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Brown Box / Variac with old tube amps

Verticos

Verticos

Don't just do something; sit there.
Gold Supporting Member
Mar 29, 2007
450
849
Nashville, TN
#1
I played a festival yesterday and the bassist performing after me had an B-18 that he was powering through a Variac of some sort (not sure if it was actually variac brand).

I didn't get a chance to talk to him about it, so I ask the knowledgeable people here: what's the point of this? Prolonging tube life? Specific tonal changes with the way the amp responds at lower voltages? Am I about to be convinced that I need one for my B-18?
 
#2
The typical line voltage in the US in the 1960s-1970s was 110 to 115 VAC. The typical line voltage in most of the US today is around 120 VAC. The reason someone may use a Variac between the outlet and an old amplifier is to be able to set the supply voltage to that which the amplifier was designed to be operated. I own quite a few Ampeg (and other tube amps) going back to 1960 and I operate them at whatever line voltage there may be without any detrimental effect, (so far) to the operation or the sound.

I own three variacs but I use them as a way to bring up the supply slowly on an old amplifier, (sometimes with an incandescent lamp limiter in series) if I am concerned that there may be problems with the power supply or output section or for other work that requires a voltage between zero, and 140 VAC.
 
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#3
Part of this comes down to mystique. It’s much more common in the guitar amp world, “it’s all about the tone man”, yet practicing generally improves tone far more reliably.

There are exceptions but not all that many IME.
 
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#6
Jim C said:
Asking the standard question: does the "brown sound" (created with a Variac) sound like poop?
Click to expand...
I'm curious about this as well. I have a Fairfield Barbershop on my board that I really like, and it's got a "sag" knob that starves the circuit and takes the harshness out of the drive in a way that I really like. It would be neat if I could use a variac to achieve the same thing on my amp, though the pedal does offer far more flexibility.

Thanks as always to the other knowledgeable people weighing in!
 
#8
I was once playing a gig where my tube amp sounded like :poop:. Some intuition made me pull out my voltmeter and check the line voltage - it was down around 100V. "Brown sound" might work for guitar, but I don't think it is good for bass.

For the OP's question - many variacs can slightly boost the voltage (I have a big one that can go to about 135V from a 120V line), so it's possible that B-18 was running at higher voltage, not lower; or it might have been used just so the voltage to the amp could be right even if the line voltage was a little off one way or the other.
 
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#9
The B-18’s supply voltage spec was 117VAC 60Hz in North America. The amp should have no problem operating at 120 or 125VAC. If the bias was optimized for a particular line voltage, someone might want to use a variac to maintain that. This is more common in studios than live though. It depends on what the stage line voltage is.
 
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#10
The variac may be to adjusted down to taste or it may be necessary for safe operation of an old amp. Another possibility. Stages sometimes have poor power with low voltage. So a variac may be used to bump the voltage back up to the proper range. This can be risky IMHO.

In my experience adjusting the voltage down can make a minor difference in the sound and feel. Most of the amps I have tested actually sounded best at 120V, including 100W vintage JMP and JCM 800 Marshalls. Some of my other vintage British amps sound better at 115V or even 110V. My belief is the power transformers in these amps are a bit too agro on 60hz. Meaning they might sound optimum on 120V/50hz.

My JMI era Vox AC30 is the the one that really needs 110V. The working voltages in the amp run significantly over spec on 120V/60hz and the output tubes idle over 100%. At 110V/60hz the working voltages are still above spec, but the tubes idle at about 90%....AFAIK 90% is widely accepted as the target for this type of amp, but many do think the sound better running 100% or higher.

A commonly recommended mod for AC30s of this era is to wire a bucking transformer in with the power transformer. The bucking transformer drops 120V down to approximately 110V. If I were going to gig or backline this amp, I would have a bucking transformer installed. Since I only play it at home, I use my variac instead.
 
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#11
The key word I see here is "festival" - a lot of festivals run off generators, and the voltage you get from those can be quite variable. If he had a voltmeter, I could see him using a variac to make sure he didn't hit the amp with too much voltage - I would consider that smart, although one also might question the wisdom of using a low power bass amp for an outside gig. I had an old Bassman that failed at one point - stopped working. Turns out the weak point in that amp wasn't the tubes, or the transformer, or the capacitors (the things you think would be weak links). What blew was the solid state diode rectifiers (early 60's diodes). A lot of things can be stressed from too much voltage.

I have, a few times in old amps, changed values in the string of resistors in the power supply feeding the preamp tubes. With the same capacitors, and higher value resistors, you can set the preamp tube plate voltages to be at original spec (where they would have been with 110 volts) at 120 volts, and you get a little more filtering (a bit less noise) in the power supply as well.
 
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#13
micguy said:
The key word I see here is "festival" - a lot of festivals run off generators, and the voltage you get from those can be quite variable. If he had a voltmeter, I could see him using a variac to make sure he didn't hit the amp with too much voltage - I would consider that smart, although one also might question the wisdom of using a low power bass amp for an outside gig.
Click to expand...

Good point. While a variac wouldn't protect the amp from the overly high voltage, at least you have options once you become aware of it.

I played a festival a couple of years ago where the stage power was approaching 130v. My surge protector shut down. I asked the sound guys why the generator was cranked up so high and they said it was to compensate for loss from long extension cords. Clearly, they weren't really paying attention.
 
#14
Verticos said:
I'm curious about this as well. I have a Fairfield Barbershop on my board that I really like, and it's got a "sag" knob that starves the circuit and takes the harshness out of the drive in a way that I really like. It would be neat if I could use a variac to achieve the same thing on my amp, though the pedal does offer far more flexibility.

Thanks as always to the other knowledgeable people weighing in!
Click to expand...


Why complicate things if you already have a pedal that does the job?
 
#15
I have a Variac but wouldn’t use it for my tube amps:
1- the dial is not that precise and I have to confirm correct voltage with a DVM;
2- I could bump the dial and don’t want to fry my gear with 140 VAC!

The only use I have for it (with the DVM verification) is to set precisely 120 VAC when testing tubes with my two old tube testers.
 
#18
Good point. I’m guessing that the B-18 fellow was using an Amp Maniac or one of the Amp RX Brown Boxes. Those devices can drop the voltage but not increase them as a variac would.

I use an Amp Maniac at most gigs. I primarily gig vintage Sunn 200s / Sorados. They were designed for 117v, but as I understand it, their capacitors are just barely adequate for that voltage and I don’t want them hit with the 123-125v from some of todays outlets.

I also play guitar and sometimes gig Fender Tweed and Brown amps from the 50’s and early 60s. Those amps were designed for 110v and I use the Amp Maniac for them as well.

At least in Tucson, wall voltages are all over the place. I had a gig last Saturday and the wall voltage was about 125v, but played recently at a different club and the outlet was giving me 113v . . .
 
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