Red Plating??

I have a CLASS A KT88 amp that has an autobias circuit.  The company rep recommended SED 6550C Winged C tubes.  I purchased a quad of them.  They bias OK and they sound wonderful but I just noticed that they red plate a bit. If I can figure out how to do it I will attach a picture.

The red plating is just in the inner corner of one of the plates on all 4 tubes.  It looks as though I am looking at the filament through a translucent plate but I know that its red plating.

Can I operate like this safely?  The bias does hold steady and does not fluctuate. The red plating does not change.  As a Class A amp this status should hold steady.  Of course, it does run hot. If all that it hurts is tube life, thats OK.  These are my best sounding tubes without a doubt.

@georgehifi  When you go from an autobias to adjustable (by definition both are considered 'fixed bias' as the bias is 'fixed' by the voltage on the grid), regardless of how the amplifier is biased, the output power is unchanged (the distortion created is another matter entirely). The bias by itself does not affect the power- it simply sets how much quiescent current is flowing through the tube.

To increase amplifier power the B+ would also have to be increased. Then of course the amplifier could not be biased for class A operation, because the dissipation of the power tubes would be exceeded. With any power tube when you look at the specs, there is a maximum plate current and maximum plate voltage that are shown. These two maximums should not occur at the same time.

Performing modifications on a new amplifier is likely to void its warranty. Its one thing to ask for support of a warranted product, its entirely another to ask for it when the product has been modified by unauthorized personnel. You might want to take this under advisement; I hope you now see that you've offered bad advice. In this case the best course is simply contact the manufacturer to see if the amp needs service or not. 
Given the ambient light level, it is a fair bit more red-plating than a little.

IF you are competent to operate inside a tube amplifier, check rated voltage, B+ and the cathode resistors. !!!__ ALWAYS KEEP ONE HAND IN YOUR POCKET __!!!

Many US HiFi products are still rated for 115v. That's OK in Los Angeles where 118v is a very good day. In Oregon 118v is a brownout where 122v+ is typical. A bucking transformer solved Oregon overvoltage  ieLogical BuckTrans

B+ could be too high due to a mains transformer mis-wiring [in the US, 115v instead of 120v power transformer input tap].

Cathode resistors could be the wrong value. [I received a pair of monoblocks and one amp had 20k & 30k plate resistors swapped]

I just checked my circuit voltage and found it to be 124 Volts.  The amplifier indicates its designed for 115V, 

Could I build a transformer to reduce the voltage??  I have built a number of electronic devices and probably have the required equipment.

chinook9 OP

Yes you have 10v higher mains, you must be nearer the big pole/sidewalk mount transformers that have many taps and boost or lower the voltage on your mains line. The nearer you are to them the higher the mains voltage, the further away the lower the mains voltage gets.
You could complain to get it bought down, but they’ll just say the houses at the end will then be too low (100v). I got it bought down from 250v to 230v for Australia because I also service medical equipment and used that as a bargaining tool, the real point was, way back I was going though many incandescent light globes, they’d last sometimes not more than a month.

As for a different mains transformer, that’s for a specialist to do.

As I said before get it changed to a self adjustable bias much cheaper, that way you set yours bias 10% lower and will stop the plates glowing red, and you’ll get more power from the amp also compared to auto biasing.
Hope also your power supply capacitors have a bit of max volt head room for this extra mains voltage you have as they will be be up 10% also.

Cheers George
A bucking transformer is one day project. Built as I did, it covers from <115 to 126 volts
Changing bias will INCREASE B+ as current will drop.

A variac is a simple solution, but one I don’t recommend.