No load connected - damage to amp?

[GregS]

Not sure if this is the right forum for this. I was just adjusting the bias on my brother's blackface Fender Bandmaster. At one point I swapped the output tubes and unplugged my resistive load while I did it. I then forgot to plug the load back in when I turned it back on. I had it on for up to 5 minutes without the load. Nothing unusual happened during this time. The current through the half of the output transformer primary that I was measuring remained at a normal value, nothing overheated.

Unfortunately I don't have a 4 Ohm speaker cabinet I can plug into to see if it still works ok for my own peace of mind. I have an 8 Ohm cabinet but I'm not sure it's a great idea. In case I haven't already messed it up I'd hate to ruin it doing that. Otherwise I have to wait a number of days before I can return it to him so he can try it out and hope for the best.

Anyway is it likely I've done something to it given the information I've provided? I've read a lot of things about how you are almost sure to ruin the amp if you turn it on without a load connected but I haven't seen any detailed explanation of why this happens.

Thanks for any input.

Greg

[R.G.]

Running without a load can be and is frequently a bad idea (1) for tube amps (2) which employ feedback in the power amp stage and (3) which do not have some kind of internal always-connected resistance. But it is not uniformly damaging or fatal.

Generally the damage comes from the amp going into internal oscillation, and ever overheating some part of the power path or generating repeating transient spikes that over voltage some part of the power section; or both.

Power amps which are (1) solid state (2) tube but don't use overall feedback on the power amp and/or (3) have some kind of internal "safety" load are generally immune to this. Even tube amps which use feedback and have no internal safety load don't always go into oscillation or get damaged by it if they do oscillate.

Plug in your speakers and play. If it sounds OK, you almost certainly got away with it. If it doesn't, it was already damaged and trying an 8 ohm speaker cab did not damage it further.

[Steve Conner]

Did you apply an input signal and crank it up without the load? That's when the damage is done. If the amp was just sitting idle, it's probably going to be fine.

Disclaimer: I haven't worked on many tube amps compared to the professional techs here, but all of the ones I've built or worked on idled happily with no load. However, it is theoretically possible for an amp to go unstable when powered up with no load, and blow itself out spectacularly.

[GregS]

Thanks for the replies.

It's good to know what it is that can actually happen by running a tube amp with no load connected.

It is a tube amp with feedback in the power amp stage without any internal always-connected resistance. However, I did not have any input signal applied and certainly nothing you could actually notice happened. I noticed it when I reached around the back to turn the power off and saw the speaker cable not plugged in.

The whole while it was on I was watching the current in one half of the OT primary and everything was totally normal. I turned it on again afterwards (with the load connected) an all seemed normal from the point of view of current in the OT primary and voltages at the tube sockets.

I'll try it out briefly with the 8 Ohm speaker. I could also look at the signal across the load (with signal generator input) with my oscilloscope but probably actually hearing it would be a better test.

One other concern I have with it is that the tubes are seriously mismatched. I ended up biasing so that the plate current was 30 mA in one and 45 in the other. That's a 50% difference and I'm not sure that difference has stabilized - they could drift farther apart with more use. Is this difference enough to be concerned about output transformer damage?

Thanks,

Greg

[pdf64]

Check the load signal on a scope. The upper and lower halves of the wave should be roughly symmetrical. If they are grossly different it may not sound too good, unlikely to get to big clean bottom end. But your brother may be fine with it.
15mA disparity in the no-signal condition won't cause a problem for the OT, it's insignificant and there likely to be a far greater disparity at full power.
But I would adjust the bias such that the hot tube could drift a few mA without going past 70% max plate dissipation.

[loudthud]

Leo's got your back. Blackface Fenders short the speaker leads when the speaker is disconnected. This prevents most damage.

[GregS]

Leo's got your back. Blackface Fenders short the speaker leads when the speaker is disconnected. This prevents most damage.

Great! That must be the purpose of the switching speaker jack. I hadn't realized that.

I'll look at the signal across the load with the oscilloscope to make sure it's normal as well.

Thanks again for everyone's help/input on this thread.

Greg

[GregS]

The signal output from the amp looks normal and symmetrical. At low to medium level it looks like a perfect sine wave. Just before clipping what appears to be cross over distortion becomes apparent. I'm concluding all is ok and no damage done. Thanks again to all that helped on this thread.

Greg

[J M Fahey]

Besides, you got lucky, good for you.
As an example, you *can* cross an avenue, blindfolded and with the red light on, and reach the other side undamaged ... or not.
Anyway, I wouldn't try it.

[GregS]

Right. The amp was played with the speaker cabinet last night and was fine so I am lucky. However, I can't believe this is a rare occurrence. It seems to me that this is an accident waiting to happen. It's very easy to have an absent minded moment like I did despite how careful I normally am about this. Also any time someone moves their amp there is the potential to forget to plug the speaker cable back in. Even if you don't move it, your dog can have pulled it out without your knowledge and unless you check it every time you turn the amp on.....

I can't believe I'm one of the few people this have ever happened to. It would be interesting to find out how many others have experienced this and what the results were - also what methods people have to make sure they never make the mistake.

Greg

[J M Fahey]

truth is, many of us have.
But on the workbench, you hear the output transformer (and even the tubes) "sing" faintly, with no corresponding sound from the speaker, and you turn it off in 10 seconds.
Now, with the chaos, noise, darkness, pressure, whatever on stage, anything can happen.
I have personally witnessed dumbfounded guitar players strumming loudly, setting all knobs on 10, worried that no sound comes out, banging it, pulling all cords, except the speaker one, for up to 5 minutes, which is a very long time.