I thought you might find tis interesting. My Juicy Music Peach preamplifier always made a high pitched "ting" sound when switching inputs or switching from stereo to mono.
I just changed the main 6H30 tube from the stock EH to a NOS Russian military 6H30DR. An expensive gamble, but it really paid off. I really love the preamp with the DR. I highly doubt I'll ever sell it.
In any event, another member was selling a pair of the 12AX7 Tube Dampers, and I figured "What the hell." I was originally going to try them on my power amp, but decided to see if one would fit the 6H30DR, which it did. The "ting" which Mark Deeneen says is from the Peach being a zero negative feedback design, isn't entirely gone, but is greatly reduced. The amplitude of the "ting" did not change when I swapped tubes.
Has the sound improved with Herbie's Tube damper? I'm not sure because the 6H30DR is still breaking in, but I am certainly having fun. I can remove the damper in a few weeks and report back if anyone is interested.
The other damper is on the NOS LM Ericsson 2C51 in my MDDT Labs Havana DAC.
In any event, Herbie's Tube Dampers work!