Using A Variac

[OlinMusic]

I have talked to 4 techs - 2 agree, 2 disagree on the use of a Variac. All four are very well known (I just don't wanna use names w/o permission).

One says that the amp doesn't like it and it can hurt the amp INCL Tranny's.
Another says that it creates some uglier harmonics.
One in Nor Cal says it won't hurt the amp a bit.
Another who sold it to me, says it not only will NOT hurt the amp, but it is a part of the brown sound. Also, at 110 it saves vintage amp parts.

I run mine between 95 - 105. I have had my amp checked out, and it got a great bill of health TWICE! At 95-100 it makes everything sound SOOOO Spongy and SILKY. With APH1's it is a DREAM!

I used it on my Mark 2b at 110 and it made the thing sound GORGEOUS!

Any users? Any thoughts?

PS - I have already seen the "Marshall Myths" article. I have heard that guy may be wayyyy off base.

 

[Rich_S]

Re: Using A Variac

The problem with variacs is that in addition to pulling down the main supply (B+) voltage (giving you the sound you like) they pull down ALL the voltages in an amp. Running the filament supply too low can be a problem - cathode stripping, worn-out tubes.

Correcting the voltage on vintage amps is a valid use for an autotransformer. Many old amps are rated at 110 volts, and run best at that point. Modern voltages can be 120 volts or higher; not only will this effect the sound or the amp, but it can stress tubes, mess up the bias, and burn out filaments.

I say to keep your amp set on the voltage if it was designed for - then if you want it browner, look into mods to make it so. There are many ways to do it - tube rectifiers that sag, resistors instead of a choke, etc.

 

[Majestic]

Re: Using A Variac

Ah, the thoughts of Eddie back in 1978 with a fan aimed at his power tubes........

Ah, to be naive again..........claimed to get something like 10 hours out of his power tubes, or something like that. Truth or not, it's funny.

 

[OlinMusic]

Re: Using A Variac

Hey, hey I have always been told 105 is actually peak effeciency and 90 is the low limit
I never go below 95

Usually 100 - 105 is my fave

I saw a grid online that showed the effects on B+ and laid it all out - and heck it made sense to me as a layman with decent math skills - so 105 seemed nice.

 

[Scott_F]

Re: Using A Variac

Well, think about old fenders and Marshalls from the 60's. Made to work on 100-110. Now we're up to 120v. All your old amps run a bit hotter these days. So, if you've got a really old amp, running a variac at 105 just gets it back to where it was designed to run?

 

[ErikH]

Re: Using A Variac

I'm no amp genius but that makes totally perfect sense and was exactly what I was thinking, Scott. If I had an old amp like that, I'd be looking to do the same thing. It just makes sense.

 

[AdmiralB]

Re: Using A Variac

Well, think about old fenders and Marshalls from the 60's. Made to work on 100-110. Now we're up to 120v. All your old amps run a bit hotter these days. So, if you've got a really old amp, running a variac at 105 just gets it back to where it was designed to run?

When was power in the US ever 100V? Or 105V?

If you look at Fender schematics - even back in the '50s - you'll see that they reference the supply as 117VAC. It's true that voltage is higher today than back then, finding 120VAC or less is pretty much unheard-of, but back then it wasn't THAT much lower.

Using a variac to reduce voltage won't hurt anything except your tubes - they'll be running cold on the heaters, and they'll strip the cathodes. But if you like the sound and don't mind shorter tube life, go for it.

Note that if your amp is correctly biased at normal wall voltage, variacing down will make them overbiased (running cold) - which won't hurt anything; however if you bias them in the reduced-voltage condition, they'll be hot when you go back to standard supply.

 

[shredaholic]

Re: Using A Variac

Theres so much bull**** surrounding variacs, it's unbelievable. Most of the techs that try and describe the effects of one on an amp have never actually tried changing plate voltage in an amp, measuring effects of variacs etc.

The main disadvantages with using a variac are on an EL34 it'd knock a few thousand hours of the life of the tube, as can be seen via this diagram of the effects of varying heater voltage:

...and that when you use a variac, you're throwing off the bias. If you want to use one permanently, I'd suggest variac'ing at your desired voltage, then setting the bias, then leaving it at that. A variac is not going to blow up your amp. Period. To be fair the loss of life on the tubes isn't really that bad, considering using a variac probably won't throw your heaters under 90%. Just depends whether you don't mind re-tubing the power amp more frequently.

