Turn on an amp w/o a speaker connected?

[bluzmn]

Sometimes when I'm cruising eBay, I see an amp head for sale, and the seller says "Turned it on and the tubes lit up; did no further testing". I wince every time I see that. Will turning on an amp without it being plugged into a speaker definitely damage a transformer, or is it just more likely than not?

[Tom Phillips]

...Will turning on an amp without it being plugged into a speaker definitely damage a transformer, or is it just more likely than not?

No. Not "definitely." It is hardest on the output trans when a signal is put through the amp such as when one keeps strumming a guitar & continuously turning up the volume because they hear no sound.

[Chuck H]

What he said^^^^^^^^^^^^^^^^^^^^^^^^^

I laughed reading it too. "Amp is on.?. Guitar is plugged in.?. Huh.?." Turning it up, and uP, and UP! Blangin' on the guitar.

I turn on my amps all the time with nothing plugged in so the tubes can warm up while I get bench gear and stuff set up. They're usually in standby but I've done it with non standby amps too and never gave it another thought. Yes, there IS current and a necessary load at idle, technically. But the OT should be able to handle that level of current just about indefinitely. Way more critical when conducting signal.

[nickb]

What he said^^^^^^^^^^^^^^^^^^^^^^^^^

I laughed reading it too. "Amp is on.?. Guitar is plugged in.?. Huh.?." Turning it up, and uP, and UP! Blangin' on the guitar.

It's the natural thing to do, especially if you just checked the speaker was plugged in but didn't know the lead had gone open circuit

[nhbassguitar]

Sometimes when I'm cruising eBay, I see an amp head for sale, and the seller says "Turned it on and the tubes lit up; did no further testing". I wince every time I see that. Will turning on an amp without it being plugged into a speaker definitely damage a transformer, or is it just more likely than not?

Being a highly suspicious bastard by nature, and trying to take a 10,000-foot view, about all I have a right to expect from such an amp is that the filament transformer and the tubes' heaters are functioning. It could be the seller is expecting potential buyers to read something into the words "did no further testing." Or maybe not. To me, the whole thing sounds like a crap shoot. If I were a tech-in-training and the price were low enough, I might buy it with the goal of self-assessment and as a way to test/further my skillset. I've done this several times in the past with amps that are advertised "as found," e.g. from a house cleanout or estate sale. Sometimes I've done it just to obtain a chassis with the "right"-sized tube socket holes so I can strip it and use it to host my own design; sometimes I'm presently pleasantly surprised by what I get and so I go with some Plan B for the unit.

[52 Bill]

And a lot of tube amps have a shorting speaker jack, designed to protect the amp from damage when there is no speaker plugged in.

[Mick Bailey]

Or flyback diodes on the OT, or a resistor across the output socket to damp oscillation.

One of my regular income streams is repairs/restoration of amps bought off E-bay. They often have a lot wrong with them, but only infrequently have failed OTs, unless it's a Marshall 20/20, or Laney VC30.