[MrFixr55]

Hi Gang,

I have seen shorted screens but only in Pentode Output tubes like 41, 6K6, 42, 6F6, etc. this usually occurs when the plate circuit is interrupted. The screen then draws all the current, which it was very much not designed for. I have seen screens glow incandescent when the plate circuit of an output tube is open. I suppose that the same thing could happen to a6K7 or 6A8 but since the curent through a 6K7 is so low, I don't think that this failure is common. There is a bit more plate dissipation in a 6A8 but Idont think that the issue is common there either.

Rod, I went back and reviewed the thread. What is interesting is that with a strong signal (your AM transmitter which is giving a much stronger signal 10 feet away than the 50 KW blowtorch that is 50 miles away) is causing the screen voltage to go higher (but still not within spec). This is likely due to the AVC action. The stronger the signal the stronger (more negative) the grid bias. The more negative the bias, the less the current. (This may not be true in a class A amplifier or in a convertor .)The less current, the higher the screen voltage.

I wonder if one of those tubes is gassy.