Sources of mechanical/ microphonic vibration

[mike-p]

So my hodge podge mess based on an 18watt with double power section, is finally sounding ace and running 4x 6p1p which I'm preferring to the 6p14p-k. But it only sounds ace sitting on top of, or beside the combo cab I built. Screwing the chassis into the box I get a metallic vibration which is definitely coming through the speakers. It sounds like a little washer or something vibrating.

I've looked for

Loose hardware
Microphonic tubes
Vibrating grid wires
Holding everything I can touch still with a chop stick whilst running a looper pedal.

Anything I haven't thought of?

To be clear, the combo cab sounds great without the chassis so I don't think it's that.

When I strike the chassis with the handle of a screwdriver without power I can hear a faint metallic/ glassy ring which I can't identify but could be just normal tube innards. With power I honestly can't detect anything.

I've spent many many hours trying to solve this and it's killing me, it's probably an annoying question and not an interesting electronic audio question but I'm seriously stuck and any suggestions greatly appreciated. I know a mechanical vibration is hard to find but I'm hoping the fact that it is being amplified will give clues.

[martin manning]

Have you tried mounting the amp in the cab and driving the speaker with another amp? If you still hear it then it's purely a mechanical issue. If not its electro-mechanical, triggered by the vibration, or perhaps purely electrical, i.e. a parasitic oscillation.

[Stevem]

Go with process of elimination.
Rap on the chassis and remove one tube at a time to see if it goes away.
I had a output transformer once that it’s coil was loose enough on its core to rattle.

[xtian]

I have a method. Plug in a sine wave generator into the amp's input. A smart phone is perfect for this. Turn up the sound and sweep the frequency until you hear the offending rattle. Now you have two hands free to poke everything with a chopstick until you find the rattle.

[GAStan]

Can you post some pics? Good pics of the mounting method may be helpful.

On my Dumble style builds the chassis mounting screws protrude into the interior of the amp. If yours is similar check that the screws aren't contacting anything when fully tightened (if you haven't already).

[mike-p]

Have you tried mounting the amp in the cab and driving the speaker with another amp? If you still hear it then it's purely a mechanical issue. If not its electro-mechanical, triggered by the vibration, or perhaps purely electrical, i.e. a parasitic oscillation.

Just tried this, less than ideal as using a 4 ohm output into 16 ohm speaker load and a 3 watt amp which isn't working quite right... but no, I can't hear the rattle.

[mike-p]

I have a method. Plug in a sine wave generator into the amp's input. A smart phone is perfect for this. Turn up the sound and sweep the frequency until you hear the offending rattle. Now you have two hands free to poke everything with a chopstick until you find the rattle.

This is basically what I'm doing with the guitar and a looper pedal. No joy.

[Stevem]

What your calling a rattle could be due to a bad solder connection and the resultant arcing .
This would be most noticeable on low notes or full Barr chords that draw a lot of current.

[martin manning]

Have you tried mounting the amp in the cab and driving the speaker with another amp? If you still hear it then it's purely a mechanical issue. If not its electro-mechanical, triggered by the vibration, or perhaps purely electrical, i.e. a parasitic oscillation.

Just tried this, less than ideal as using a 4 ohm output into 16 ohm speaker load and a 3 watt amp which isn't working quite right... but no, I can't hear the rattle.

It'd be more conclusive if you could drive the speaker in a way that is closer to your double 18W.

[mike-p]

What your calling a rattle could be due to a bad solder connection and the resultant arcing .
This would be most noticeable on low notes or full Barr chords that draw a lot of current.

I'm the first to suspect my solder joints but it's frequency dependent, present on the high strings and volume doesn't need to be very high.

[Stevem]

You have a oscillation problem then and likely due to layout reasons.

Just for S & G try reversing the output tube plate wires or the wires off the secondary side of the PT to the speaker jack, whichever is easier for you.

[mike-p]

You have a oscillation problem then and likely due to layout reasons.

Just for S & G try reversing the output tube plate wires or the wires off the secondary side of the PT to the speaker jack, whichever is easier for you.

Can an oscillation be triggered by mechanical vibration? Did you mean the output transformer?

Sat the chassis in the back of the other combo cab, managed to get the vibration there so it's definitely in the chassis, really sure its not tubes. Gonna remove the ss recto switch to tidy things up. Feel like it's around the pt/ recto/ speaker output end of the chassis.

[R.G.]

> have you tried a different set of tubes?

> do you have an oscilloscope?

[mike-p]

I have several options for pre amp tubes which I've been through. Only have the quad of 6p1p but I had 6p14p-k in before worth the same problem.

[mike-p]

> have you tried a different set of tubes?

> do you have an oscilloscope?

No scope sadly.