Music Reference RM10 MkII tube question

[Jflijohn]

I’ve been occasionally blowing a Left channel 160 mA fuse on my Music Reference RM10 Mk II. As I was testing it and checking for voltages, I input a tone at 1kHz at 1 volt RMS. I had two 100 watt, 8 ohm dummy loads connected to the speaker terminals. There was an audible 1kHz tone coming from the four EL84 output tubes. When I put my probe on the grid lug (#2) the amplitude decreased but still at 1kHz. It looks to me like the tube elements were resonant at1K…
I guess I can’t explain this, the impedance of the volt meter shouldn’t affect the load and if pushing the probe on the lug was dampening the vibration, the frequency would have reduced as well. This was the case on all 4 EL84 output tubes.

Are tubes resonant at audible frequencies? I guess it would make sense, the elements are pretty thin.

 

[MaxwellsEq]

Are tubes resonant at audible frequencies? I guess it would make sense, the elements are pretty thin

Tubes are microphonic at audio frequencies.

 

[Jflijohn]

Tubes are microphonic at audio frequencies.

Never heard of that before. Found this on the web.

Thanks

Microphonics

This is a deeper discussion for those that want to learn more about how tubes should be tested. If you are new to tubes, don’t read more into this than there is to read. In other words don’t worry about it. If you are buying your tubes from Upscale Audio, you don’t even need to read this because...

upscaleaudio.com

 

[MaxwellsEq]

Never heard of that before. Found this on the web.

Thanks

Microphonics

This is a deeper discussion for those that want to learn more about how tubes should be tested. If you are new to tubes, don’t read more into this than there is to read. In other words don’t worry about it. If you are buying your tubes from Upscale Audio, you don’t even need to read this because...

upscaleaudio.com

Ideally you wouldn't operate a tube-based device in the same room as the loudspeakers are in.

In general, motors are the same as generators, which means loudspeakers are the same as microphones. You can record something using a loudspeaker as an input, and you can listen to music from a microphone. BUT, this is suboptimal! Each device is designed to be optimal for the task in hand, so music played on a microphone is poor, as is music recorded using a loudspeaker.

Tubes being microphonic means that they inject audio signals around them into the circuit. Some consider this adds to the "bloom" associated with tubes, but I don't know of any objective measures that confirm this. There are tube dampers, but I've never seen objective measurements for any benefit.

Obviously, given the generator/motor analogy this work both ways - tubes can oscillate when fed a signal, which is what I think you've experienced.