I've read this a few times and I'm curious, people seem to be saying that if you turn on a tube head without a cab connected that you could damage the head. But I really don't see why this is, and why amp manufactures wouldn't have solved this problem yet. Like what if theres a wiring problem or somehow a wire didn't get plugged in when you're turning on a head, I'm sure this has happened 1,000's of times in the consumer and professional world, does this mean everytime that happens, that head is dead? Or is this one of those urban legends that used to happen maybe 50 years ago but doesn't happen anymore?

I don't currently own or plan on buying in the near future a tube head and cabinet but I'm just very curious.
it builds up power and has no where to send it. so it fries.

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yes it is very true, the head will spontanously explode if switched on wwithout a cab
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yea, you don't want to do that. Its not an urban legend. Your amplifier amplifies the out-going signal, and if this large current has nowhere to go it will just keep increasing untill you blow your output transformer or something and cost you a lot of money to get fixed. This is one of the golden rules in tube amps. Always make sure you have a proper speaker load attached to the amp
Yes but this isn't something spontanious then is it, like if you turn it on and then realize a minute later that the cabinet's not working or it's not plugged in you're not gonn have a dead amp are you?

Why haven't amplifier manufactures come up with a solution to such a problem then?
Well they could put it an 8ohm high power resistor (note: a 100W resistor would be ****ing huge) thats connected to the output transformer when you don't have any cabinet hooked up

I think Mesa Boogie have have something where you don't have to have a cabinet pluggged in, not entirely sure
There are a few amps (THD for one) that have a dummy load circuit so you can have the amp on and play through it for direct recording without damaging the amp.
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I think it blows the transformer specifically.

output tranformer and the power tubes are the things at risk (though I'd worry more about the transformer as thats $100+ for a new one if you have a +30W amp)
It's basically the same thing that'll happen with gross impedance mismatching. There's something called maximum power transfer, it occurs when you match impedances. When you mismatch impedances, then you have output loss. The higher the load impedance, the greater the loss. When you don't attach the cab, that's basically equivalent to a load of infinity, so the power is dissipated in the amp.

It's also not a problem you can just solve. World hunger has existed since the dawn of mankind. We haven't solved it yet. Tube amps blow when you don't have a load connected. We haven't solved that yet either.

Here's a graph for those of you wondering how it works (it's really big...):
as you can see, the most you'd want to mismatch is at a ratio of 2 or 1/2
Last edited by greenbox at Jul 2, 2006,