B+ when the amp is variac'ed at 90-100v does have quite a pronounced noticeable effect on tone - it adds compression and rolls off high end. This is due to the effect on the preamp B+, and as provided the bias is tracked with the B+ you can reduce the B+ in the power stage as much as you like and it'll sound exactly the same (just quieter). This is the principle of power scaling. Power Scaling is like using a variac on the power tubes alone, and maintaning bias current, heater voltage and preamp/PI voltage. The only tonal change in using a variac is the effect on the preamp voltage. The effect is that you get a 100% pure reduction in volume output. Bit off topic though...

What I'm getting at is that the power being dissipated by the tubes in a variac'ed amp that has had it's bias adjusted accordingly is going to be less than normal, therefore you're less likely to blow something in the amp.

Because a variac only affects the preamp in terms of tone (B+ in the splitter and output tubes does not affect tone in any way, just volume), a good tech should be able to modify your amp to lower the voltage in the preamp on its own, so you don't have to use a variac. This mod would not strip cathodes on the tubes, would not throw off the bias, and would not change the volume. The mod just consists of adding a fixed (or variable if you'd like that) resistance in the preamp B+ line, and costs about 50p.

 

[AdmiralB]

Re: Using A Variac

A variac is not going to blow up your amp. Period.

So far we've mostly been talking about lowering voltage. If you recall, EVH used to claim that he raised voltage with it (I believe this was intentional dishonesty, but I bet there are plenty of people who believe it), and you certainly can damage things going that way.

Lowering the voltage to some amps will totally change the character - the Komet is one example of this, it runs very high preamp voltages and variacing down loses the touch-sensitivity.

 

[shredaholic]

Re: Using A Variac

Oh yeah, raising the voltage is definately a stupid thing to do on the amp, unless you rebias it. Then you're at the mercy of how much voltage your power tubes can take on the plates and screens, and if we're talking current production tubes, it's not likely to be much over 500v. In addition, I can't imagine giving a power transformer a higher input voltage than it's expecting is a particularly good idea, pure speculation but I'd expect that might damage the windings.

One interesting thing is the loss of life resulting from higher heater voltage is actually less at the same percentage of voltage raised instead of lowered though, so cathode stripping isn't as much an issue as screwing with the bias current and feeding the power tubes and transformers voltages they can't handle in that case.

 

[OlinMusic]

Re: Using A Variac

SHRED you are a MONSTER

Thanks


So hmm I should just permanently mod it? i dunno, I kinda like having both sounds or th ability to go from 95 or 98 to say 105 or just go 110 when I want the good old sound.

 

[LesStrat]

Re: Using A Variac

:doh: I thought this thread said, "Variax."

*self-flagellates*

 

[ErikH]

Re: Using A Variac

I thought EVH lowered the voltage on his Marshall to something like 90V. Not only did it pull all the voltage down but the volume as well, because he always ran it out full bore.

 

[AdmiralB]

Re: Using A Variac

I thought EVH lowered the voltage on his Marshall to something like 90V. Not only did it pull all the voltage down but the volume as well, because he always ran it out full bore.

I believe that's true, but that's not what he said when asked about it - he said he "ran it up to 130V and watched the tubes melt".

Randall Smith has spoken of seeing early VH soundchecks where Ed will play, then tell the guy at the varaic "down a little" - plays a bit - "a little more" - plays - "now up a bit" - plays - "OK, that's it".

 

[ErikH]

Re: Using A Variac

I believe that's true, but that's not what he said when asked about it - he said he "ran it up to 130V and watched the tubes melt".

Randall Smith has spoken of seeing early VH soundchecks where Ed will play, then tell the guy at the varaic "down a little" - plays a bit - "a little more" - plays - "now up a bit" - plays - "OK, that's it".

LOL....yeah, I've read those too. I wonder if going up in voltage was part of his whacky experiments. Just seeing what he did to that Destroyer and other whacked out projects, that sounds like something he'd do just for the hell of it.

