Use THIS as a variac?

[xtian]

This box says 20 amps. Why wouldn't it work as a variac for our guitar amps?

http://amzn.com/B001NIK6PC

[R.G.]

This box says 20 amps. Why wouldn't it work as a variac for our guitar amps?

It's tempting, isn't it?

It won't work, at least not very well, because it's based on phase controlled SCRs and/or Triacs. It doesn't control the amount of voltage that goes out (although that *is* a minor side effect) so much as controlling how much of each 8.667mS AC half cycle is let through. It's a technique that's specific to a few things, notably incandescent lights and universal-wound electric motors.

An amp fed this would not respond quietly or well. It would lower the voltage at some settings, but not others. From zero to the half-way point on the dimmer pot would correspond to zero to full on in the amp's power supply, and the sudden turn-on at each partial position would generate an ugly transient as the thyristors turned on - every half-cycle.

Once the half-cycle width includes the AC voltage peak, the amp power supply no longer cares how much earlier it turns on, as the power supply in the amp sucks most of its electricity out of the peak voltage it gets.

It would "turn the voltage down" some, but it's not what you want.

[vibratoking]

This box says 20 amps. Why wouldn't it work as a variac for our guitar amps?

http://amzn.com/B001NIK6PC

Hard to say without any real specs. Looks like Asian junk to me. That's a mighty small transformer, if there's one in there at all. The amazon reviews aren't inspiring.

[R.G.]

Sorry - my browser didn't tell me it had completed submitting the message, and I hit "submit" again.

[David Root]

I had one of those when I used my router. They work fine with routers. I don't think there is a transformer in them.

[xtian]

Thanks, RG!

[Leo_Gnardo]

What RG says ^^^. You know that ugly buzz that shows up in audio equipment when SCR dimmers are on the same circuit, you'll have LOTS of that. No cheap lunch. Variac wins (and its copies, under other brand names.)

[roberto]

Smelling it, it remembers me of the dimmer that my father used years ago with the drill.

Like the ones used to soften the lights.
Making a long story short, don't use it.

[img:750:500]http://cdn.instructables.com/FPH/BA8Z/H ... .LARGE.jpg[/img]