building a tube amp

[km-r]

ill be buying 3x 12ax7 and 2x el84 for my very first tube amp, DIY!
you cant warn me enough of high voltages, ive been working on tesla coils and neon signs since highschool...

so, my question is about the output transformer.

my supply transformer would be around 235vac 20w.
so is my output tranny be rated 235-0-235vac on the primary and 8ohms on the sec?

or could you link me with sites that has a comprehensive and concise[sp?] details about building output trannies?

thanks,

[sfx1999]

Tubes are an inexact science. The impedance and such does not need to be matched perfectly. Anyway, there are various options. If you are considering buying one, you could get a Hammond 1650F.

One thing to note, don't make an ultralinear design. I haven't really heard good things about them.

The people over at http://www.ax84.com/ should be able to help you more with this.

[d95err]

my supply transformer would be around 235vac 20w.
so is my output tranny be rated 235-0-235vac on the primary and 8ohms on the sec?

or could you link me with sites that has a comprehensive and concise[sp?] details about building output trannies?

With 235VAC on the secondary, you'll get a B+ of about 300-340V. The voltage on the output transformer will be B+ plus the maximum peak signal voltage. So the transformer insulation must be able to handle at least 450-500V to be on the safe side.

Specifications like "235-0-235" are only used for power transformers. Ouput transformers are listed for their impedance and power rating. Perhaps you are getting output and power transformers mixed up somehow.

For a push-pull 2x EL84 stage at 300V, output transformer primary impedance should be about 8k ohms. Somewhere between 5k-10k will probably work fine. The secondary should be according to the speaker impedance you want, e.g. 8 ohms.

I recommend getting a Hammond 125E output transformer. It is very cheap, sounds great and has multiple choice of primary and secondary impedances. It is rated for 15W, but for guitar purposes it works well for a lot more than that (20-25W should be fine). Your 2xEL84 state should produce around 10-20W.

I wouldn't recommend winding your own output transformer unless you *really* know what you're doing.

[km-r]

i know some people from a shop that winds POWER transformers... so i would just specify them the primary impedance [8k you suggested], power rating+headroom... and interleaving the coil turns... etc etc...

oh btw, if its going to be 8k, would it be a dual 4k winding?