how do i use a variac?

mr ohmite · Post by **mr ohmite** » Wed Sep 02, 2009 6:42 pm

hey guys
all replies much appreciated
i have a plush amp that has 4 out put tubes and 3 pre tubes
its a 400g tuck and roll special
i have had a master volume mod done on it
this amp only breaks up a lil w/the mod
BUT as i crank it it starts to sound better
this amp sounds horrible w/any distortion boxes[hate em}
the transformers on this amp are flippin HUGE
i was told that this amp pushes 200 watts but i am not sure if this is correct or my memory is fried
i live in an apt but i can crank up loud during the day
so
which variac do i buy off ebay?
how do i set it up?
what cables do i need?
i have a 2 x 12 cabnet w/75 watt celestions in it
it is a home made cab so i don't know what the ohms are

thanks dudes

flemingmras · Post by **flemingmras** » Wed Sep 02, 2009 8:10 pm

You need an attenuator, not a Variac.

A Variac is a variable AC transformer (hence the name VARI-AC) that allows you to vary the wall voltage from 0-140 volts. However, they're made solely for the purposes of testing a possibly unstable amplifier that may have a major internal short. Running a variac on the mains doesn't change your output volume a whole lot. It does change the "feel" of the amp and cause it to break up earlier. However, it is not good for the amp at all. It drops not only the main supply voltage, but the bias and heater supplies as well. Eventually you reach a point where the tube heaters don't have enough voltage and you end up with what's called "cathode stripping". This is where the heater can't get hot enough to heat the cathode in the tube sufficiently enough to emit electrons and the high voltage from the plate ends up trying to "strip" electrons from the "not so hot" cathode.

However, running an attenuator between the speaker and amp will get you the result you're after. The Marshall Power Brake and the THD Hotplate seem to be the most popular ones. All you'll need are regular speaker cables...one from the amp to the attenuator, and one from the attenuator to the speaker cab.

As far as your cabinet's impedance -

If the speakers are both the same ohms rating and they're hooked up in parallel (+ terminal of speaker 1 hooked to + terminal of speaker 2; - terminal of speaker 1 hooked to - terminal of speaker 2; input jack hooked across one of the speakers) then the total impedance is half the value of 1 speaker (if they're both 8 ohms then it's a total 4 ohm cab whereas if they're both 16 ohm speakers then total load will be 8 ohms).

Hope this helps.

Jon

tonepilgrim · Post by **tonepilgrim** » Wed Sep 02, 2009 9:31 pm

If you're amp is pushing 200 watts, you will probably want a variac as well. Not too many if any attenators out there that are going to take that kind of power without risk. Do a searchon variac...lots of info on this forum.

mr ohmite · Post by **mr ohmite** » Sat Sep 05, 2009 7:38 pm

thanks for all of your suggestions
I am gonna buy a hotplate off of ebay
peace

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