Ghost notes

[Tonegeek]

I am getting ghost notes when my amp is sitting on my speaker cab. The note is an octave or 2 below the note I am playing and is sometimes not the same note. C# on the 11th fret, D string sets it off more than any other note I play. My cab resonates more around this frequency so it makes sense that it would cause more vibration. It does not do it when the amp is located away from the cab. I can grab my power tubes with a rag and the ghost note does not ring as long but I cannot get it to completely go away. I replaced the JJ EL-34 tubes with some old Sylvania 6l6's and it got worse so i am thinking it is something mechanical in the tubes. I tried tapping the pre's and they seem fine, but you can hear a bit of response from the power tubes when tapped. I wonder if it could be the wiring in the amp vibrating, or some other component in the amp? Has anyone else experienced this? I see lots of threads about ghost notes with responses about filtering, etc. but this seems mechanical since it does not happen without being vibrated by the speaker cab. The note is not that loud and you would never hear it in a band situation, but I would like to track it down. BTW it does it in clean or OD and is dependent on volume and frequency.

[Firestorm]

Well, it could be a parasitic oscillation, but you are likely right that it is mechanical in nature. A cap is essentially a microphone so one could pick up vibrations -- maybe try a bit of flexible glue here and there.
Another possibility -- I once had exactly the same thing in a Twin Reverb combo (impossible to move the chassis away from the speaker in that ) and it turned out to be a preamp tube vibrating and sending the "rattle" all the way to the power amp. Certain notes set it off -- others didn't

[Richie]

sometimes if the amp doesn't have a shielding plate, you can have these problems happen when sitting on a cab. And as others mentioned a part vibrating etc.. I've seen amps where a single cap would cause a noise on certian notes or vibrations,when the cap was kind of laying on its side..and no noise or problem whe it was sitting straight up.

[butwhatif]

most likely a tube, usually power tube grid vibration

[Tubetwang]

Deep Purple's Live in Concert 72/73 were making use of 2" wood planks between Marshall heads and cabs...

Blackmore and Glover were on platform shoes as well...buffering the stage against lift-off.

Raw, kick-assed jamming band...although Blackmore was off on more than a few occasions...

I could do without the banging around guitar bit though, thank you very much.

[Tonegeek]

And the winner is...

most likely a tube, usually power tube grid vibration

Finally got around to diagnosing this issue. It is one of the 2 JJ EL-34 power tubes that is becoming increasingly microphonic. I started subbing tubes until I found it. I was hoping to get more than a year out of these tubes. Maybe I will try some SED Winged C next time.