OT damage ??

The risk of no load on a tube amp is high current. The tubes usually start sparking. Transformer damage usually occurs when the current oscillates and "runs wild", which doesn't always occur. It takes a fairly large current and time to burn a transformer, so your probably OK, although your tubes are taking a beating.

 

Something I'm wondering:

Are there physical indicators of a burned out transformer? How do you check. Just measure resistence across the windings? Shouldn't read open or short. Anything else?

 

No, it's the exact opposite. With no load, no current can flow in the secondary circuit, so the energy in the transformer's magnetic field is turned into a high voltage instead, which can arc through the winding insulation.

Sometimes, but not always. If they do, there's a reasonable (but not certain) chance that they'll protect the OT by absorbing the voltage spikes.

That's definitely true if the amp doesn't actually have a signal going through it - damage will only occur if the amp self-oscillates. Most amps aren't actually unstable with no load, so there's usually only a risk if you were actually playing through it, and at substantial power, too - running an amp at low power will not damage it even with no load.

Not at all - if the amp is already producing high power and the load suddenly goes open (ie if the speaker cable gets pulled out or something like that) the damage can occur in a fraction of a second. It's the initial voltage spike as the transformer's magnetic field collapses and the stored energy has to go somewhere that is the biggest risk. (This is exactly how a car ignition circuit works BTW .)

If the amp is self-oscillating, definitely.

If it reads open or short, it's definitely dead. But they don't normally do that... the normal failure is a partial short on the primary side. This can be difficult to tell by measurement because the difference between the correct resistance and the damaged reading can be quite small - even a single shorted turn is enough to stop the transformer working properly. The two sides of the primary are normally slightly different resistances anyway, so comparing the two doesn't always help much either.

Really the only ways to be completely sure are to temporarily patch in a new one, or to calculate the power input to the transformer (from the voltage swing across the power tube plates and the average tube current) and measure the power output. If there's a drastic difference - a healthy transformer should be about 90% efficient - the transformer is dead.

 

It's kind of 'hidden' in all that stuff I wrote...

If the amp is not unstable into an open circuit, and there was no significant signal going through the amp, there is no danger.

It's unlikely that any 'cumulative' damage has occured. It's possible that the winding insulation could be partly damaged but not make a permanent short, but unless the amp was putting out a lot of power this cannot have happened either. There needs to be enough power being developed to create a high voltage in the OT.


It's not a Mesa Dual Rectifier or Trem-o-verb though is it?

(These are unstable with no load.)

Anyway, try not to do it. While probably nothing bad has happened, it really isn't a good idea .