JCM800 with variac

  • Thread starter Thread starter Kapo_Polenton
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tpruitt said:
Evh amps come from the factory with a low ass voltage. I’ve seen them at 15ma.
Yes you should rebias if you plan on leaving it at a low voltage but mine is within range how I run it.

Actually it’s easier on the tube. And it lowers the overall volume. Which is why Evh used it to begin with.
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Has nothing to do with my post.
 
Cordoba said:
I just read the YT description and there is not much info there. Do you know any specifics on the amp by chance? What year, has it been modded, etc?
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No, he was not clear on what mods were done to the amp so I also asked him on another video if he was running it in more or less stock 800 mode.
 
Would this not do the trick? Mu understanding is that the slow blow Fuse in the Marshall dictates the Amp needs of the variac. I might just get a 15 Amp variac to be sure though but not interested in spending hundreds on something i may not use that often. It's more to scratch the itch.

VEVOR 2000VA Auto Variable Voltage Transformer, 15.3 Amp, 110V Input 0-130V Output AC Voltage Regulator, with LCD Display 4 Extra Fuses Thermal Control Switch for Home Industrial Office (107$ CAD)​

 
I never would have thought that an 800 can nail that tone.
 
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Kapo_Polenton said:
Good question! I edited my post as it is not my video..i was just thinking about that so i put it into youtube. Seems like it does the trick.
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I couldn't find info on what pickup was used. What pickup was used in that video?
 
This made me throw a Variac on my stock 100w Plexi today. I dropped it down to 90v and the indicator lamp was totally dark. I grabbed an EBMM EVH, tuned down 1/2 step and started running through the old VH catalogue. Oh my, this was awesome. I kind of want to do this some more and maybe at band gigs. Do I need to rebias while running at 90v or is it safe to leave things as biased at normal plate voltage, when my wall sockets are usually 123VAC?
 
Most amps sound amazing right before they die.
Dropping your 6.3v heater voltage to zilch is going to strip the cathodes on your tubes and stress your PT to the point it gives out.
 
Rick Lee said:
This made me throw a Variac on my stock 100w Plexi today. I dropped it down to 90v and the indicator lamp was totally dark. I grabbed an EBMM EVH, tuned down 1/2 step and started running through the old VH catalogue. Oh my, this was awesome. I kind of want to do this some more and maybe at band gigs. Do I need to rebias while running at 90v or is it safe to leave things as biased at normal plate voltage, when my wall sockets are usually 123VAC?
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I think the trick is to run the bias up hot all the way and hit it with the variac like you said.

My 100w Plexi was biased hot when I got it likely for that reason.
 
Just checked wall voltage again for the first time in years and it was 120 now. I understand that's common in the summer months in AZ. And the dial on my Variac was off too. I re-indexed it to be accurate and will try 117VAC tomorrow when Mrs. Lee is out of the house.
 
Some guys don't even re-bias the amp for a variac and it seems not to kill their amps. Maybe just kill tubes more often running the volume open
 
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Ok, so do I need to rebias when dropping the voltage with a Variac? I don't care about shortened tube life, but I care about keeping my original trannies.
 
Rick Lee said:
Ok, so do I need to rebias when dropping the voltage with a Variac? I don't care about shortened tube life, but I care about keeping my original trannies.
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You may not be able to w/o modification as there is a specific range with the bias range resistor.
Honestly man id drop the whole romance with the variac.
 
in the Jim Gaustad videos he talks about not adjusting the bias. He may or may not have brought it up in his Tone Talk Episode w/ Dave Friedman. I also recall he ran the voltage as low as 67 volts.

I hear more sources I trust telling me not to worry about it than those sources telling me the sky is falling.
 
Jim gaustad had a specific transformer made for that application
 
SoooRad said:
in the Jim Gaustad videos he talks about not adjusting the bias. He may or may not have brought it up in his Tone Talk Episode w/ Dave Friedman. I also recall he ran the voltage as low as 67 volts.

I hear more sources I trust telling me not to worry about it than those sources telling me the sky is falling.
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Same. Toobz gonna give before transformer does from what I "hear" on the youToobZ
 
romanianreaper said:
Ok, so let's analyze this. I don't want to get too EVH psycho, tone chasing, etc. but that is a JCM 800 with that tone. Maybe everyone has been so focused on "Plexi" and really it cones down to a Marshall eith a variac. I'm not saying a Plexi isn't a part of that but maybe a variac was a bigger part of the Brown Sound than the other parts of the puzzle people have been analyzing to death.
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You're not wrong.

To me, a good JCM 800 tone has always sounded like somebody took a stock Plexi and tweaked it to be just a bit tighter, brighter, stiffer, and more gainy. They definitely share the same DNA.

If you take an 800, max the master, and use the preamp gain as your volume knob, you'll really start to hear the Plexi come out.
 
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