justinsweber wrote:Should I assume its the resistors below?
R17,R20 1.2K, 3W
R18,R19 1.5K, 3W
Change these to 800ohm 3W?
R17 and R19 are in parallel with each other, as are R18 and R20. 1.2K in parallel with 1.5K gives an effective resistance of 1/(1/1200 + 1/1500) = 667 ohms. Your amp is already setup partway between Shannon's second and third simulated examples. If you study the numbers in the "Load" section of each simulation, I think that gives you an idea of what you could theoretically expect to achieve from the given circuit. I say theoretically, since if I'm reading these charts correctly it is predicting an output power between 8.42 watts and 9.25 watts. You say you are measuring 4 watts from your amp. Perhaps your measurement technique is imperfect, or the model is optimistic.
Anyhoo, there are other numbers to study besides just the predicted load power. There are also numbers for output impedance (Z out) and distortion figures (2nd and 3rd). Plus, as Shannon points out, you need to consider the driver stage's ability to swing sufficient voltage and/or current into its load. In short, if you want to follow Shannon's suggestion and optimize this circuit for KT88, you would replace R17 through R20 all with 800 ohm parts. I would not expect to hear a dramatic difference.
If it were my amp, I would probably just leave it as is. I've played quite a bit with another single ended, octal tube, circuit board amp. It started life with 560 ohm cathode resistors, but ran just slightly too hot with EL34 or 6L6 family tubes. I ended up increasing the cathode resistors to 810 ohms. It still runs KT88 just fine, but no longer cherries the plates of the smaller tubes. Another option for you to consider might be to keep the 1.2K parts you already have and replace the other two with something like the 620 ohm
Xicon 286-680-RC. They are five watt rated - check first to make sure the physical dimensions will fit the board. Put them in parallel with the 1.2K parts, and you get an effective 430 ohms which is better optimized for your KT88. If you want to swap out your tubes and go back to EL34 or 6L6, remove the 1.2K parts (and maybe install a DPST switch in series with them, so you can toggle it later) and you are back to 680 ohms.
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