Red Plating??

+1 George. But how do you  manually bias?

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

Interesting, the manufacture designs an auto bias amp around the kt88 tube and then recommends the user to use 6550 tubes.  

Interesting, the manufacture designs an auto bias amp around the kt88 tube and then recommends the user to use 6550 tubes.

Functionally they are nearly the same. No-one makes 6550s anymore; KT88s work in all the same places. But a 6550 will be a vintage tube and most vintage tubes are better than new production.

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.

Its important to understand the difference between autobias and cathode bias. Autobias is a form of ’fixed bias’ in that usually there is a servo circuit of some sort that monitors the bias of the power tubes and applies correction. ’Cathode bias’ is often confused as being the same, because it is also ’automatic’. But to do cathode bias you need a fairly large resistance in the cathode circuit; as the tube conducts the voltage dropped across the resistor is the bias voltage (since the grid resistor is tied to ground).


If you were to convert to some form of fixed bias (such as autobias...) from cathode bias, the cathode resistor value would be vastly reduced and the resistors in the grid circuit tied to a negative voltage source instead (which might be controlled by the autobias servo). Because the voltage drop across the cathode resistor is gone, you would have more plate voltage on the power tube and so ’more power’. But if the amp is supposed to be class A, at this point the dissipation in the power tubes would be exceeded, leading to short term failure!

For this reason, I would ignore George’s comments entirely!


Since the tubes are clearly running a bit hot, I think it would be a good idea to contact the manufacturer as others here have advised and see if this is OK. Its not surprising that with some modern tubes they might be a bit fudged on some of the numbers; if the plate dissipation of the tubes is lower than spec, you would see a bit of red-plating even if the tube is biased correctly for class A.

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.