[Johnthurston]

I built a Bijou amp a few years ago, and find it performs well for me and provides a satisfying sound. It sits on my desk, and provides me my sonic distractions during my workday.

Today, I noticed a high-pitched squeal. It wasn't in the audio output. It was in the room. After hunting around, I was able to locate it at the EZ81 rectifier tube on the power supply. I touched the tip of the tube and the sound stopped. But started again a few minutes later. I powered down, plucked the tube, and examined it visually. I could find nothing by eye indicating any mechanical defects inside the tube. I reinstalled the tube, and have had no more occurrences.

The sound I was hearing was very high frequency. A bit like the flyback squeal from an 'old' CRT. What was the frequency? I don't know, but it was up there. I had the Bijou on the bench a couple of months ago to replace a failing input connector. At that time, I measured the power supply output and confirmed it was still at its specified voltage.

If this squeal returns, where should I be looking? Is this an indication my rectifier is on its way out?

 

[HiGHFLYiN9]

I've experienced the same thing. Some tubes just get noisy over time and have to be replaced. You can try damping them with O-rings and/or shields, but it only slightly masks the problem.

If you want to do measurements, this thread has some info: https://www.diyaudio.com/forums/tubes-valves/118110-sound-failing-tube-rectifier.html