Amp making sound when hooked up to dummy resistor

[peesinstew]

I just got an oscilloscope and wanted to trace some signals. I don't have a proper high power dummy load yet, but I temporarily hooked up my 18Watt to a 20W 8ohm resistor, sent a sine wave through the input, and scoped the output.

The thing is, when I started turning the volume up I could hear the sine wave coming from the amp itself, I think from the power tubes. I've never sent a pure sine wave through an amp before, and I've never hooked up an amp for silent operation either, so I've never heard this happen before. Is there something wrong? Do amps just make noise when they're not hooked up to a speaker? Could it possibly be because of the nearly undersized dummy resistor?

Maybe I have some sort of special reverse microphonic tubes that I can sell on ebay for $5k.

[v846]

they do that, screaming at ya to turn it down

[Bob-I]

Perfectly normal. The elements in the tube will vibrate when pushed really hard.

[mhuss]

Actually, I usually hear the OT singing when cranked and hooked to a load. The windings actually vibrate slightly trying to push the core back and forth.

--mark

[CaseyJones]

Oooh, that raises possibilities. Eliminate the speaker, use a dummy load to keep from blowing the OT and throw a mic on the transformer. Betcha there's some tweezed tones in there. With more vocalists using a bullhorn between themselves and the mic (Neil Young, Michael Stipe w/ REM) what's a poor guitar player to do?

How 'bout a piezo stuck right to the OT? Or maybe a guitar pickup in close proximity.

[peesinstew]

Actually, I usually hear the OT singing when cranked and hooked to a load. The windings actually vibrate slightly trying to push the core back and forth.

--mark

That's probably what it is. I couldn't tell if it was coming from the tubes or the OT. Good to hear that it's not the sound of things braking.

[rfgordon]

Some OTs will "sing" more than others. In the very first amp I built, which had iron from an old Harmon Kardon mono amp, the OT had two secondaries, one for conventional speakers, the other for a 70 volt distributed speaker line. I had picked the wrong black wire as common, so there was nothing in the speaker, but the tranny sang like crazy! Swapped wires and it came to life.

I don't know why some do and some don't. Maybe someone with more engineering design knowledge would know.

[Vanessa]

It depends on varnishing and type of varnish,transformer which is light on varnish will be more alive
and it would be also harmonicaly richer ,should we say in KF terminology "on the edge"/more live... when speaker comes to play with him.
Simple physics