What would be the advantages of using half of a 12AU7 as the triode for a cathodyne phase inverter as opposed to a 12AX7 (disregarding the fact the other triode would probably be the preceding gain stage with a lot less gain)?
I see on the data sheet for this tube that it lists a grid resistance of 1Mohm (with cathode bias) and that it does not list a maximum positive or negative grid voltage. The 12AX7 on the other hand lists max neg and pos Vg and nothing about grid resistance. This is leading me to believe the 12AU7 would be a better design for the Cathodyne PI function as opposed to a 12AX7. I have an amp with a cathodyne pi and have already added a 1meg grid stopper, but if I was using a 12AU7 type triode in this spot it seems as this would not be needed. The theory of adding the grid stopper on the 12AX7 was to try to limit grid current if the grid voltage swing tries to go positive.
Strangely enough if a 12AU7 type triode is the better triode for a Cathodyne PI, there is the 12DW7 which is half 12AX7 and half 12AU7, but every cathodyne PI amp circuit I've seen (including mine!) has the wrong sections wired for the 12DW7! In my case its a PCB, if it was HW I could try it out easily, just getting a 1M grid stopper in there required some creativity. I have seen that JJ makes a non standard tube the ECC823 which is actually a reverse wired 12DW7, so I could grab one of those to check out if there is further reason to do so. (wish I could say I like JJ tubes, that's another issue tho).
The 12Au7 triode has other differences (from 12AX7) so again I'm looking to see if there are other advantages in this particular application.
I see on the data sheet for this tube that it lists a grid resistance of 1Mohm (with cathode bias) and that it does not list a maximum positive or negative grid voltage. The 12AX7 on the other hand lists max neg and pos Vg and nothing about grid resistance. This is leading me to believe the 12AU7 would be a better design for the Cathodyne PI function as opposed to a 12AX7. I have an amp with a cathodyne pi and have already added a 1meg grid stopper, but if I was using a 12AU7 type triode in this spot it seems as this would not be needed. The theory of adding the grid stopper on the 12AX7 was to try to limit grid current if the grid voltage swing tries to go positive.
Strangely enough if a 12AU7 type triode is the better triode for a Cathodyne PI, there is the 12DW7 which is half 12AX7 and half 12AU7, but every cathodyne PI amp circuit I've seen (including mine!) has the wrong sections wired for the 12DW7! In my case its a PCB, if it was HW I could try it out easily, just getting a 1M grid stopper in there required some creativity. I have seen that JJ makes a non standard tube the ECC823 which is actually a reverse wired 12DW7, so I could grab one of those to check out if there is further reason to do so. (wish I could say I like JJ tubes, that's another issue tho).
The 12Au7 triode has other differences (from 12AX7) so again I'm looking to see if there are other advantages in this particular application.
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