Could this harm OT?

[nosaj]

I have read somewhere that you can turn a tube amp on without a speaker(to check voltages) an not run a signal through it an your OT will be ok.
It is when signal is applied that you can fry your OT. I have gotten answers on both sides on another site and wanted to check here to see which way is correct.

Also does it differ from SE to PP? I understand it is best to always have a load speaker or dummy, but in the instances it gets forgotten what would be the correct answer?

Thanks,
Jason

[g1]

In the instances that it gets forgotten, there will be no damage until some kind of signal is applied.

[Tom Phillips]

Or until something happens to cause the amp to self generate a signal such as a parasitic oscillation or large signal level caused by a broken ground, intermittent part or probing done by the repair tech.

[g1]

Thanks Tom, I meant to lump that in under "some kind" of signal but I'm glad you clarified.
Definitely not worth the risk when checking voltages, better off to short the output completely than leave it open.

[nosaj]

Thanks guys. It seems to hapen sometimes I'll power an amp on then look over it and realize oh crap no load. Someone caught me on that the other day and I was sure it wouldn't hurt it but I see your points.

Thanks,
Jason

[Tom Phillips]

Yep. I've done that too and, contrary to some internet lore, the OT doesn't instantly get damaged. I've always found it interesting that no one talks about connecting a load to the reverb drive output when bench testing an amp that utilizes a reverb drive transformer.

[eschertron]

Thanks guys. It seems to hapen sometimes I'll power an amp on then look over it and realize oh crap no load. Someone caught me on that the other day and I was sure it wouldn't hurt it but I see your points.

Thanks,
Jason

I gave myself a scare last weekend; got no sound from the amp during sound check. Turns out the 4-Ohm / 16-Ohm selector switch on my speaker cabinet was in the middle position (open). Yeah, stupid me for using that switch. Once the speakers were switched in, the amp ran OK. So I can definitely say a certain 5W SE amp will not kill the OT within 20 seconds. Whether or not permanent damage was actually inflicted I can't say.

On a side note, every amp I've worked with has a shorting jack on the speaker connection, so with nothing plugged in there's a load present on the OT. Isn't this universally done?

[R.G.]

The devil is always in the details. In developing the Workhorse amps, I (ooops, did *I* do that?) left an amp idling without a speaker or resistor load for over an hour. Result? No change. No damage, no change in anything and nothing was even unduly hot.

The Workhorse amps deliberately did not use feedback, to allow the sounds of the tubes to show through - for good or bad. I speculate, but have never been able to prove that this is why it did not go into self oscillation and SDS (Sudden Death Syndrome).

It's probably true that without a signal more than X, an amp will not be damaged if run without a speaker load. The problem with that is twofold:
1. X varies from amp to amp for the threshold of damage
2. Some amps will oscillate, providing the maximum signal of which they're capable, with no input signal at all, when unloaded.

The result is that in my toolkit of Personal Rules is the following:
1. Don't intentionally unload an amp, even though you may get away with it.
2. Tell people who ask not to unload an amp, as they probably won't get away with it, and will blame you for the misinformation if they're unlucky.

[Tom Phillips]

The shorting jack is not universal. Some damaged OTs I have seen happened when the user had an open cord plugged into the speaker output or forgot to plug into the speaker cab end. Then he kept strumming the guitar and turning the amp volume up higher trying to get it to work. I like to tell users to stop trying to play thru the amp right away if they don't hear sound from the speaker. Then they can check things out and just put an ear close to the speaker and listen for hum/hiss to verify that the speaker is connected to the amp.

[Enzo]

So I can definitely say a certain 5W SE amp will not kill the OT within 20 seconds.

But it isn't a time thing, it is an event thing. Driving without your seat belt is dangerous and can kill you, but it isn'lt a matter of how long you left it off, it is a matter of whether certain events take place while you are without it.

Signal through the unloaded transformer MIGHT generate a kick large enough to arc over some windings or to frame. It could happen 10 seconds from now, or at the end of MacArthur Park. Your no signal condition won;t hurt the thing, but all that needs happen is some transient noise from the power environment makes a loud pop with nowhere to go.

[eschertron]

Your no signal condition won't hurt the thing

My point (and my confusion) exactly! I was strumming the guitar and wondering why I couldn't hear anything... and course I had to turn the volume up to 12 (it's two bigger, innit?) before I thought there might be another cause for the silence. Fingers were definitely crossed when I tried again after bringing the speakers on line.

[Leo_Gnardo]

I've noticed on some models of Mesa/Boogie, without a speaker load the output tubes flash on & off blue (as if there's a big output signal, not an arc). Apparently no load conditions set up a very low frequency 3-4-5 Hz oscillation. Can't be good for the tubes or OT.
No shorting bar on outputs of these amps. Fuse doesn't blow & amp operates normally after speaker attached to output.

[Steve Conner]

Yikes. :/

As far as my own personal homebuilt amps go, they are all stable without a speaker. They don't seem to mind producing signal without a speaker either, as long as it stays below clipping. When the power amp is overdriven without a load, that's when things go horribly wrong. Years ago I did some careful experiments to investigate this, and the first sign of trouble was always grossly excessive screen current.

Enzo likes car analogies, so here's another one. You can sit and rev your engine with the transmission in neutral. But if you give it too much gas you'll wreck it by exceeding the maximum safe RPM. Modern engines have a rev limit function in the engine management computer to prevent damage, and I think it would be nice to have a guitar amp equivalent. It would probably take the form of a current limit on the screen supply. There are lots of low-powered 240V halogen light bulbs on the market now, maybe a couple of these in series would protect the screens, similar to the 12V ones used for tweeter protection in PA cabinets.

[J M Fahey]

I was strumming the guitar and wondering why I couldn't hear anything... and course I had to turn the volume up to 12

[Merlinb]

Yep. I've done that too and, contrary to some internet lore, the OT doesn't instantly get damaged. I've always found it interesting that no one talks about connecting a load to the reverb drive output when bench testing an amp that utilizes a reverb drive transformer.

I once fried a 12AT7 by doing that (flashover due to flyback). The transformer survived though!