EL84 red plating

[musicrepairs]

I have a question for the tube experts.
I repaired a tube amp running a pair of EL84 in class AB fixed bias.
I have found in the past loss of control voltage to the control grid
will cause the tubes to Red plate due to high plate currents.
In this case I had a break in the screen wiring Pin 9.
Why did the tube draw more current?

 

[DF96]

Pin 7 is the anode. If this connection breaks then all the cathode current goes to g2 and it gets very hot. Maybe you saw this and thought it was the anode getting hot?

 

[TonyTecson]

fixed bias required lower grid leak resistor,
if that resistor has drifted in value to a higher one, red plating will be the result...

 

[musicrepairs]

Sorry Guys
i don't know why I typed pin 7 it was pin 9 screen.
The PCB track to pin "9" was intermitant due to a crack
my customer saw the tube glowing a cherry red around the plates "not heaters"
Plate pin 7
Control pin 2.

 

[Osvaldo de Banfield]

Red plating can also be found if the tube is oscillating hard, in some circumstances.

 

[hooman]

brand of your amp ?
brand of el84?
check dc voltage between g1 and k .

 

[DF96]

What happens will depend on what potential g2 picks up from leakage currents etc. If low potential then little current will flow to the anode. If it picks up something nearer the anode potential then a higher current will flow. Oscillation is possible, although unlikely.

 

[hooman]

need information