grid stopper resistors on power tubes

[Mooreamps]

My calculations were based for the EL-84. Maybe it's not clear to me the total grid to cathode capacitance, thus using the value that was suggested.
-g

[Merlinb]

Good treatment of this subject is given in Radio Engineering, Terman, 1937 p231-239.
In brief, oscillation can result from the combination of stray inductance in the grid circuit, and stray capacitance across the output transformer, plus interelectrode capacitance. In other words, there are stray resonances in both grid and anode, and if they coincide the sh*t hits the fan.

p238:
"If trouble is encountered from this residual grid-plate capacity... place a resistance of a few thousand ohms in series with the grid.... such a resistance introduces proportionately more loss at the higher frequencies...and thereby tends to counteract the lower input admittance as the frequency is increased."

[zaphod_phil]

The purpose of the grid stoppers is also to help prevent blocking distortion.