Amperes w/Variac

[Goat]

Will a 50 watt tube amp exceed 1.75 amperage draw?

 

[fusedbrain]

Pretty sure the mains fuse in a 50watt el34 amp is 2 amps, and 100watt is 4 amps, and you may want a little extra headroom, so your not running the variac right at the limit and stressing the unit. ( it'll get hot, and fail quicker )
When I was looking for a variac, I didn't find a big price difference until you got into the high current ones rated at 10 amps +.
I think I paid about $100 Canadian for an 8 amp unit new.

 

[fusedbrain]

Lol, you nuked it while I was posting!

 

[Goat]

Lol, you nuked it while I was posting!

Thanks! 50 watts @ 2 amps, but would the amp actually pull that much current?

 

[fusedbrain]

Lol, you nuked it while I was posting!

Thanks! 50 watts @ 2 amps, but would the amp actually pull that much current?

Well..... under normal operating conditions, it probably wouldn't be pulling the full 2 amps continuously, but I don't see any reason not to go bigger on the variac given the cost and reliability factor.
Then you have to consider the unit's ability to dissipate heat. Physically larger units are better. If you are only going to drop 5 volts, then not so bad, but do the power calc on the voltage drop you want and see how many watts of power the variac will be dissipating.

Should also mention for brevity that if your 50 watt EL34 amp has a tube rectifier, the mains fuse will usually be 3 amp, but I don't imagine their are too many guys that want to variac their JTM45's.

 

[Goat]

Lol, you nuked it while I was posting!

Thanks! 50 watts @ 2 amps, but would the amp actually pull that much current?

Well..... under normal operating conditions, it probably wouldn't be pulling the full 2 amps continuously, but I don't see any reason not to go bigger on the variac given the cost and reliability factor.
Then you have to consider the unit's ability to dissipate heat. Physically larger units are better. If you are only going to drop 5 volts, then not so bad, but do the power calc on the voltage drop you want and see how many watts of power the variac will be dissipating.

Should also mention for brevity that if your 50 watt EL34 amp has a tube rectifier, the mains fuse will usually be 3 amp, but I don't imagine their are too many guys that want to variac their JTM45's.

Thanks. I think I'll just get a variable bucking transformer w/meter. This one looks cool... It's cheaper than the AmpRx Brownbox too.

https://vintagesoundworkbench.com/amp-maniac

 

[fusedbrain]

OK cool.
That looks like a nice unit, and it's designed for use with tube amps

 

[Goat]

I was looking at variacs on Amazon last night... According to comments, some of them arrived from the manufacturer, wired in reverse!

 

[CornNutz]

It wont pull that much nominal current, you'll have a 2 amp fuse because you'll get a current spike when you first turn the amp on as the empty filter caps cause a current inrush. If you had a smaller fuse the caps would pop it. This sometimes happens anyways, even if there are no problems. Fuses wear out, and an old fuse could pop.

1amp x 120v = 120w. Of course your amp is 50w but you have to account for the losses.

 

[Goat]

It wont pull that much nominal current, you'll have a 2 amp fuse because you'll get a current spike when you first turn the amp on as the empty filter caps cause a current inrush. If you had a smaller fuse the caps would pop it. This sometimes happens anyways, even if there are no problems. Fuses wear out, and an old fuse could pop.

1amp x 120v = 120w. Of course your amp is 50w but you have to account for the losses.

It would be cool if a could use the 1.75A variac. It's cheaper than a variable bucking unit, though I suppose I'd be limited on devices. I'd need the variac to handle the amp, and my pedal power + pedals. I didn't consider the in-rush voltage to the caps.

 

[CornNutz]

It wont pull that much nominal current, you'll have a 2 amp fuse because you'll get a current spike when you first turn the amp on as the empty filter caps cause a current inrush. If you had a smaller fuse the caps would pop it. This sometimes happens anyways, even if there are no problems. Fuses wear out, and an old fuse could pop.

1amp x 120v = 120w. Of course your amp is 50w but you have to account for the losses.

It would be cool if a could use the 1.75A variac. It's cheaper than a variable bucking unit, though I suppose I'd be limited on devices. I'd need the variac to handle the amp, and my pedal power + pedals. I didn't consider the in-rush voltage to the caps.

Most common variacs are 10-20amps. The difficult part is what rating did the manufacture quote, is it 1.75A peak, or continuous? If it's Chinese that number is probably peak, which means it can handle that for a very short amount of time. It's also not uncommon for Chinese to WAY overrate ratings.

 

[Goat]

It wont pull that much nominal current, you'll have a 2 amp fuse because you'll get a current spike when you first turn the amp on as the empty filter caps cause a current inrush. If you had a smaller fuse the caps would pop it. This sometimes happens anyways, even if there are no problems. Fuses wear out, and an old fuse could pop.

1amp x 120v = 120w. Of course your amp is 50w but you have to account for the losses.

It would be cool if a could use the 1.75A variac. It's cheaper than a variable bucking unit, though I suppose I'd be limited on devices. I'd need the variac to handle the amp, and my pedal power + pedals. I didn't consider the in-rush voltage to the caps.

Most common variacs are 10-20amps. The difficult part is what rating did the manufacture quote, is it 1.75A peak, or continuous? If it's Chinese that number is probably peak, which means it can handle that for a very short amount of time. It's also not uncommon for Chinese to WAY overrate ratings.

It's an older Staco variac.

 

[paulyc]

I have a cheap Chinese one that’s rated at 5 amps, I’ve used it forever with a 100 watt Marshall with two tubes pulled, so the amp behaves like a 50 but with a “beefier” power supply, never cooked anything.

 

[Goat]

I have a cheap Chinese one that’s rated at 5 amps, I’ve used it forever with a 100 watt Marshall with two tubes pulled, so the amp behaves like a 50 but with a “beefier” power supply, never cooked anything.

I've got a Tenma variac which I believe is China-made. It's been a great unit, but I use it with my keyboards and PA in a separate room.