The only time I have seen Mullard EL34's redplating is in origional BIG FAT FACE amps running 800 volt plates and 400 volt screens.
With regard to anti oscillation caps, ALWAYS use the best ceramics you can buy. NPO being recommended!! They are all of sixpence a dozen!! I kid you not.
The phenomena Dangerman describes is due to "shadows" cast by screen grid wires, and is 100% due to lazy/poor/bad construction. The "blocks" being caused by free electron flow where the screen doesnt "sheild" the plate.
Its kinda hard with self bias/auto bias / cathode bias, but a really excellent way to stop EL34/6CA7 from red plating, is to tie the suppressor grid direct to hard negative bias voltage. By this I mean,
when fixed bias is used, there is a negative supply available, usually "about" minus 50 or 60 volts, Without ANY resistance in circuit, the suppressor grid/s are connected directly to this negative supply!!!.
IF the valve is correctly biassed on the control grid, and the valve is overdriven, it can NEVER dissipate MORE than its maximum plate dissipation.
IF the bias voltage is lost, i.e. total failure of bias supply, the valve will be toast.
I have changed them out of Marshall amps, where the vacuum has "sucked" the glass envelope onto the plate!!!!!! when the glass melted !!!!!
Another useless piece of information for you
Joe