for two tubes.
hih

[lightningfingers]

Thanks :mrgreen:

[puretube]

interesting thread:
http://diystompboxes.com/sboxforum/viewtopic.php?t=22846