Eliminating a gain stage

[dude]

Can one just by-pass half of a 12ax7, go directly to the next 12ax7's grid?

Can half a 12ax7 tube have no connections?

I think I read somewhere that something has to going to the grid even if that half is not being used. 

al

[thermion]

q1. make sure there is an appropriate value, non-leaking coupling cap between the gain stages and you should be good.
q2. check out v2 of the bassman aa165 schematic...an unused 7025 section sits there with no connection (well, the heater is connected).
happy hunting.

[jjasilli]

Can one just by-pass half of a 12ax7, go directly to the next 12ax7's grid?  Yes assuming there's a coupling cap to block DC from the plate of the driving stage -to- the grid of the following stage; and a grid leak resistor for the following stage.  (You may also need a voltage divider to prevent the following stage form being swamped with excessive signal voltage.  So it kind of depends upon what  you consider the "directly connected". But note that if a live tube's input is merely left unconnected, it may generate noise -- that's why input jack's are usually grounded.  

Can half a 12ax7 tube have no connections?  Yes.  E.g.:  Traynor Bassmate YBA-2B, 6V6 version.  If the "bypassed" stage is left inoperative, then there's no issue with noise generation.

[phsyconoodler]

Take a look at a 2204/JCM800. It bypasses one gain stage for the lo-gain input.It simply means that as long as you have a grid to ground reference on the first stage it won't run away by inputting into the next stage.
  Is that what you had in mind?

[dude]

Too much gain in a 15/18 watt crate VC20, three gain stages, just wanted to take out the middle stage running the first stage after a .022 cap and grid leak to the third stages input. And let the second stage hang there with no grid connection in, and no plate out.

thanks,

al

[HotBluePlates]

Can half a 12ax7 tube have no connections?

I think I read somewhere that something has to going to the grid even if that half is not being used. 

Look at the AB165 Bassman schematic. One triode of a 12AX7 is not used, and simply has no connections made to that triode's plate, grid or cathode.

If you were talking about switching on/off a triode (as for an extra gain stage), and there will continue to be voltage applied to the plate, then you'd want to connect the grid to ground through a resistor to be sure plate current remained at an idle value.

But if you just want to stop using a triode, just disconnect the plate, grid and cathode.

[dude]

Got it.

Thanks,
al