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Variac help

Randy

Silver Supporting Member
Messages
3,932
Originally posted by fifty9 335
How many amp variac is required to run a 100 watt amp???
Thanks :)
I think a 5 amp will handle a 100 watt head. But having said that, I'd get a 10 amp if I had the choice.
 

fifty9 335

Member
Messages
374
To run an old Marshall that is use to 240v, do I require a variac or will a step up transformer be adequate and safe...i'm worried about too much voltage comming out of the wall, resulting in too hi a voltage after step up.
:)
 

John Phillips

Member
Messages
13,038
A typical tube amp will draw a maximum of around four times the power that it puts out.

So a 100W amp needs about 400W, which at 110VAC is just under 4A. A 5A Variac is enough, but a 10A is better.

If you're using a transformer, just be sure to get a 120-240 model, not a 110-240 one - that way, even if the wall voltage is up as far as 125V, you won't get more than 250 at the amp (which they're rated for). Also, make sure it is at least 500VA.

Doesn't this amp have a 120V tap on the PT?
 

fifty9 335

Member
Messages
374
Thanks for the replies, yes it does have a 120 tap but I was told that I should continue to run it on 240....less chance of a problem.
 

jbh

Member
Messages
77
Power = volts * amps

If max power is 400 watts = 110V*amps
A = ~4A

If you have 240 V, yes you should run it there.
 

John Phillips

Member
Messages
13,038
Originally posted by fifty9 335
Thanks for the replies, yes it does have a 120 tap but I was told that I should continue to run it on 240....less chance of a problem.
I don't agree with that at all. Running at 240V increases the chance of a winding insulation breakdown (the main cause of failure in old transformers) compared to running at 120V, simply because the voltage is higher.

Yes, running at 120V increases the current that the PT will draw, but the wire in the lower windings is thicker to compensate, and unlike the insulation, copper wire does not degrade with age, so it's no more likely to fail now than when it was new - and the transformer was designed to run at 120 as well as 240V.

All the other internal workings of the amp will be the same regardless, since the whole point of the multi-tap transformer is to match the amp to the supply voltage.
 




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