Admiral 12x12

[rbc7snc]

Hi All,

Thanks for all of the helpful suggestions.

I replaced the vertical linearity pot, which seemed to fix the problem. The old pot tested fine on the bench, so I'm not really sure the exact issue. I wonder if it might have been shorting under load. In that case, the cathode voltage would approach ground, which would increase the tube current. As the grid and cathode approached the same voltage, the grid would also start to act like a cathode, which would raise it's voltage even further via current through the grid resistor. The end result is a run away tube. I've got a fried 12AU7 to suggest something along these lines probably happened.

The other minor detail I ran into is that I was trying to adjust the height and vertical linearity by looking at the picture. I got much better results when I looked at the vertical refresh waveform on my scope, adjusted for good linearity, then readjusted the vertical sync.

Here are a couple of screenshots of the Admiral on the bench.

http://metro706.hostmetro.com/~archr...r/working1.jpg
http://metro706.hostmetro.com/~archr...r/working2.jpg

Thanks again!

Roger