Output transformer ringing

[Esquirefreak]

So I just learned about this phenomenon.

I built an amp based on the normal channel of a 6G6-B and initially thought that I had some severe oscillation going on. I had yet to test the amp with a guitar and just put a cable in to make sure I had signal passing through (tapping the cable tip with my finger). Even with a 100pF cap between the PI plates and grid stoppers on the power tubes, the issue persisted. I then noted that grabbing and clamping the OT with my hand made the noise go away completely. So I'm gonna try to tighten the bells/laminates. The amp sounds really good with a guitar, but I had never encountered this problem. I might put some o-rings between the mounting brackets and chassis to reduce mechanical vibration being fed into the preamp tubes.

The more you build, the more you learn, obivously.

This happen to any of you guys?

/Max