Proper chassis alignment in Princeton style cabinet

[jjang1993]

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[WRGKMC]

Its not an Alignment issue, its a contact issue. The screws need to pull the chassis firmly against the side of the cabinet to prevent oddball vibrations. Yes, maybe if the alignment is off enough, the screws may be pulling at an angle and failing to pull the chassis tight with the cab. Otherwise alignment is purely cosmetic.

 

I can say you do have rotten luck with your amps. What is this, the 10th or so post you've had with oddball vibrations from tubes, to speakers, to chassis rattle now.

 

My question would normally be, why did you take the head out in the first place if you cant put it back in properly, but I'm guessing since this is a custom build the wood of the cabinet was made of green wood and the vibration has developed as the heat from the tubes dry the wood out and warp it slightly.

 

The fix for it is simple. You need some single sided foam tape to put along the edge of the chassis frame that contacts the cabinet.

 

You can buy this foam tape in most hardware stores in gray and black too. You simply want the single sides stuff so it only sticks on one side, not both. You want the thin dense stuff not the pours spongy foam stuff typically used for weather stripping.

 

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I also have to make sure you understand this critical point too.

 

Many amp heads either have a piece of steel screen or thick aluminum foil tacked to the cabinet frame. When you screw the head down it provides a grounded cover to the open chassis. The head must make an electrical contact with it to act as a "Faraday Cage" to ground out hum before it gets inside the chassis and gets amplified.

 

If your amp doesn't have the screen then no big deal. Just use the foam and the vibration issue is solved.

 

If your amp "does" have the aluminum foil or a screen - the foam tape is going to act as an electrical insulator, preventing the foil/screen from being electrically grounded. In order to ground the chassis you have to bridge the chassis to the screen/foil. This can be done in several ways. Normally I'd just take a couple of strips of copper foil maybe 1" x 1". Stick it to the foam tape so it has an ear sticking out. Then after you stick the tape to the chassis you simply fold the ear sticking out over the top of the tape.

 

This will provide the electrical contact between chassis and the cabinet screen.

You could also use a piece of spring steel between the screen and chassis or an actual ground wire.

 

Last combo I built was a 4X10 open backed cab like a Bassman Tweed. I mounted a solid state Fender head inside and it rattled badly. Didn't matter how tight the screws were. I only had a single layer of foil at first so the surface was quite hard and neither the head or the wood was ideally level.

 

To absorb the vibration I needed to get the entire frame tight against the foil and have it give where needed. I left the foil in place and stuck the foam strips along the foil where the chassis edge would be then I put a second layer of foil over the foam, sandwiching it in between the foil.

 

Then I simply tightened the head down into place. The foam under the foil had a firm pushback on all edges of the chassis and absorbed all the vibrations. Never has a problem after that.

 

That head was designed for a separate head cab so it was never designed to see the vibrations a combo produces.

A little creative energy based on experience and the problem was quickly solved.

 

[WRGKMC]

No the width really isn't that important. The thickness should be around 1mm, maybe 2. It will compress as you wang the screws down, but that's the main idea. The dots of felt didn't work because the chassis between the dots were free to resonate from the speaker. If you dampen the entire side any vibrations between the edges of the chassis and the cabinet will have that foam to dampen it.

 

If need be do all 4 sides of the chassis but I'd leave a couple of air gaps so heat can escape and extend the life of components inside. The components are of course rated for high temps in tube amps so there's no short term issues if the sides are completely sealed but a little extra air flow may make the difference between the caps inside lasting 30 years vs 50 years before they need to be replaced, especially in amps where the tubes are under the chassis where the heat rises up to heat the chassis like a frying pan.

 

Since the amp doesn't have a foil shield I wouldn't worry about it. So long as there aren't any abnormally high hum levels, or radio stations being amplified there's no problem just using the foam. Just avoid setting any electrical devices on top of the amp like wall warts or cell phones that might emit magnetic waves which could be amplified by the circuits.

 

Tube amps in general use high voltages so the waves need to be quite strong to cause any problems. Allot of it involves the circuit layouts too. On solid state amps good shielding is often important because the circuits run on low voltages and weak magnetic waves can cause bigger problems.