Hey mate,

Tried rolling back the pre gain?

Have you checked your output tube voltage and bias?

First of all here's the disclaimer : Tube amps have high voltage and amperage and can kill you so be careful!!

Now The best way to find your Plate and Screen Voltage on your amp is to drop the chassis, set it up on your bench. Print out the layout for your amp so you can see which pin is which, then put all the tubes back in. now plug a speaker or a good dummy load into the speaker output and fire her up. allow everything to heat up. Take your multimeter, plug the negative lead into the black bias ground spot, and set your meter to read DC voltage in the 100's range. Very carefully check Pin 3. You will hear a slight pop through the speaker, this is normal. The value you see will be your Plate Voltage write that value down for future reference.  Now carefully check Pin 4. This will be your screen voltage. Write that value down for future reference. Do this for both tubes. Now that you have your correct Plate voltage, you can more correctly set your Bias. The Plate and Screen may be about the same, depending on your model of OTS, or exactly the same. Some amps have vastly different Plate and Screen Voltages. Also, it's a good idea to check your wall voltage to see what you have coming out of there. On the OTS series Plate Voltage should average around 450 VDC, some run a lil lower, some run a lil higher. If the Plate Voltage is way high, it can cause your output tubes to overdrive at lower volumes. If the screen voltage is way off, you could have a tube problem or Once you know your plate voltage, you can properly set your bias as well, using the forumula in the manual. a good starting point is 55% Idle current. If your bias is too low you can get crossover distortion, if it is too high it will be "hot" and tend to overdive more.  Hope this helps!!

Gregg

so the tubes were running at 32 miliamps, so too low and were giving some crossover distortion on the cleans. Now rebiased and running up at 40 miliamps with 70% disipation.

(to the amp tech gurus i apologise if the above wasnt expressed in the correct way lol!)

anyway, now all good!

yup it was the power amp tubes. im using NOS sylvanias and they sound sweeeeeeeeeeeeeeeeeet!