Interesting Oscillation Experience

[Munga]

I put my Ceriatone chassis on top of my new 4x12, fired it up, and was pissed to find extremely extremely bad oscillation and weird frequencies. I tested about every preamp tube combination I could, with mixed results. So, I decided I needed to chopstick it. I put it the chassis on its side and fired it up one last time. NO OSCILLATION! I then placed it back down flat thinking that cured it. Oscillation again. Then I remembered reading on here to shield the cab to try and prevent this kind of thing. I put a sheet of tin foil under the chassis, and ALL of the oscillation is gone.

I wanted to post this experience for people that may be having similar problems. I wasn't aware that the tin foil would make that much of a difference.

[sliberty]

I had a similar experience with my JTM45. I used roof flashing material t which I bought at Home Depot (I forget who recommended this, but thanks) inside the bottom of the head cabine, and that took care of my problems.

[bnwitt]

This is very common with high gain amps. I've had to use aluminum shielding on several marshalls and trainwreck circuits. I recently had this same issue with a Marshall 36 watt head.

Shielding a head cabinet is just good practice anyway. If you've seen the Komet amp head cabinets, they have the shielding top, bottom. front and back. A nice little cocoon from extemporanious waves.

[jaysg]

I used roof flashing material t which I bought at Home Depot (I forget who recommended this, but thanks) inside the bottom of the head cabine, and that took care of my problems.

Richie recommended flashing to me. I had been using stainless steel sheet, which was a royal pain.

[guitarsnguns04]

I use a foilf faced tape that is used in the heating and a/c trades.

[Munga]

This is very common with high gain amps. I've had to use aluminum shielding on several marshalls and trainwreck circuits. I recently had this same issue with a Marshall 36 watt head.

Shielding a head cabinet is just good practice anyway. If you've seen the Komet amp head cabinets, they have the shielding top, bottom. front and back. A nice little cocoon from extemporanious waves.

Do you think shielding more than the bottom would have much of an effect? I though the chassis already acted as a (good enough) shield.

[bnwitt]

Do you think shielding more than the bottom would have much of an effect? I though the chassis already acted as a (good enough) shield.

Shielding just the bottom on a head will eliminate the squeal you described and is usually all I do. If your chassis is steel and not aluminum you might need to shield the other areas too. The best shielding material is non magnetic like copper, aluminum or brass. I've never used roof flashing as someone stated as it is usually made of galvanized steel. Of course it probably works fine in guitar amp applications as these are AF amplifiers we're talking about and the shield material is not as critical as it would be in an RF amp. If your squeal is gone (assuming there isn't any above or below your hearing range) you're done.

[muchxs]

There's some really nice copper foil available at craft shops who cater to stained glass craftspeople. It's available either with a gummed back or plain, I think I'd prefer the plain material. Glassworkers apply it to the edges of stained glass, there's no reason why it couldn't be applied to the inside of an amp cabinet.

The nice thing about copper foil is it's easy to solder, that's why it's used for stained glass work. You could also use a short length of de-soldering wick and a couple ring terminals to securely ground your shielding to a star ground.

[mumford]

I glued tin foil to a sheet of posterboard. Seriously. It worked fine, and cost less than a dollar.

[lastwinj]

have never needed it, even running an open bottom rack chassis on top of a 4x12, super high gain, too.

results may vary. go figure.