While I know this not new , I just learned about the 1612 Pentagrid Amplifier. The pentagrid amplifer is a type of radio vacuum tube with five grids and was often used as the frequency mixer stage of a superheterodyne radio receivers.

The Data Sheet is here:
https://rtellason.com/tubedata/1612.PDF

It is is unique in the sense that it has 4 grids. Most penotde tubes I have used have This allows for a some interesting options for compression and limiting. The circuit below highlights how to connect the Grid 1 (Top Cap) to a rectified (through the 6H6) signal. This allows the rectified input signal to control the flow of electrons from the cathode to anode(plate). The larger the input signal, the more voltage on the Grid 1 pin. This increased voltage on grid 1 chokes off the flow to the load more or less proportionally.

This concept was used in the Gates 39B and 39A, although it was implement using Grid 3 and the input was on Grid 1.

These tubes are still out there but the 1612 was a bit hard to source. I believe I will be experimenting with this concept so more...

I learned the PFL trick on aux busses to keep Soundtoys PrimalTap freeze going and going, really really cool.

A few years back I learnt something that I hadn't realised.

When sine-wave testing a new pre-amp, I did a quick re-patch of my panel, but omitted to unplug the output of my 1176 compressor from the panel. The rack that held the 1176 was unpowered, so I was open-mouthed when the 1176 meter started showing the test pre-amp's output level in dBu. I quickly dug out the 1176 schematics and found that, in the signal level display positions, the moving-coil meter is wired across the secondary of the output transformer. Even with no power applied, it was responding to my test signal coming into it via the output connector.

What I learnt was how to make (an expensive) VU meter.

Here is the video I talked about ! ;)

Hi, I'm an engineer from India. An interesting thing I've picked up from a recent session is that many ethnic string instruments naturally produce a reverb like effect due to their sympathetic strings.
It's something I picked up while recording an Indian instrument called Sarangi which has 3 main strings that are played with a horse hair bow and 35-37 sympathetic strings. When the main strings are played, the sympathetic strings also vibrate giving off a natural reverb like effect which is a beautiful sound.

Interesting, maybe like the snare drum snares vibrating from a bass guitar...

Most multi string acoustic instruments will do this.. For exemple if you pick the first string of an acoustic guitar (the high E) and stop it right away, you'll hear sympatic notes right away.
Some will say that the more it's doing it more high quality the instrument is.
Of course it goes the same way with different instrument influencing each other like bass and snare. Actually snares are bound to reac to most vibration in the room.
Here's a funny video I found about sympathetic vibrations :

Wow that was a really neat video to explain this concept. Yes sympathetic vibrations are prevalent in all string instruments. It's a good thought to use it creatively in a session instead of drowning them in reverb afterwards. For this, a talk with the artist goes a long way I've found.