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Re: Chassis ping


 :
6/2/2000 10:19 PM
Ric Re: Chassis ping
Tubes,  
 
12ax7 EI's and Mullard  
12at7 Mullard  
KT-66 Valve Art and Genelex  
 
I'm looking for some grommets to do just what you've suggested.
 
6/3/2000 6:30 AM
Bruce

To do that up right you need four grommets for each tube socket.  
Two go between the socket and chassis and the other two go under the heads of the screws.  
Don't over tighten the screws or nuts and it doesn't hurt to use some Locktite in order to keep the loosely tightned nuts from falling off.  
Bruce
 
6/2/2000 4:13 AM
Sweetfinger

Yeah, I've noticed that caps can sometimes be surprisingly microphonic. Also, all sorts of things can transmit vibration to preamp and power tubes. In a high gain amp it may get to the point where you NEED some kind of damping system. There's more than one reason Dumbles are full of goo!
 
6/2/2000 7:50 PM
Gil Ayan

quote:
"Yeah, I've noticed that caps can sometimes be surprisingly microphonic. Also, all sorts of things can transmit vibration to preamp and power tubes. In a high gain amp it may get to the point where you NEED some kind of damping system. There's more than one reason Dumbles are full of goo!"
 
 
The only other place where you can get some mechanical noise induced would be the wires themselves. Using tightly shielded wire works great, but if you use something like microphone cable, you may find that it is very noisy. Tap that and you will get a lot of noise.  
 
Regarding the Dumbles, since those amps are actually pretty low gain in comparison to something like a Boogie (with at least 2 times the number of stages, or a Soldano.... eith 2.5 times the number of stages), I think the goop is hardly there to prevent anything from shaking. The amp's are simple and their layout a lot more straight forward than any Fender or Boogie I have ever seen.  
 
Gil
 
6/2/2000 10:36 PM
Ric
I've tried swappin' out all the tubes, and have subed out most of the caps with no improvment.  
 
It's a puzzle!
 
6/3/2000 5:46 PM
Carlo

Those EI tubes are known for their good tone as well as their microphonics. I'd say more than 50% of the EI 12AX7's I've purchased have been microphonic. The First stage will be most critical. Like RG said, gently tap the tube (with a pencil, sharpie, fingernail...) and listen. The culprit will be the loudest.  
 
Carlo
 
6/4/2000 11:14 PM
Ric What goes on here, or "ping" con't...
OK, in the last installment I had a microphonic problem that manifested itself when I tapped on the front of the chassis. I changed ALL the tubes to known good ones, changed all the caps one by one and still no improvement. The amp section is your basic Fender/ Marshall set up with KT-66s but I deleted the NFB loop. So to reduce the gain and hopefully solve my microphonic problem I added a 100k series resistor to the 16 ohm output and a 5.6K shunt resistor, I didn't have a pot handy, w/a .1 cap across it . Problem solved right? Not by a long shot! It now feeds back uncontrolably. What's up with that? Any suggestions?
 

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