Earlier this year my nephew was after a valve guitar amp and after picking up a box of 6CM5 valves fairly cheaply, I had a look round on the 'net for amp projects based around these valves and found: ECONOMICAL VALVE BASED AUDIO POWER AMPLIFIER and Grant Wills
I built a stereo version of the first amplifier fairly cheaply, probably well under $200 as I had most of the parts lying around already. It was fairly straight forward to make and after letting a few guitar players try it out the general consensus was that it was no good as a guitar amp: it sounded "too clean" and "no distortion there" which made sense as I built it with a fairly stiff power supply with plenty of 47 microfarad 400 volt electrolytics. I also used a 20 Watt PA transformer instead of the 15 Watt unit for the later guitar amp which should extend the ful power bandwidth a bit.
https://www.woodworkforums.com/attach...cm5-amp15a.jpg
