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Variac
Feb 9, 2006 14:49:09 GMT -5
Post by Ripper on Feb 9, 2006 14:49:09 GMT -5
A friend of mine at work offered me a Variac. Do I risk frying my Marshall with this gizmo? He said no, but he wont be the one replacing it if it doesnt go well.
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Variac
Feb 9, 2006 16:00:00 GMT -5
Post by UnklMickey on Feb 9, 2006 16:00:00 GMT -5
Do I risk frying my Marshall with this gizmo?
ABSOLUTELY!
first, it's common for a variac to be variable from 0 to about 115% of input voltage. at max, that means 140 volts. some of them can be wired internally to have an even higher increase.
if you accidentally increase the voltage to your marshall, you run the risk of shorted caps, shorted windings in your transformers, arcing tubes, and who knows what other evil things.
but you will no doubt be using it to decrease the voltage.
the big risk there is short tube life. the cathodes don't like running on reduced heating current. but if you don't mind more frequent tube changes, you can get some truly interesting tones running at about 90 volts.
if that variac can be wired so the maximum output is the same as the input, that would be reasonably safe.
at least, you'll want one with a built-in meter, so you can measure the output voltage.
and when you set it for a level you like, tape that big fat knob, so it doesn't accidentally get twisted.
use with caution.
unk
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Variac
Feb 9, 2006 16:11:09 GMT -5
Post by Ripper on Feb 9, 2006 16:11:09 GMT -5
Thanks Unk... Maybe ill pass on it. I read years ago that EVH used one to get his " Brown" sound. Im not a big fan of his sound per say but figured I could tweak it to my own liking. Besides, Eddie can afford to make costly mistakes! You saved my bacon Unk...Thanks again.
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Variac
Feb 9, 2006 17:25:00 GMT -5
Post by UnklMickey on Feb 9, 2006 17:25:00 GMT -5
...You saved my bacon Unk... that must mean a lot. cause we all know candian bacon is so much more valuable than the stuff we call bacon here in the states. lol
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Variac
Feb 9, 2006 21:09:12 GMT -5
Post by ChrisK on Feb 9, 2006 21:09:12 GMT -5
My Mesa MK IV has the "tweed" switch. It does make for some "browner" sounds, especially w/ 4-6V6's installed.
I'm also quite fond of the "brown" sounds in Class A (two outside 6L6's) and with Triode mode selected.
I agree that the variac is dangerous, with flexibility comes responsibility (as opposed to my Variax, which is also dangerous differently). A trick from the TV days in the 60's is to make your own (anti) booster. We used to take a 6.3 volt secondary filament transformer with a secondary current rating equal to the loads AC input current. We would wire the transformer primary to the AC line voltage and place the secondary in series with the line voltage feed to the load. In essence, this would increase the voltage supplied to the load by 6.3 VAC to help with line brownouts.
(?See where I'm going with this?)
If'n the transformer secondary is reversed, the load is supplied with the line voltage decreased by 6.3 VAC. Coincidently, about half of the time that I made a "booster", I had to reverse either transformer winding's connections 'cuz I had a "bucker" on my hands.
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Variac
Feb 9, 2006 21:45:10 GMT -5
Post by UnklMickey on Feb 9, 2006 21:45:10 GMT -5
My Mesa MK IV has the "tweed" switch. It does make for some "browner" sounds, especially w/ 4-6V6's installed.... hey Chris, i guess that might be more of a "tan" sound, eh? ________________________________________ doing a voltage bucker with a 6.3 v transformer probably wouldn't go far enough to make all that much difference. 30 volts or so, now you're in EVH brown-out territory! unk
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Variac
Feb 12, 2006 0:16:00 GMT -5
Post by ChrisK on Feb 12, 2006 0:16:00 GMT -5
Well, They make 12.6 VAC (and other) transformers too...
Remember, the heat generated varies w/ the square of the applied voltage (and the filament is a heater w/ a positive tempco). I don't really know much about tube amp practice, but my engineering background makes me wonder about too big a voltage drop. At -30%, I'd be concerned that the tubes won't "tubulate". But then again, some acoustic guitar maker has a (nearly) cold filament tube-based in-body preamp module, dont' they?
An inquisitive person (perchance w/ an amp to let go) might try a lamp dimmer (or AC motor speed controller).....
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Variac
Feb 13, 2006 17:36:34 GMT -5
Post by UnklMickey on Feb 13, 2006 17:36:34 GMT -5
Well, They make 12.6 VAC (and other) transformers too... Remember, the heat generated varies w/ the square of the applied voltage (and the filament is a heater w/ a positive tempco). I don't really know much about tube amp practice, but my engineering background makes me wonder about too big a voltage drop. At -30%, I'd be concerned that the tubes won't "tubulate". But then again, some acoustic guitar maker has a (nearly) cold filament tube-based in-body preamp module, dont' they? An inquisitive person (perchance w/ an amp to let go) might try a lamp dimmer (or AC motor speed controller)..... i'd guess 24v would probably be the best choice for an off-the-shelf transformer value to approximate the EVH thing. i'm told, the best effect is at about 90 volts. that lamp-dimmer idea sounds pretty good on the surface, but since those things increase the "on" threshold for switching on the triac each half cycle, the dimmer you go, the narrower and spikier the voltage going to the load. could be difficult to measure the "effective voltage". but since it's always going to be less than the input it probably might not be too unsafe. my guess is you would introduce more noise than the transformer and p.s. caps could filter out. could get kinda ugly in that respect. but hey, dimmers are way more plentiful and way less expensive compared to variacs. so if it did work, even reasonably well.... OH, BTW: consider the power requirements (not the output power) of the amp, when determining the dimmer size required for such an experiment. and: PROCEED AT YOUR OWN RISK!
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Variac
Feb 14, 2006 19:52:39 GMT -5
Post by ChrisK on Feb 14, 2006 19:52:39 GMT -5
Yeppers!. Tube amps ain't efficient. A class A amp draws nearly the same power at no output as compared to full output (the same plate current is flowing at no output, it just ain't "wigglin' about").
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