Grey smoke from a KEL84 - Help!

[bikerhifinut]

Personally I don't think playing around with the cathode resistor is of any benefit. The KEL84 is not, of itself, an inherently unreliable design. The OP just got unlucky I think with a set of dodgy valves. He never said if they were new or "Previously enjoyed".
If it was my amplifier I'd be satisfied with a replacement of the 270R cathode resistors, perhaps uprated to 2 or 3W, and their associated bypass capacitors x4 as they literally cost pennies for good quality non "boutique" components. Along with the 100R screen grid resistors.
Plug in a set of known good EL84 and measure the voltages on anode, screen grid and cathode to check all in tolerance.
and enjoy.

Now on the Stereo20, which is only similar to a KEL84 in that it uses the classic 270R cathode resistor on its output stage along with 50uF bypass and theres a reason for that, there were substantial mods made to the St20 namely a reduction in HT volts,grid leak reduced to 470k, and a reworked output transformer the 8778 which had the UL tap moved from 50% to 25%, all of which made it an easier life for the EL84.
Owners of the older Stereo20 that didnt get modified with expensive mains and output transformers can just use the modified circuit values and that usually suffices along with a coupling cap mod on the LTP phase splitter and also reduce the first stage anode R to 47k from 100k. Some also had a frequency correction network on the first stage anode but this was by no means universal.

Andy