mini step up transformer wiring

[mouldiwarp]

Please forgive me for what is probably a stupid question. I have tried to search for an answer already, but the search function crashes my internet browser....

I am looking to build a tube based guitar pre-amp pedal / effect. In order to power the thing I am looking to use a 3ma 12vac wall-wart supply. I will be taking the heater feed straight from the power input then connecting up to a transformer (in reverse) to step up the voltage to 240V.

My question is around the fact I will essentially be wiring a step down transformer in reverse and I am a bit unsure in my mind how this works. My 12VAC supply runs from 0-12v, the transformer I have selected has a 0-240V primary and I have the choice of a 6v, 9v or 12v secondary. In each case the secondary is center tapped (6-0-6, 9-0-9 or 12-0-12). I know that to get this to work properly I need to match the supply and the transformer properly but, in order to do this, do I use the 12-0-12 transformer, wiring up between one end and the center tap (12-0)?? or, do i use a 6-0-6 transformer giving me an effective 12v across the coil whole coil?? Or am I simplifying things too much here??

Thanks in advance

[wakibaki]

Originally posted by mouldiwarp
do i use a 6-0-6 transformer giving me an effective 12v across the whole coil??

Yes.

3mA is not enough to drive a heater.

3mA @ 12V is 0.15mA @ 240V. Which is not a lot of current, even before you subtract the heater.

I'm not sure someone who has to ask these questions should really be dicking around with tubes, they can be dangerous. A guitar usually provides a pretty good connection to a musician (or even a non-musician, so a lack of talent is no safeguard).

w

[mouldiwarp]

Fair one, and I am happy to say on this occasion it is less a case of me not knowing what I am doing, and more a case of me not knowing what I am typing . It should have read 3A 12Vac.
I have been through a lot of the maths, and it all seems to stack up. Running 2 * 12ax7's at 12Vac across the heaters (150mA per tube) and with pretty low anode current there should be enough overhead with 150mA @ 240V to run both tubes.

I am still very much in the learning stage and am not going to build anything until I am completely satisfied it is going to be safe... Which unfortunately means asking a few questions like these along the way. Sometimes the most obvious bits can also be the most confusing.

I have done a reasonable amount of work with tube circuits but this is the first time I have needed to put together a power supply, so its another small step in the never ending search for knowledge and improvement... but yes, they have to be safe steps..!! and I do appreciate your concerns and somewhat brutal honesty.