Subbing 12AU7 for 12AX7

[Chevy]

Used to know how to do tube load lines and all that, but am getting older and slower !
I subbed in a nice old USA made 12AU7 for a 12AX7 in my Bassman just for experimentation, and checked the DC voltages at the tube, found that the plate resistor now had very roughly about double the voltage across it, maybe 200 volts if I remember correctly. (B+ for that preamp tube around 300V; usual plate R 100K, and cathode R 1.5K.) This put the plate R dissipation near .3-.4 watts.
My question is this... if I start fooling around with tube subbing a 12AU7 type in other amps, is there a risk of burning out the plate R ? I wasn't comfortable with my Bassman running the plate R at .4 W with a 1/2 W resistor in there, so I put a larger W value in there just to be safe, even if I don't keep the 12AU7 in there. But if I do this on other amps, who knows what wattage rating the plate R will be ? Maybe they would cheap out on modern amps and go with smaller values.... dunno.

[Jammin'John]

is there a risk of burning out the plate R ?

Yes.
au7's & at7's have to have a different bias than ax7's. they don't belong in the first stage.

JJ

[stokes]

As JJ says,you are running that plate resistor too close to design max.If its idling at .4 watts,when you put a signal into it,it will rise from there.I know there was an issue with the Bassman reissues that used 1/4 watt plate resistors.I actually have a RI Bassman on the bench right now I'm modding for a harp player and it has 1/2 watt,it was made in '90,so I guess they started cutting corners using 1/4 watts later sometime.I did a lot of experimenting with A_7 type subs and always found the AT7 and AU7 to be very dull and lifeless in the V1 position.The best tubes for lowering the gain a bit is the 5751 or 12AY7 in V1