What is Variac?

[TS808]

I was looking at some youtube videos of the Bogner Alchemist tonight, and it's switchable between 40 and 20 watts. In 20 watts it's in "variac" mode. Can someone explain what that means? I've seen the term before with other amps but never really knew what it was.

Thanks in advance!

 

[tunghaichuan]

A Variac is actually a trade name for a variable AC transformer. I have a stand alone unit that plugs into the wall. There is a socket on the front that you plug an amp into. There is a rotary knob that allows you to dial in a range of voltages. Mine goes from 0v to 130.

More info here:

http://en.wikipedia.org/wiki/Variac#Variable_autotransformers

I'm not sure if the Bogner has a real autotransformer in it or not, but what it sounds like is that there is a circuit that switches between two sets of voltages to allow either 20W or 40W operation.

Guitar players have used Variacs in the past to both lower and raise the voltages in an amp. The down side is that when you use a Variac with an amp, it affects all the voltages, high voltage and filament. The closer the filament voltage is to its spec, the longer the tube last. It is just as bad for tubes to run the filament voltage too low as it is to run them too high.

A Variac is useful to match voltage coming out of the wall to the voltage specified for the primary of the power transformer. For example, if you have a vintage amp and it was made for 110v operation, running it on 125V out of the wall is bad for it. With a Variac you can dial down the voltage to 110 coming out of the unit to send to the amp. It also works in reverse. If you have 100v coming out of the wall and your amp is made to work with 120v, then you can increase the voltage coming out of the unit to interface better with the amp.

tung

I was looking at some youtube videos of the Bogner Alchemist tonight, and it's switchable between 40 and 20 watts. In 20 watts it's in "variac" mode. Can someone explain what that means? I've seen the term before with other amps but never really knew what it was.

Thanks in advance!

 

[TS808]

Got it! Thanks!