Warming-Up Amps w/o Inputs or Speakers Attached

Herman,

Even when driving input 20W should be enough since average music power delivered to speakers is only a few percent of peak power.

Herman is correct in referring to inductive kickback that will occur in the output transformer of a tube amp in the absence of a load, and Rodman is correct in that damage is unlikely to occur in the absence of an input signal.

However, "unlikely" does not mean that the chances of damage in the absence of an input signal, when there is no load, are zero. Depending on the specific design it is conceivable that transient "signals" which the amp's circuitry may create internally at turn-on or turn-off will induce significant inductive kickback, which could cause damage cumulatively over time.

The most significant concern is not damage to tubes, but the possibility of damage to the transformer itself, due to the extremely high voltages that can be briefly produced when the current through an inductance changes abruptly.

Concerning SS amps, my understanding is also that in most or all cases the absence of a load will not be harmful.

Regards,
-- Al

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I'll respectfully disagree with that but have no hard science to back me up.

"Most tubes are cheap compared to the price of good output transformers. "

Either tubes or transformers are more likely to be damaged.
The price has nothing to do with it.

I have seen circuits designed to protect output tubes from kickback. I have not seen any designed to protect OPTs. I have seen quite a few output tubes fail and very few OPTs fail.

For those reasons I think it is the tubes that are in danger, but in any case, running tube amps without a load is a bad idea.

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I agree with Herman. It is the output tubes that are in danger and running a tube amplifier without a load is a bad idea.

Why not use an 8 ohm resistor in the place of speakers?