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. 

https://imgur.com/a/RVLWPcf

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.

chinook9
I just noticed that they red plate a bit.
Drop the bias 10%

Cheers George


Sorry didn’t see you have cathode biasing, (auto bias).
You could get them converted to fixed biasing, by someone close to you that’s good with tubes, could cost around 2-3hrs labour, up side is you’ll get a little more wattage also.

Cheers George