amp damage from having no speaker hooked up

[tonemandan]

We do a lot of tone testing in my studio and the question came up about damaging an amp from having it on with no load. There was disagreement about what conditions will damage the amp. Obviously playing the amp with no load is bad news, but is it OK to switch from one cab to another without turning the amp off and back on? Is it damaging the amp to have it ON and no signal into it with no speaker hooked up? Does flipping it to standby protect it? We routinely unplug the amp from one cab and plug it into another without shutting the amp off and no problems so far but don't want to damage anything.

Thanks

Dan

[Fuchsaudio]

If the amp is idling and not playing any sound, it should not cause any harm to do this.

[VaughnC]

Originally posted by tonemandan
We do a lot of tone testing in my studio and the question came up about damaging an amp from having it on with no load. There was disagreement about what conditions will damage the amp. Obviously playing the amp with no load is bad news, but is it OK to switch from one cab to another without turning the amp off and back on? Is it damaging the amp to have it ON and no signal into it with no speaker hooked up? Does flipping it to standby protect it? We routinely unplug the amp from one cab and plug it into another without shutting the amp off and no problems so far but don't want to damage anything.

Thanks

Dan

Due to flyback voltage effects, damaging a tube amp is possible with an input signal applied and no load attached. However, with no input signal applied to an amp, no output transformer flyback voltage can be generated, thus any damage is unlikely under no load conditions. But, you never know when an amp will all of a sudden develop a parasitic oscillation or malfunction in some way that it will internally generate its own signal.....and the output transformer will see the amps self-generating signal and react accordingly by producing a flyback voltage under no load conditions, with no actual input signal applied. So, it's bettter to be safe than sorry and never run a tube amp "off standby" with no load attached, input signal applied or not. However, if the amp is in standby mode, no signal can get to the output transformer because, in standby mode, there's typically no high voltage applied to the tubes.

[John Phillips]

I totally agree with VaughnC. Putting the amp on standby is 100% safe; leaving it 'on' may not be, under rare circumstances.

Putting it to standby is better than turning it right off, as that introduces another cooling/heating cycle which is not good for the amp either.

[tonemandan]

Thanks guys. That's pretty much the way I understood it but appreciate the validation from the experts.

Dan

[garyrogue]

What type of damage will occur? What will amp sound or not sound like if in a situation like this? Just curious.
Thanks Gary

[aleclee]

Originally posted by garyrogue
What type of damage will occur?

Worst case is a blown tranny. Effects of flyback voltage can be cumulative as the insulation between windings breaks down so a little flyback here and a little there can eventually cause catastrophic failure.

[garyrogue]

Thank You

[KLB]

If I am checking bias on the bench, I don't use a load.
I turn the master volume and/or input gain to Zero.
KLB

[VaughnC]

Originally posted by KLB
If I am checking bias on the bench, I don't use a load.
I turn the master volume and/or input gain to Zero.
KLB

Still risky....because a malfunction can cause a post volume control internal signal to be generated, which will then be passed on to the output transformer.