Vintage restoration- very odd problem!

[Shorty]

Hey there, first off let me apologise for making my first post a cry for help (you know how it is).

I've been restoring an old philips hifi amp and converting it for guitar use. So far so good, and it sounds amazing 99% of the time -BUT-!
When I play a C# (E string fret 9 or A string fret 4) I get an odd sort of fuzzy distortion- sounds a little bit like an amp that is biased cold, like a gated fuzz effect. But only on this one note!

My first thought was that something in the amp is being physically moved by vibration at this frequency. However, the control settings on the amp do not effect this problem- if I pluck the note quietly it doesn't really happen, even at loud volume, whereas if I strum very hard at quiet volume it will cut out and fuzz straight away.

So there's the problem. Now the clues to the mystery. Guitar is a Maton hollowbody electric, humbuckers, going straight in. The amp uses a 12ax7 and 2 6bm8's, I have restored it based on the Univox U-45b, with tremelo section removed.

Schematic here: http://music-electronics-forum.com/a...4-u45_orig.jpg

Voltages coming soon, I kinda lost the paper I wrote them on the first time.

Thanks for any ideas! (even if they are outlandish)
Shorty

[RWood]

My first thought is inadequate power supply filtering, and looking at the schematic I see only 10uF for the reservoir cap, then another 10uF for the screens. I would replace the first 10uF with a 33uF or at least 22uF. Then 22uF for the screens.

At least that is a quick thing to try to see if eliminates what is probably your guitar note's frequency interacting with unfiltered AC mains frequency.

Rwood

[hasserl]

My first thought was that particular note is a resonant frequency of the guitar that is causing an oscillation. It may be an electrical problem with the amp, but I'd rule out the guitar first.

[Enzo]

My first thought is to isolate the problem. Is it guitar, amp, or speaker? Try a different guitar, any change? Possible resonance or pickup adjustment issue. Connect a different speaker to the amp, any change? Possible voice coil rubbing or even cone damage or tear.

[RWood]

Doesn't look like the OP gave a second thought to our suggestions!