12ax7 Pedal Power Transformer

[sbirkenstock]

Hi everybody,

I want to build an overdrive "pedal" with 12ax7 tubes.
I was looking for power transformers for the 6.3V heater supply.
What I found is an transformer called TRA200
https://www.die-wuestens.de/eindex.htm

This one delivers 6.3 volt + 230v secundary + 20v secundary.
You can add up the 230 + 20 = 250v.
I do have 230v supply.
So can I directly rectify the 230v line supply? (for the plate supply of the 12ax7)
What advantage does an transformer have to transform from 230 to 230?

Possibly just avoiding hum?

If I can directly use the 230v from the line, I would only need an 6.3v transformer.
I actually did not find any....
Any hint where to source one?

Best regards,

Stephan

[Stevem]

Yes you can ,and you will end up with about 320 volts which is way more then you need, but this is a good thing !
This will allow you to set up at least 2 , maybe 3 RC sections of filtering which will leave you once you work it resistor wise with about 180 volts of pure AC ripple free D.C for plate voltage!
If you want even less hum you can yet add another RC section to get about 40 volts to apply to the heater o sheild the cathode from the AC heater .flux
180 vdc for the first gain stage is a good starting point if you ask me , and for the second gain stage if you want the first stage to be able to clip it then about 80 to 90 volts on the plate should be good.

Just note that the lower your plate voltage no only will that gain stage clip sooner, but the high end will roll off also so concider that as a fine tuning aid also.

Note that you can go on line and view the tube info section that starts on page 641 of the RCA RC-30 tube manual, and this should help you a ton in setting up your two gain stages!

[sluckey]

A 12AX7 can also be lit up using 12.6VAC or 12VDC. Take a look at this link for another approach to powering a tube pedal...

http://el34world.com/projects/tube_box.htm

This method uses a 12VAC wall wart (available in USA or Euro style) to power the pedal, so no dangerous line voltages need to be run to the pedal. The 12VAC is rectified to provide 12VDC for the filaments and it is also connected to a small toroidal transformer that steps the 12VAC up to about 240VAC to be used with a bridge rectifier to produce B+. This is really a neat and safe way to power a tube pedal. Definitely worth considering.

[martin manning]

So can I directly rectify the 230v line supply? (for the plate supply of the 12ax7)
What advantage does an transformer have to transform from 230 to 230?

Isolation! Using the line voltage directly will create a dangerous situation where a fault will expose you to line voltage potential with respect to any grounded object, such as your guitar strings. A transformer such as you described seems like a good option for a compact solution, but the one you linked is much larger than needed for a couple of 12AX7s. Another common arrangement is two 6.3V (or 12.6V) transformers back-to-back, with the low-voltage windings connected in parallel. The filament supply is taken from the middle, with the line being isolated from the HT on the other end.