Fender Princeton making strange purring sound

[adathead 53]

I recently purchased a 1970 Fender Princeton. The 10" speaker has been replaced with a 12" Eminence Patriot. Everything else is original. It sounded great in the store and I got it for a great price. I bought this for recording purposes as my Hot Rod Deluxe was just too much power for my small room. Anyway, the amp is making a strange sound very much like a cat purring, even when nothing is plugged in to it. It's plugged in to a good power conditioner, so I've eliminated that possibility. A friend suggested I leave it turned on for several hours, as it hadn't been used in some time. Didn't really help much though. This purring sound makes it impossible to close mike for recording. I'm hoping to avoid an expensive trip to my local amp tech. Any suggestions on what might be the culprit? Thanks for any and all help.

Larry

[ktf]

Filter caps? Is it a 60 cycle purr?

[adathead 53]

Definitely not a 60 cycle hum. Not really a hum or buzz. The best I know to describe it is a sort of purring sound. I wish I was set up to do a sample and download it. I'd mike it and post it on here, cause it's kind of hard to describe. Thanks for the input though.

L.P.

[Wavebourn]

Like motorboating?

[JTR]

Cat sleeping in the back?



Failing that, check for tubes on their way out.

[corworld]

Sounds like a parasitic oscillation, more commonly known as motorboating. Check the tubes first by swapping in known good ones, then find out if any of the front panel controls change the sound. It would be a volume or treble pot that would do this. If the tubes are good and you hear a change in the sound when turning a knob, then you need to check all the caps starting with the filter caps. Sometimes a bad ground on a cap can cause this.

[adathead 53]

Motorboating-what a great term. That's definitely it (why didn't I think of the word oscillating?). I'll check out all the tubes and if that's not it I'll have my local tech check out the caps, cause I'm not really qualified to do that. Thanks so much for the responses. I knew you guys would come through, as always.

Larry

[JTR]

Motorboating is also used to describe the noise from a condenser mic; often when humidity content of air is very high.

[Wavebourn]

As people already said, there may be most probably either an odd contact between a ground wire and a chassis, or lost contact between an electrolytic filtering cap and ground.