Isolation transformers....

[Riscchip]

Greetings,

I've read that an isolation transformer is necessary to be 100% safe using a variac. I generally run my old amps with a variac because my wall current puts out way too much voltage.

Can anyone point me in the direction of a suitable isolation transformer for reducing the chance of electric shock with a variac?

So far, I've turned up this one. Would it be suitable for use with a 100 watt Marshall?

http://www.tubesandmore.com/scripts/foxweb.dll/moreinfo@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?item=P-T171B

Thanks for your time.

[traynor_garnet]

I have a pro grade Hammond Isolation Transformer I'm not using. It's a really heavy duty unit, the type a tech would use on his/her bench.

I was using it with a 50C5 transformersless amp I had, but the amp is long gone.

Sorry for the spam, but tf you are interested let me know and perhaps we can work out a trade for a pedal or something. The thing is very heavy so shipping must be factored in.

TG

[vibroverbus]

if budget isn't a big concern, spring for an isolated variac, all in one solution, nice and clean.

if budget is a concern, look around antique radio / ham sites for how to build one via two identical back-to-back transformers - possibly the easiest piece of 'test' equipment to build, short of a 'dim bulb current limiter'. if you get transformers rated for 3A or more, should be plenty for your application. they might drop a bit of voltage, so if you want to get deluxe on it you could build in a voltage meter on the output so you know exactly whats being delivered.

[abro163]

A variac is in itself an isolation transformer I believe

[vibroverbus]

A variac is in itself an isolation transformer I believe

Some are, most of the ones that electronics/music hobbyists end up with aren't.

Basic variacs are 'autotransformers', meaning there's not really a secondary, it's just a self-tapped single coil, no isolation factor. I have a beefy 10A Genrad (aka REAL Variac 'tm') with current and voltage meters and even it is a basic autotransformer.


http://www.eng.cam.ac.uk/DesignOffice/mdp/electric_web/AC/02146.png

[VacuumVoodoo]

Power transformer in your tube amp is in itself an isolation transformer.
An external isolation transformer may be called for in two instances:
- it's an old directly powered amp without internal power transformer
- the power cord is a two prong without earthing prong in which case replacing it with a three prong and safety grounding the chassis is what you need to do.

[Riscchip]

Thanks for the additional info guys. Much appreciated.

An isolated variac sounds like a great idea...I wish I didn't already have two non-isolated ones, hehe. I'd love to at least keep my old Staco in service, as I'm rather fond of it. :) Perhaps down the road I'll drop the coin on an isolated unit. I'd love to build one, too, if I had a little more time on my hands to do a nice job of it.

Setting aside the issue of whether or not I need to use one...can anyone suggest a specific off-the-shelf isolation transformer that they use? The Stancor GIS-500 is rated 4.35 amps. Hammond 171E perhaps? I'm assuming as long as it's 1:1 and rated for 3-5 amps, it's probably sufficient for most cases?