hearing sound when running into a dummy load

[black_labb]

i've just been troubleshooting my bass amp build and have been looking at the signal as it passes through on a scope while the amp is on a dummy load.

when there is some signal passing through output of the amp you can hear the signal. i was running a 1 khz signal most of the time, but i tried some other frequencies which also came through. at first i thought it must be the valve holder (one of the spring mounted ones to hold the valves in place) as there were 2 near the output transformer that didnt have a tube in them. i then got a bit frustrated with the amp and decided to test the power of another amp (5e3 styled) to make sure i am measuring the power right. to my surprise the frequency on the test signal was able to be heard again.
also the signal didnt seem to transfer to the signal going into the dummy load very well (seemed like an on off thing this time when viewing on the scope).

is this normal? could it be that the resistors i bought to make a dummy load have alot of inductance and the load at 1khz is much higher than the dc resistance causing a similar condition to having an open load? (6x 24ohm 50w resistors in parrallel for 4 ohms, in parrallel/series for 16 and half of them for 8ohms (150w only) here are the resistors. WELWYN|WH50 24R JI|RESISTOR, 50W 5% 24R | Farnell Australia

i was pretty happy to be able to get these resistors for a dummy load as they seemed more reasonable than most, but a dummy load needs to work correctly.

edit: i measured the resistance of the dummy load, and the the dummyload and lead connecting to it measured 9.3ohms when i was aiming for a 8ohm load, so its not a missed connection or similar.

[Phostenix]

I would say that it is normal to be able to hear the dummy load resistors singing with power into them. Mine do it, and others here have said the same.

What do you you mean by this:
"also the signal didnt seem to transfer to the signal going into the dummy load very well (seemed like an on off thing this time when viewing on the scope)."

[black_labb]

I would say that it is normal to be able to hear the dummy load resistors singing with power into them. Mine do it, and others here have said the same.

What do you you mean by this:
"also the signal didnt seem to transfer to the signal going into the dummy load very well (seemed like an on off thing this time when viewing on the scope)."

thanks, good to hear. i was worried it was a problem.

i meant that the signal didnt seem to be showing up across the dummy load. i then realised that i had put a bad 12ax7 in the amp (tubes were out of the amp before i decided to test it).

since then i got signal across it, but the signal wasnt as strong as i would have expected for the amp, but thats a different issue.

[Jazz P Bass]

Singing transformer

The output transformer also will reflect the signal.

[Enzo]

Yep, output transformers will sing, you just usually can;t hear it over the sound from the speaker.

[black_labb]

thanks everyone. good to know.

[J M Fahey]

I've even heard power transistors sing.
They were those very old, aluminum encased TO3 Motorolas (2N3055 and MJ2955)
In this case there must be some piezoelectric effect involved, not electromagnetic as in wirewound resistors.

[Gunny]

I've experienced the same thing dozens of times - solid state and tube amps. You can also hear the point where the output starts to distort. The pitch of the signal changes (confirmed by oscilloscope waveform).

[brucec59]

Don't forget the tubes themselves. Output tubes can make a lot of noise at full tilt.