I've seen a video from Upscale Audio https://www.upscaleaudio.com/pages/Microphonics.html that describes microphony makes a sound and my tubes do not make any noise; they just seem to be acting similar to a microphone so I'm not sure if calling my issue a microphonics issue is correct.
That said, if I tap on certain KT-120 tubes I hear the tap through the speaker. Details below.
I exchanged four KT-120 tubes in place of the VTA ST-120's 6550's. One of the tubes (right front) will not bias below .515, after a few hours more the rear tube of the right channel will not bias below .512. Until I could sort and test further I listened. All seemed OK until I noticed I heard my footsteps from the right channel speaker even with the attenuator at 0. When I tapped on the table the amp is on I heard it from the right speaker I then tapped the amp case and finally the tube. All tapping was heard from the right speaker.
Next day I swapped the tubes between channels. The sound followed the tubes. Now the tapping can be heard from the left speaker. Inability to bias below .515 followed the tubes as well. The now front left tube has the most sever "micophonics", although I think I hear it from the left rear now as well. The tubes swapped into the right channel will bias and also seem to have a much lower volume tapping sound.
I'm wondering if others have experienced tubes acting as a microphone so to speak. What about the inability to get correct bias?
Thanks,
Jimme
That said, if I tap on certain KT-120 tubes I hear the tap through the speaker. Details below.
I exchanged four KT-120 tubes in place of the VTA ST-120's 6550's. One of the tubes (right front) will not bias below .515, after a few hours more the rear tube of the right channel will not bias below .512. Until I could sort and test further I listened. All seemed OK until I noticed I heard my footsteps from the right channel speaker even with the attenuator at 0. When I tapped on the table the amp is on I heard it from the right speaker I then tapped the amp case and finally the tube. All tapping was heard from the right speaker.
Next day I swapped the tubes between channels. The sound followed the tubes. Now the tapping can be heard from the left speaker. Inability to bias below .515 followed the tubes as well. The now front left tube has the most sever "micophonics", although I think I hear it from the left rear now as well. The tubes swapped into the right channel will bias and also seem to have a much lower volume tapping sound.
I'm wondering if others have experienced tubes acting as a microphone so to speak. What about the inability to get correct bias?
Thanks,
Jimme