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I am working on an Fender Hot Rod Deville 212 and am running into
something weird. If I put my hand close to the 6l6's it makes a high
pich squealing that seems to change in volume and pitch as I move my
hand. Anyone ever run into something similar? Any suggestions for
troubleshooting?

[Chris Hornbeck]

On Thu, 14 Jun 2007 16:48:25 -0700, wrote:

I am working on an Fender Hot Rod Deville 212 and am running into
something weird. If I put my hand close to the 6l6's it makes a high
pich squealing that seems to change in volume and pitch as I move my
hand. Anyone ever run into something similar? Any suggestions for
troubleshooting?

Assuming that your description is complete, that is, that
there is no obvious acoustic feedback path, then a good
first guess might be that you're seeing parasitic oscillation
being interrupted at an audible rate (by bias change, etc).
This even has a name, "squeeging".

The solution is to stop the parasitic oscillation. Does this
amp have grid stops? An output Zobel network? How are the
power supply bypass caps? The usual stuff.

All good fortune,

Chris Hornbeck

[Phil Allison]

I am working on an Fender Hot Rod Deville 212 and am running into
something weird. If I put my hand close to the 6l6's it makes a high
pich squealing that seems to change in volume and pitch as I move my
hand. Anyone ever run into something similar? Any suggestions for
troubleshooting?


** Nothing is wrong.

The back cover has an aluminium foil shield stuck all over it which needs
to be earthed and is when fitted.

Keep your hand away from the 6L6s tubes when playing a guitar - cos that
creates electrical feedback and make the whole amp oscillate at a high audio
frequency.



....... Phil