 

[AdmiralB]

Re: Using A Variac

LOL....yeah, I've read those too. I wonder if going up in voltage was part of his whacky experiments. Just seeing what he did to that Destroyer and other whacked out projects, that sounds like something he'd do just for the hell of it.

The Pasadena equivalent of "Hey y'all, watch this"?

Supposedly they had stacks of replacement OTs on hand at all times, too, since he was running into a big power resistor with some serious mismatch.

Kinda makes you think about the boutique transformer industry, certainly whatever he was using would have been an off-the-shelf 4xEL34 replacement, nothing fancy around back then, and it didn't seem to adversely affect his tone....

 

[ErikH]

Re: Using A Variac

The Pasadena equivalent of "Hey y'all, watch this"?

:chairfall That's priceless.

Supposedly they had stacks of replacement OTs on hand at all times, too, since he was running into a big power resistor with some serious mismatch.

Kinda makes you think about the boutique transformer industry, certainly whatever he was using would have been an off-the-shelf 4xEL34 replacement, nothing fancy around back then, and it didn't seem to adversely affect his tone....

I'm always wondering about the entire boutique industry sometimes. There's some things that are worth it and others that make you go, "why, those guys didn't need it." My Marshall needs a new power tranny, and maybe an output tranny. My plan is to go with the Marshall replacements.

 

[OlinMusic]

Re: Using A Variac

All I can say is the effect of the Variac is pretty impressive. I really like the way it plays, feels and records. It's a nice option to have available. It's a VERY underrated component in "browning out" an amp, if you some people incl. me. It's not the only solution, but it works fast and easy.

As I also stated, while dark amps can lose articulation - I found my Mark IIB Mesa sounded sooooo much better and more saturated running at 110 than to go around without it.

I took a Variac to a lot of gigs, and it was deadly heavy so I bought a hardshell case at GC meant for RECORDS or OLD VINYL. The case was great for Hot Plates, Variac, and some mics and guitar accessories. This was my main rig for over a year, but I found I could not get a PURE clean. Then again with a Marshall you always need 2 amps, unless you want to roll back volume and get sublt break up.

 

[shredaholic]

Re: Using A Variac

SHRED you are a MONSTER

Thanks


So hmm I should just permanently mod it? i dunno, I kinda like having both sounds or th ability to go from 95 or 98 to say 105 or just go 110 when I want the good old sound.

Assuming the B+ rail is either a Marshall or Marshall style setup, and your PT doesn't put out rediculously high plate voltage, you should be able to stick a 25k pot in there to control it - when the pot is maxed it'll be full preamp voltage, when it's min'ed it'll be lower than it would be at 90v mains (exactly how lower, you're tech would have to find out by measuring current draw or reading the voltage after the pot is installed), and turning the pot gives you infinite variation between the two voltages. The wattage needed on the pot would only be about .35w, and most amp pots are at least 1w, PEC pots being 2w. I'll get a diagram worked up on PTPCircuits.com for this so you can give it to a tech if you want and if it's a Marshall or similar amp he can work out a way of implementing it.

 

[shredaholic]

Re: Using A Variac

The Pasadena equivalent of "Hey y'all, watch this"?

Supposedly they had stacks of replacement OTs on hand at all times, too, since he was running into a big power resistor with some serious mismatch.

Kinda makes you think about the boutique transformer industry, certainly whatever he was using would have been an off-the-shelf 4xEL34 replacement, nothing fancy around back then, and it didn't seem to adversely affect his tone....

THATS why he used the variac in the first place - to reduce the stress on the output tubes caused by the resistor, and also the reason for the 6CA7's - they were more rugged. Presumably he found he liked the tone better with the variac and decided to keep it after he ditched the power resistor.

Try sticking a bigass 1K resistor between the OT primaries of a Marshall and you'll discover Van Halen I tone right there and then - the only problem is not that the resistor caused OT mismatch (as it was placed between the OT primary, not after the amp), but that when it causes the tubes to work harder than normal you will get a lot of tube failures, thus often taking out the OT. The big power resistor and his dummy load are two different things - the power resistor massively reduces the volume, so he needed a dummy load and slave rig to INCREASE the volume for gigs.

http://www.ptpcircuits.com/mods2.html

Have a read up on the 'Cerrem Mod'