OT: Short vs Speaker load

Hi folks,

I'm studying a Bassman AB165 circuit and have arrived at a question about how the output transformer works.

The main speaker output jack has a different type of jack which has an additional contact that seperates from tip lead on the jack when the speaker cable is plugged in.

So, how I see it based on the schematic, the OT output sees the speaker impedence as the load when a speaker is plugged in (assuming there is a speaker on the other end of the speaker cable!), but according to how I'm reading the schematic, one lead of the the output winding is connected straight to ground if the speaker is NOT plugged in.

So, 0 ohms vs <speaker load> ohms at the output.

There is a lot of discussion as to whether or not mismatched output loads can harm the OT.

I'd like to understand why having NO speaker cable connected doesn't harm the OT. Or does it if one were to try to play with no speaker connected? I've always been carefull not to run an amp with no speaker connected, but I am presuming that since the jack has that ground disconnect element built in that, that is some sort of safety feature.

Please clarify this.




Platinum Supporting Member
Im sure some one will be along who understands this however I know my Rivera shorts the speaker connections together if nothing is plugged into the amp (0 ohms) as a "safety" feature. This is harder on the tubes but much safer for the output transformer- something about preventing "flyback" voltage from damaging the OT with no load/speaker connected I believe. Bob


If you short the output of the amp, the only load the amp "sees" is the secondary winding of the transformer. It's usually only a few Ohms, but this is something at least, and means that nothing will blow up immediately. But, if you play through it for any period of time this way, it will most likely melt the transformer and ruin the amp.

If you leave the output open, then the only load the amp sees is the primary of the transformer. The primary usually has a high resistance and inductance, and since an inductor stores and releases energy, large voltages get produced within the coil and on the plates of the output tubes. This causes the arching and damage to the amplifier and may ruin the transformer.

Bottom line: have a speaker plugged in at all times and match the impedance to what it should be.

John Phillips

Both a dead short and an open circuit can potentially damage a tube amp, but the big difference is that while a dead short will take some time even at full amp power - during which you're almost certain to notice that it isn't working, and stop - an open circuit can damage the amp instantly, so it's much safer to deliberately short the output if a speaker isn't connected.

A short may eventually overheat the transformer, and also stress the tubes very heavily to the point they might fail (which is also a hazard to the amp, even if it doesn't usually do damage), but it will take a long time - several minutes with most amps. The efficiency of the amp actually falls because the load is so mismatched, so the rise in current becomes self-limiting to some extent.

But an open circuit at full power can cause a voltage spike which can arc inside the transformer winding and destroy it in a fraction of a second. Even with no input signal, not all amps are stable with no load and will self-oscillate, which will produce a single frequency at full power - sometimes outside the audio range, where the components are not designed to operate. Shorting the output also stops any self-oscillation.

This only applies to tube amps. Solid-state amps are almost always safe with no load, because they don't have an output transformer to generate and be harmed by voltage spikes - but you must never short the output, unless the amp is specifically designed to handle it and protect itself - you'll blow the power transistors because without a transformer and a mismatch to reduce efficiency, the current through them will simply rise without an effective limit and exceed their ratings, and unlike tubes they don't usually recover from this.

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