Variac size to buy

salesmonster · 8th July 2009, 04:32 AM

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?

Thanks

lgreen · 8th July 2009, 04:59 AM

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

Hi,
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...

Elwood

salesmonster · 17th July 2009, 09:41 PM

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!

http://amptechtools.com/variac.htm

8th July 2009, 04:32 AM	#1
salesmonster diyAudio Member Join Date: Aug 2007	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? Thanks

8th July 2009, 04:59 AM	#2
lgreen diyAudio Member Join Date: May 2003 Location: San Diego, USA	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. __________________ My DIY audio projects- PartTimeProjects.com.

8th July 2009, 07:31 AM	#3
h_a diyAudio Member Join Date: Sep 2006 Location: Graz, Austria	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 __________________ fresh matched IRFP240/IRFP9240 fets \|\| AlephJ/JX-kits F5 transistor kits \|\| Burning Amp BA-1/2/3 transistor kits

8th July 2009, 10:38 AM	#4
h_a diyAudio Member Join Date: Sep 2006 Location: Graz, Austria	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 __________________ fresh matched IRFP240/IRFP9240 fets \|\| AlephJ/JX-kits F5 transistor kits \|\| Burning Amp BA-1/2/3 transistor kits

8th July 2009, 10:59 AM	#5
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, 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? __________________ regards Andrew T.


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8th July 2009, 07:08 PM	#6
Nelson Pass The one and only Join Date: Mar 2001	Look at Marlin P Jones for cheap variacs.

9th July 2009, 01:47 AM	#7
eyoung diyAudio Member Join Date: Aug 2007 Location: Sunny SC,USA 15 min south of Charlotte NC	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... Elwood __________________ "when you open your mind to the imposible, soon you will find the truth...."

17th July 2009, 09:41 PM	#8
salesmonster diyAudio Member Join Date: Aug 2007	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! http://amptechtools.com/variac.htm

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