Unused 1/2 -12ax7

[studioit]

If you are only using 1/2 of a dual triode- 12ax7 in this case- what needs to be done to the other half? can it just sit there as long as there are n. c. to the second unused plate, grid and cathode? I wired the heater out of habit by tying pins 4 and 5 together. Just occurred to me before I fire up this circuit.

Thanks,
Ian

[dai h.]

from what I recall,

a) only connect the side of the heater you are using (so from htr.cnt. tap 9 to either 4 or 5)

b) tie the unused grid, cathode, and plate together with a wire but not connected to anything else (can't remember the exact technical reason but there was some reasoning that this was correct)

[studioit]

Right, I figured I would disconnect the other heater and I found another thread elsewhere that suggests tying all other unused elements to ground so I guess that's what I'll do.
Thanks

[Enzo]

I've seen any number of amps with the unused section just sitting there - three socket lugs with nothing connected. If Fender gets away with it, so can you.

[Chuck H]

How about the most coveted of boutique amps!?! the TrainWreck express. I can't comment directly on the usefullness of the circuit, but they do seem to have any noise and stability well under control. And there is NOTHING connected to the unused pins of V2a on that amp.

Chuck

[dai h.]

grounding the unused grid, plate, and cathode would seem to be better since they are metal (generally not good to have floating, or not connected to anything metal masses). Tried to look up the post by kg where he made some remark on this in the archives but couldn't find it. Tying the unused pins together and not connected to anything is something I saw in Bruce Egnater's Marshall mod in an old issue of GP mag.

[drewl]

Why not parallel a stage?
Especially the first stage, adds a nice bit of gain and increased touch response.

[Chuck H]

Second that...Or...

Set up a cathodyne PI (with the unused triode) instead of a LTP and then feed a pair of virtual earth mixers (made from the triodes that used to be the PI) You can adjust the return feedback on the virtual earth mixers for the level of gain you want and most likely still get a lower impedance output for pummeling the power tube grids without fear of crossover or blocking distortion caused by grid conduction. New idea of mine, It works, but I havent had time to fine tune it yet.

Chuck