I had the same problem. My solution was to remove some cathode bypass caps, bypass V1b and turn V2b into a cathode follower--in other words, to turn it into a low-gain amp. If you're looking for cleans without major surgery, you might replace V1 with a 12AU7.

Or you could get a new amp. Just sayin'.
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Quote by Levi79
Don't get a Windsor.

I second this, as a previous Windsor owner. The Windsor is great for balls to the wall rock, but cleans are pretty nonexistent on this amp.
Roll back the volume on your guitar, it will help. Also, try the low gain input. There's not a whole lot of clean headroom but it does have some. There are some amazing tones that you can actually get out of one. Might want to check this out:

Guess it depends on what you want for cleans...

At it's cleanest, with your volume all the way up on your guitar, it's going to have a light little bluesy grit to it..

If you wanted Fender Cleans, like a Twin Reverb, well.. ya go the wrong amp...

I use a volume pedal, and it helps quite a bit.. I set the volume on the amp at where I want it for solo's then back off the vol pedal for rhythm.. it cleans up pretty good..
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Low gain input. turn the pre amp all the way down and crank the output. Slowly bring up the pre gain. Whamo, it's clean.