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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

Showing 4 responses by ieales

ieales's avatar
ieales
1,201 posts
 
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]
ieales's avatar
ieales
1,201 posts
 
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.
ieales's avatar
ieales
1,201 posts
 
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 (pge.com)

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

Build a variable bucker and be done with it!
ieales's avatar
ieales
1,201 posts
 
A CV or Isolation Transform is a 1:1 device. It does not reduce the input voltage unless running above rated current. For proper operation, the load must match the device capacity.

It provides isolation and reduces some noise components, some by a lot.


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