Red Plating??

Thank you all for your input. I have sent a message to the company rep.  He was the one who recommended these tubes. 

atmasphere, you mention "a bit of red plating."  Would you consider the red plating shown in my photo a "bit"?  The amp "bias- checking" lights/circuit indicate that it is correctly biased. All four tubes have a similar amount of red plating.

@atmasphere some really good info in your post.

Thank you.

Would you consider the red plating shown in my photo a "bit"?

@chinook9  Technically this would qualify as 'a bit' :)  It would certainly have me nervous! I would contact the manufacturer of the amplifier.

chinook9 OP

Red plating is not good even slight, because both are doing the same it means the tubes are being pushed to their max, either because the tubes are overrated or the mains voltage in your area is higher, than where the amp was built, in Au we can have up to 30vac mains differences in different areas.
Best is do what I said and get it converted by a competent tech to a bias system you can adjust yourself (to 10% less), which we in Au call fixed biasing as you fix the bias point manually, and you'll get more power.

Cheers George

@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.