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salesmonster 8th July 2009 04:32 AM

Variac size to buy
I'm am near completion of an Aleph J, and I soon plan to build the F3-5. I want to slowly fire the amp up with a variac the first time. What size variac would you buy? 5A? 10? Larger?!?!

Any suggestions?


lgreen 8th July 2009 04:59 AM

1000 VA
I have build a bunch of stuff. I think a 1000 VA is fine, less and you cannot startup your big amps, and more is unnecessary because its not taking the startup transient, you start from zero and go slowly up.

here is a link, I think I bought one of these red dealies, but not one that they have now, about 1000 VA.

h_a 8th July 2009 07:31 AM

Don't forget that it's a good idea to set bias to minimum before starting up for the first time - so you don't need that much current ability. I wouldn't use a variac for full power testing.

Have fun, Hannes

h_a 8th July 2009 10:38 AM

Oh I forgot: poor man's variac - a light bulb (100W) wired in series with the transformer primaries. It has no effect if the amp comes up ok, but lights up nicely if your amp features a dead short somewhere.

Have fun, Hannes

AndrewT 8th July 2009 10:59 AM

fit a bulb tester between the mains and the new project.
Test the transformer alone.
Add the PSU to the transformer and test again.
Add one amplifier channel with output bias set to near zero and test again.
Remove bulb tester and substitute soft start.
Increase bias and check voltages and temperatures throughout the amplifier.

Save up for a small Variac (2A to 3A @ 240Vac) does almost all other jobs that need a variable supply voltage. Is that equivalent to a 4A or 6A @ 120Vac?

Nelson Pass 8th July 2009 07:08 PM

Look at Marlin P Jones for cheap variacs.


eyoung 9th July 2009 01:47 AM

Regards all...

I'm with Andrew on this not only does a light bulb act as a current limiter it can be a visual cue that something is not kosher ... I use a variac and a light bulb when cranking up a new amp.

keep the faith baby...


salesmonster 17th July 2009 09:41 PM

I bought a 1kva model
I couldn't find a 1kva initially. Which is odd, because I remember seeing them for sale at the usual websites.

Anyway, I found one at a tremendous price. Ted Weber (the guitar amp guru) has a tool site where I found the variac for 50 bucks!

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