Sound from dummy load?

[2004ex]

Planning to look at signals with a scope, I had replaced both loudspeakers from a home blew 6L6 PP amp with 8 ohm power resistors. The input signal to the amp is obtained from the earphone output of a portable cd player. To my surprise, even no speakers are connected, I can still hear music coming out at a very (very) low level. What is vibrating? Are the output transformer coils making the sound? Is this what people have referred to as "tube vibrations" and what the ebay tube damper rings are for? Any one had seen/hear this before?

[peranders]

Current = Magnetic forces = sounds

The transformer can sound quite a bit but surely other parts too.
When I put out 600 W into watercooled resistors my amp was not quiet (It's cool(!) to boil water with an amp )

[Sherman]

My guess is that the sound is coming from your tubes, though as Per-Anders said it could also be the transformers. I've had the same experience and made a little cone from cardboard and put the small end in my ear and used the other end as a sort of "directional sound finder". It turned out my output tubes were happily cranking out quiet but audible tunes.


<Aside to Per-Anders>
I don't have an amp that powerful but it does sound like a fun thing to do- boiling water with your amp! (Hmmm... I wonder how hot my tubes get?)
</Aside to Per-Anders>

[beamnet]

experianced the same. Tried to test fire my first ever amp into a resisitive load. REALLY uncertain if i did things the right way ( i did the solder the thing entirely and then turn it on approach)

Hooked up my wave gen and was shockes by a very loud tone that got louder and louder.

hurried to turn it off

did not touch it for a month because i thought i had just misconnected $1000 worth of stuff and had nobody to look at it

then i realized that it must be the xformers

next i toasted a 4ohm 5 watt resistor. mistaking the voltage of my tone generator by a factor 10, making the amp run at 75 watt instead of 5...


Water cooled resisitors....as in put them into a can of water?

how much could -for instance- a 10 watt wirewound take in water?

Bas

[tubelab.com]

Most output transformers will make some sound especially SE transformers.

The abillity of a resistor to accept overpower when in water depends on the construction of the resistor. Even if the heat is removed by the water it is still possible to blow the internal wire in half. I have a pair of the brown glass covered wirewound resistors that have lived for years being used as load resistors. I have fed over 100 watts to these 20 watt resistors. The water will begin to boil in about 5 minutes. I do this to test power supplies under load during development. I would not use them as a dummy load for a tube amp. When a resistor fails it goes open. Running a tube amp at high power and suddenly removing the load WILL blow stuff up. Expensive stuff.

[beamnet]

does it!

damn, i suspected so

Also pp?

My speaker is very near to my door to my balcony. My roommates want to smoke in summer on my balcony, repeatingly stepping on the wires and pulling it out of the speakers...

what happens?

the load falls away...so the opt becomes a choke....so the tube disippation goes up....? am i right?


Bas

[Tweeker]

If you need a really tough AC resistor, look into water resistors.

Not suitable for DC circuits.

[imix500]

Hmm, may also be useful to generate hydrogen gas.

[ilimzn]

Even worse, an ideal mixture of hydrogen and oxygen, which ignites easily and makes a really big bang. Very dangerous stuff!

Regarding noises - no, tubes do not tend to make noise, not directly (but can pick up noise by microphonics).
Magnetic components in general exhibit magnetostrictive effects and will produce sound to some degree, even if they use a completely solid core. Laminated cores in general make sounds, especially if gapped with a small gap. Regarding audio trensformers, you could say the better the quality, the less noise. Magnetostriction produces force in an elastic medium, which means there is a resonance frequency involved. This is why laminated audio transformers tend to be well impregnated, especially if they are gapped. Vibrations in the core produce variations in the gap size, which in turn modulates inductance at the mechanical resonance frequencies of the core.
Power transformers in class B amps also make sound, since they experience a pulse width modulated version of the rectified output current (this is actually a form of intermodulation).
Windings can also vibrate and produce sound, by means of magnetoreluctance, or just plainly forces acting between windings (consider each turn as a separate electromagnet). In the same manner, some wirewound resistors will 'sing' especially if close to ferromagnetic material.
Finally, there is also electrostriction, the electric field equivalent of magnetostriction, where materials change shape in an electric field. Some types of non-polarised capacitors can exhibit this (ceramics mostly) but foil types in general use foil that is specially selected for least electrostriction, becuase the phenomenon modulates capacitance with voltage. Electrolytics however tend to sing, especially if subjected to large currents or voltage swings. Filter caps in amplifier PSUs tend to make music quite well at high power outputs

[Tweeker]

This is the reason water resistors are not suited to DC circuits, no significant electrolysis occurs with AC circuits.