need for decoupling capacitor in a Fender Bassman
The anode of triode 1 and the grid of triode 2 are connected externally between pins 1 and 7 but breaking this connection with a capacitor would mean that you need to add bias back to the grid of triode 2. You can't AC bias a grid since the grid leakage current would eventually turn off the tube. – ScienceGeyser