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Warming-Up Amps w/o Inputs or Speakers Attached

How dangerous is it to warm-up amps without inputs and speakers connected? When running A/B comparisions between gear it's difficult to truly gauge differences when one piece is good and hot and the other stone cold. What about the differences between doing this with SS vs tube amps? Can you just plug them in and let them cook?
stickman451
herman
2,288 posts
 
FYI for anyone interested

I will not be drawn into some endless debate but Mr Rodman is wrong on most counts. Kijanki is correct. SS voltage sources don't care if they have a load or not.

Transformers do indeed store and release energy. That is why tube manufacturers caution you to always have a load connected. That is why many guitar amps have spike protection across the output tubes in case there is a voltage kickback from the inductance in the transformer.

Radio Shack has 20W 8 ohm resistors which are plenty for the purpose of protecting the output tubes unless you want to drive the amp with an input signal

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kijanki's avatar
kijanki
4,477 posts
 
Herman,

Even when driving input 20W should be enough since average music power delivered to speakers is only a few percent of peak power.
almarg's avatar
almarg
9,670 posts
 
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
swampwalker's avatar
swampwalker
5,138 posts
 
What Al said. Most tubes are cheap compared to the price of good output transformers.
herman
2,288 posts
 
.

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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