Originally Posted by Audiofanboy
Meanwhile, I think I've made sense of this whole jumper business, pin out, and why some tubes are compatible or not in the LD MK III/IV, after reading through a number of datasheets (that I barely understand), other google sources, the pics of the LD amp PCBs on the LD website, and this document on google, that finally helped me makes sense of the whole thing.
http://www.kaponk.com/~yanyong/ETF06TS.pdf (pages 11 onwards)
The amp uses pentodes as driver tubes, strapped as triodes, meaning that instead of having a cathode, grid (1), grid 2 (screen), grid 3 (suppressor) and anode, it straps grids 2 and 3 to either cathode or anode, leaving only cathode+anode+grid = triode.
On the 6AK5 tube, this is pretty easy, as the grid 3 is internally connected to the cathode (pin 2 = k+g3 is connected to pin 7 = k+g3). On the amp PCB the grid 2 is connected to the anode (pin 5 = a is connected to pin 6 = g2), thereby forming a perfect triode strapped pentode we've learned to love, with a by the book execution (g3 connected to k, g2 to a).
On the EF92/91, things get more complicated as pins 6 and 7 are inverted (g2 and g3 are inverted), and no pins are hardwired together internally (k is not connected to g3). But on the LD circuit board, pins 5 (still anode) and 6 (now g3) are still connected, leaving g2 connected to nothing. This is where the jumper comes in handy. The jumper apparently connects pins 6 and 7 (g3 and g2) to pin 5 (anode), thereby creating another alternative triode strapped pentode by the book example with both extra grids connected to the anode.
Are you following me so far? I barely am. So, this is what the jumper does to make the amp work with the usual types of tubes. Enter, 6CB6 tubes, and most of the other tubes I want to try for that matter, pins 1 to 5 are still the same, but pin 6 is now g2 again and pin 7 is g3 (like for the 6AK5), but nothing is hardwired internally (like the EF92/91), so it won't be triode-strapped with the EF95 jumper setting (and sound all flabby like when you tried your EF92 with the wrong jumpers). With the EF92 jumper setting, again pin 5 (a), 6 (g2) and 7 (g3) are connected, and you should get a normal triode strapped pentode, which is the only reason it works in the amp in the first place. The alternative solution to use these tubes with the 6AK5 setting, would be to hand-wire pin 2 (k) and pin 7 (g3), perhaps with a small flat wire soldered between the pins, which might offer better or similar triode performance like a real 6AK5, different noise rejection and such (and connect the internal shield to the cathode which should be better theoretically).
This explains, why EF92 tubes with the 6AK5 setting don't explode (they just don't work well) and why 6AK5 tubes on the EF92 setting light up and try to kill themselves (anode and cathode become connected through internal and PCB wiring, which can't be good).
One last thing, I've seen a few tubes with a 7 pin base and the right filament voltage, where the cathode (k) is wired to pin 7 and g3 to pin 2 (pins 2 and 7 are inverted basically), and not hardwired together. The 6AU6 tube (0.3A, sharp-cutoff pentode), which a lot of people like to use in DIY circuits, is like this. With a simple flat wire soldered between pins 2 and 7, and using the 6AK5 jumper setting (EF92 setting would make it explode again), these types of tubes could perfectly well work in the LD amps.
If you have no idea what I'm talking about or haven't understood any of this, either ignore my post, or pull up a few tube datasheets from google and look at the pin layout for the tube, it might make much more sense then.
EDIT: 6AG5 tubes are wired the same way as 6AK5s. That is, pin 2 (k+g3) is hardwired internally to pin 7 (k+g3), so it should work fine with the 6AK5 jumper setting as a triode-strapped pentode, but will surely explode if used with the wrong EF92 jumper setting (cathode and anode connected = bad).
EDIT 2: The 6J38P tubes also have their cathodes connected to grid 3 (pin 2 connected to pin7). I should therefore be used with the 6AK5 setting, unless you want to see some pretty fireworks...