Guitar amplifier valves fall into two main groups (if we ignore power supply valves) and red plating only occurs on one group. The two groups are preamplifier valves such as ECC83 (12AX7) and power output valves (such as 6L6, EL34 etc.).
Red plating can occur in the output valves but you will never see red plating in a preamplifier valves.
Red plating Explained
Valves are actually fairly simple devices. You have an ANODE which is also known as the ‘plate’. (You have probably guessed that it is the ‘plate’ or anode which glows red hot if your valve suffers from red plating.) The anode is the positive terminal, usually at several hundred volts.
Then there is the cathode which is close to ground potential. In between the anode (+ve) and cathode (ground) there is a control grid. Varying the voltage on the control grid varies the current which flows between the anode and cathode.
In a normal (non red plating) condition, the DC voltage on this control grid is set at a level which causes about 40mA of steady current to flow from the anode to the cathode. At this level of current, no red plating would occur.
But if a large current is allowed to flow, the anode (plate) gets very hot and starts to glow cherry red. This is called red plating.
Red plating is always a BAD sign and if you ever see red plating, turn your amplifier off immediately.
If a valve suffers from red plating for more than a minute or so you should probably consider it written off.
What Causes Red plating?
The important thing about red plating is that it always indicates a fault condition, so turn the amplifier off as soon as you see it.
The main cause of red plating is having an incorrect voltage on the control grid. This voltage is also known as the ‘bias’. If the bias is too low, too much current flows from anode to cathode and red plating occurs. The bias circuit could be faulty and that will need repairing. If there is a bias control, it’s possible to set this too low and observe red plating.
Red plating is not often caused by a faulty valve alone, so changing the valve probably won’t help cure your red plating.
How to Cure Red plating
99% of the time red plating is a bias problem. It will need sorting out by a tech but if you have access to a bias control on your amp, it’s worth adjusting this to see if the red plating can be reduced.
If your amp has been working fine and suddenly starts red plating (particularly if red plating happens in more than one output valve) then it usually indicates a problem with the bias circuit itself. If you know what you are doing and can measure the voltage on pin 5 of the output valve to ground (the control grid) then it should be a negative voltage of around 30-40V. E.g. for a 6L6GC it will be about -38V.
You can see some valves red plating here http://www.thefretboard.co.uk/discussion/3185/ever-wondered-what-red-plating-valves-look-like
Also this is worth a look http://www.tdpri.com/forum/amp-central-station/189485-red-plating-tubes.html
If you need new output valves as a result of red plating you can buy them here: www.ampvalves.co.uk