I've been running 6AV6 tubes for about a month and can vouch for their qualities, especially in the treble department but not only (realism, soundstage, and bass for at least on pair I own). Treble will most likely get "nicer" after 10-20 hours, like on the "hotter" ones I tested though, so I wouldn't worry.
OK, it's been bugging me to not know why 6BE6 and 6BY6 heptode -or more precisely pentagrid converters- work better with pin 7 -grid 3- left unplugged or open; not that I've tested it myself yet, but it's the theory behind this that bugs me.
Those pentagrid converters have, well, five grids, grid 5 is very simply the same as grid 3 in a pentode, it is just there to catch stray electrons -and send them back to the cathode, to which it is internally connected in this case- so that's easy.
Grid 1 is the oscillator grid, and you've all been using it like the typical signal grid in both pentodes (grid 1) and triode (grid.), and it happens to be made to receive one signal anyway, so that makes sense. Grid 3 is the mixer grid and is apparently made to also receive a signal. Grids 2 and 4, internally connected, act both as the screen grid in a pentode (grid 2) and anode for the oscillator grid (grid 1 in a pentagrid converter/heptode).
I assume the usual purpose of these tubes is to amplify a signal on grid 1, received and "screened" by grid 2 (and 4), "mixed" to the signal on grid 3, and eventually caught by the anode to be useful to some other electronic section. Not that any of this matters for our purposes.
Now, it appears that these tubes don't give their best when you tie grids 2+3+4 to the anode, and yet they seem to work the same way as triode-strapped pentodes when you just ignore grid 3 and strap grid 2+4 to the anode -grid 5 being hardwired to the cathode regardless- an use grid 1 to send the audio signal.
Thinking from different sources I've just read, things I'd read about the use of 6AU6 pentodes in microphones -more on that below- and the using the coincidencial discovery above, I'm pretty sure there are alternative ways to plug these tubes, that could work just as well and perhaps better -which would be the obvious goal here.
Apparently, people have been using 6AU6 tubes triode-strapped in funny ways in for microphone buffer/amplification purposes for years. In order to get the lowest noise possible -pretty important when you're recording, say, a voice- they often grounded the anode, and used the grid 2 as the de facto anode; I assume grid 3 was still strapped to the cathode.
On these pentagrid converters, we have a total of five grids -but only three pins- and one anode to play with. So, here are a few ideas that I'd like to test or at least brainstorm, most of which have little reason to explode lol:
- Using grid 2 and 4 as the anode, with both grid 3 and anode left "open" (in theory they should be grounded in this case scenario). So, chopping pin 5 (anode) and running the tube in the EF95 setting basically. Should produce sound, may be good or bad, should not explode.
- Using both grid 1 and grid 3 as for the audio signal. I have no idea is this can work properly since you have grid 2 sandwhiched between them. Basically, it would imply wire-modding pins 1 and 7 together and running the tube on the EF95 setting.
- Using grid 3 instead of grid 1 for the audio signal. Again, may be difficult since grid 2 would be in the way between the cathode and the audio signal. Would mean chopping pin 1 and wire-modding pin 7 to socket hole 1, which in and of itself is complicated. EF95 setting.
- Using grids 2+3+4 as the audio signal grid and leaving grid 1 disconnected. Same issue as above, no idea if it would work well, and same pin mod but EF92 setting.
- Using grids 1 and 3 as the audio signal grid and using grids 2 and 4 as the de facto anode, with official anode disconnected. Again, maybe it would work, maybe not, hard to say. Would mean chopping pin 5 off, and wire-modding pins 1 and 7 together.
Anyway, I need to investigate this some more, but you get the idea, there may be more possibilities to plug heptodes -and I'm starting to understand why they're not meant to have grid 3 connected to grids 2 and 4 and not running a signal (the tube is meant to run two signals).
Some resources, half of which is beyond me:
https://en.wikipedia.org/wiki/Pentagrid_converter
http://www.diyaudio.com/forums/tubes-valves/163842-possible-use-pentagrid-tubes.html
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