Allen accomplice making intermittent chirping and static

[Fred G.]

Hi All,

My allen accomplice (I built it in 1999) has started making intermittent various noises. It started as an unusual static, and I've been swapping out preamp tubes, now it's morphed into intermittent chirping and tea kettle whistles. Only happens when the amp is hot. I haven't swapped out the power tubes yet. I can sometimes alter/stop/induce the noises when slightly juggling the PI tube; I haven't swapped out the power tubes as yet. Ideas, comments?

Thanks in advance,

Fred G.

[Chuck H]

It may help to see a schematic. Sans that...

The filter caps are 15 years old. Maybe they are starting to wane in performance and coupling signal on the B+ rail (don't forget the bias supply filters). Or maybe the contacts are oxidized. The tube pin holes (may also need re tensioning) or jacks. If it's an aluminum chassis there's a possibility of faulty grounds due to oxide formation at the screw mounted chassis grounds. Maybe more than one of these things is happening at the same time. These are basic maintenance issues that can pop up on a 15 year old amp. I might start by cleaning contacts and re tensioning the pin holes and mechanically cleaning any oxide from chassis grounds and re doing them. If you still have a problem you should parallel a new filter capacitor across each old cap one at a time. If paralleling a good cap across any of the old ones stops the oscillation you should probably replace all the electrolytic caps in the amp.

[Leo_Gnardo]

What Chuck said ^^^ plus, who constructed the amp. If it was kit built, could be a bad solder connection, and even if "factory built", well I've found badly soldered, non-soldered connections, even backwards electrolytic caps in Fender & other brands, even venerated McIntosh (!?!), both tube & SS gear. Allen has an excellent reputation but anything can happen. Try Chuck's filter cap check, also poke around with your calibrated chopstick & look as closely as you can at every connection under good light, I'm sure you'll find what's wrong & get it sorted.

[lowell]

Do what they said....but you also havent tried new power tubes yet. This is potentially a microphonic or bad power tube. If it were me I'd try that first. Tubes are initial culprits in any tube diagnosis IMO.