Warm it up without anything plugged in...?

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Warm it up without anything plugged in...?

Postby jeffl » Wed Jul 05, 2006 4:36 pm

When I'm warming up my Champ, I've noticed that if I don't have my harp mic plugged in,it hums. If I plug the jack in,the hum goes away. Is it harmful to warm a tube amp up without anything plugged into one of the inputs? And,does it warm up faster if you warm it up with the volume dimed.
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Postby t bone bruce » Wed Jul 05, 2006 7:02 pm

To my knowledge it shouldn't cause any harm to the amp not to have a mic or guitar plugged in while being switched on. As your Champ is "Class A" it will draw the same amount of current if it's "dimed" or if it's not, so I'd imagine that it won't warm up any quicker with all the knobs wide open. The tube rectifier gives it a bit of a "soft start" as it warms up, protecting the other tubes. I installed a standby switch on my amp to prolong tube life as I have a SS rectifier, but they're not essential with a tube rec.
The hum you're getting may be caused by a faulting grounding on the input jack. As soon as you plug in, it grounds properly, and the hum goes away.
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Postby guitarslim101 » Wed Jul 05, 2006 8:35 pm

It won't hurt the amp if you don't have anything in the input...but it will hurt your amp if the speaker isn't connected.
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Postby ricochet » Thu Jul 06, 2006 1:18 am

Bruce is exactly right, although I'd point out that the "cathode stripping" from warming up tubes with voltage already applied has never been demonstrated to occur in the small vacuum tubes we use. It's a theoretical risk that's not worth worrying about, though on amps that have a standby switch I use it the same way Bruce does. A harmless bit of anal-compulsivity. The "soft start" of tube rectifiers makes more of a difference to electrolytic filter capacitors, which experience a considerable rise in applied voltage with solid state rectifiers and tubes that haven't yet warmed up and started conducting. If the caps are rated for high enough voltage to stand this, though, there's no harm, and amp designers should be well aware of the phenomenon. New caps are rated for a surge of 20% or so above the working voltage for a few seconds, but as they get older they have less reserve.
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