Reverse power transformer.

[Hyld]

Hello!

I have a small tube amp, about 3 watts, that uses a Hammond PT269EX transformer (http://www.hammondmfg.com/pdf/EDB269EX.pdf). I'm thinking about replacing the Hammond an external wall wart that supplies 12VAC 3A to a 230V:9V transformer wired backwards. The B+ of the amp is 250V, so it may take a bit of play with filtering to get right.

I have only read a couple of posts about using transformers backwards like this. The set up will be about the same price as the Hammond transformer when using a wall wart and an antek torroidal transformer. I have a question of safety. One of my biggest fears is to plug my guitar in and go up in smoke. Is using an external AC adapter on the mains and stepping it up inside the enclosure safer if things go wrong than bringing mains power into the enclosure, closer to the guitar strings, or is that just wishful thinking?

Thanks!

 

[PRR]

_IF_ you can wire wall-power SAFELY, _I_ would just use the 269EX.

It may be a better part than some mystery wall-wart.

AC output wall-warts are becoming scarce.

12VAC to 9VAC is just wrong. Excess voltage on a winding causes higher core flux and more heat. Yes, small iron usually sags before it burns-up. I know a guy sells a setup like this. But it is clearly not good practice.

> closer to the guitar strings

My power comes 500 feet from the street, 10 miles from the dam, and mostly 50 miles from a boiler. Wires carry power pretty good.  I don't see how moving the lump a few feet along a wire is "safer".  A tenth-inch of air is pretty good insulation gap between wall-voltage and guitar jack; you probably want several inches anyway.

From the level of your questions, I do think you should be very cautious about working with wall-power. Indeed a pre-tested 12VAC lump may avoid that aspect.

A geetar amp normally wants a "SOLID" ground connection to the 3rd pin of the outlet. For your safety! And to absorb the buzz in the air. Wall-warts rarely support that.

 

[Hyld]

For sure the wallwart is only a two pronger.

I have built three other valve amplifiers, but they have been with written instruction, or at least a diagram. I am confident I could draw a scheme that would be appropriate, but not confident in construction. I have done some safety reading, but not sure if I'd like to put my life in the hands of reading. I suppose the most reliable way is to find someone with experience to check over the whole thing.

Thanks PRR.