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Chassis ping

6/1/2000 9:55 PM
Ric Chassis ping
I recently finished a project that yielded an odd problem. When you tap on the front panel of the chassis you hear a sound like two glasses clanking together. Like tapping directly on a pre-amp tube. It seems like a mechanical coupling problem of some kind. Anybody ever run across this?
6/1/2000 11:27 PM
All the time. Microphonic tubes (likely) or capacitors(less common) are picking up the vibration from the chassis and the movement is being converted to (unwanted) signal.  
Tap individual tubes with a pencil to find the one(s) that are most susceptible and see if replacing them does any good. Some tubes are worse than others.  
If replacements don't help, you may have to mechanically damp either the chassis, the tube sockets, or both.
6/2/2000 2:30 AM
R.G.,I swapped all the tubes but no difference.  
Regarding the caps, would you look at the filter or signal caps next? I've thought about trying to damp the sockets but how?
6/2/2000 2:39 AM
Peter S

signal caps and cathode bypass caps can sometimes be very microphonic......I just got finished tracking down a microphonic component in my own amp about 5 minutes of my homebuilt jobs....It's not even a high gain amp, just based on the '59 bassman/Marshall thing. It was the cathode bypass cap on the muddy channels preamp. I had used a monolithic cap there because I wanted a 2.2uf and the Monolithic cap was small. It was very microphonic so I swapped in a 2.2uf electrolytic. No more Ping......good word for that sound too by the way. Happy microphonics hunting.  
Peter S
6/2/2000 4:18 AM

If you used any ceramic caps tap them and see if they are a problem. I just fixed a '71 bassman with two ceramics that would sing.  
6/2/2000 7:38 PM
Dave James

Tube sockets can be damped by inserting a rubber gromment (over the socket mounting screw) in between the chassis and socket. This means a little longer screw is required and possibly some lengthening of leads if your sockect wiring is tight.  
P.S. what type of tubes are you using? Some are more prone to microphonics than others.
6/2/2000 7:46 PM
Gil Ayan

One last thing you can try: use some shrink tubing on the preamp tubes themselves. Yes, this will help to isolate a bit of the mechanical vibrations and may make the difference between the tube being microphonic or not.  

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