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.

This all sounds scary to me the kt88 tube runs at a much higher voltage than the 6550 and the bias current is different also.
The two are not that different. In looking at the TungSol plate current, its rated at 140mA (max) where the KT88 is rated 100mA. They have similar maximum plate voltages too. This suggests that the dissipations are similar. I've not seen a circuit so far that supported 6550s that didn't also work with KT88s. My H/K Citation 2 has its power tube sockets marked 'KT88/6550'. Its generally accepted that if you design for one, the other can be substituted. The difference is that 'KT' stands for 'Kinkless Tetrode'; the kinkless bit being in the linearity curve. So 6550s should work fine but be higher distortion. Or KT88s should work fine but be lower distortion.
Great information being posted in this thread.  For the time being I am waiting to hear from the company rep but I'm not holding my breath. I'll use the KT88s until I get this resolved. 

Not sure if the bucking transformer will solve the problem but its worth probably worth the investment of $90 if it might work. 

I'd like to hear opinions on potential of the bucking transformer to solve the problem.  They are available in 3% or 7% reduction in voltage and it looks as though I should go for the 7% reduction if I get one.
chinook9, log your voltage over a few days. It is not uncommon to have a higher level for short periods. Base your required drop percentage on average. Note that voltage levels can vary drastically by season dependent on the local requirements. In L.A. summer is the high load, but the Oregon coast winter load is higher.

A device rated for 115v is typically ±5% which is the line voltage standard. However, local utilities may struggle to maintain. This is a good read  Voltage Tolerance Boundary (

The eBay link above has no current spec. The box is plastic. Pass...

Build a variable bucker and be done with it!
That is 5VA, not 5A. This is a low voltage transformer for things like doorbells.

Ebay has premade bucking transformers:

Good catch! - thanks.

I was not aware of the product for guitar amps but its nice to know about! That would be perfect for this sort of thing, if line voltage is all there is to it.