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Old 13th October 2011, 10:44 PM   #1
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Default variac variable transformer

How many amps are necessary for trouble-shooting a tube set-up such as a fisher or sherwood amp? 10, 15, 20, more?
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Old 13th October 2011, 11:32 PM   #2
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with 10 or 20 amps you can arc weld...

Bigger is always better, but its generally more expensive too. A 500W variac should be ample, 350W would be ok. Less than that is likely a toy.
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Old 13th October 2011, 11:37 PM   #3
Pano is offline Pano  United States
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I use a 10A model and know a few other tube guys who do as well. No problems. Probably overkill.
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Old 13th October 2011, 11:39 PM   #4
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+1 on Aardvark's advice.
OT: I've got variac(s) but seldom use 'em for troubleshooting.
I find that the 'dim bulb tester' is more useful (and protects the 'test subject' better, too).
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Old 13th October 2011, 11:57 PM   #5
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thanks
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Old 14th October 2011, 12:04 AM   #6
Dagwood is offline Dagwood  New Zealand
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Hi John

Not sure on the rating of mine (about an amp) but it works fine
I agree with VictoriaGuy but have my Variac setup with a filament lamp in series. Almost covers all bases, just need to add a ammeter.

Cheers
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Old 14th October 2011, 12:21 AM   #7
Enzo is offline Enzo  United States
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I use the variac in my shop all the time, an ammeter takes the place of a bulb. Either method works.

The variac is used mainly when things are wanting to bow fuses, I don't need it to be able to power the unit under test to its full output. Once we have established that the amp is stable and ready for high power testing, I no longer need what the variac does, and I power the UUT from the wall.

All that is the long way to say that I don;t ned a 1000 watt variac to service 1000 watt amplifiers. A smaller one will still be useful.
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Old 14th October 2011, 02:51 AM   #8
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I always wanted to make a variac test box that had meters for voltage and current and also switches to put in a 100 or 25 watt lamp. Maybe add a receptacle and also binding posts...
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Old 14th October 2011, 03:16 AM   #9
Dagwood is offline Dagwood  New Zealand
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Default variac variable transformer

Like this?

I added a isolation transformer inside to protect the binding posts, would still like to add a ammeter.

Lamp can be switched on or off and I just swap the lamps from 40, 60 and 100 watts
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Old 14th October 2011, 03:29 AM   #10
TheGimp is offline TheGimp  United States
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I tried using a 2A variac once, and proceeded to blow the windings.

Best to use an over-rated one in case something shorts out.
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