Warren

Ok good info once again, thanks.

From what I hear of the 4-400A's they will glow a somewhat cherry red / orange right at or near their max comfortable operating conditions.
So in my case for sure it looks like a bias problem as in the meantime (2) of the gas regulator tubes, a 105 and a 150V type had white film
inside the glass , indicating a bad seal. Today I have got some NOS replacements, plus a new 5U4. Though my 5U4 is ok, its also a 1959 date code... (back when sunspots were supreme) so figured may as well because of sheer age.

Also pondering the 160m mod in the back of my mind. Am told there is a Jan 1991 issue of electric radio that covers a mod like this to the Johnson desk KW amp and its design circuitry is similar. Would be interesting to read that, however looks like it would have to be purchased as a back issue.

I am thinking that my plate choke L7 needs another 25-50 uh or so. Its a National R-175A I am told, and apparently if I can find a way to add some more turns in line with it , enough to cover the extra L, keep it shorted with a relay until needed, would be the goal.

I also think we need more coupling capacitance, and the coupling switch SW2 seems to be a good spot to put it.
I have seen others using HUGE vacuum capacitors in their mods, but I think thats kinda overkill here and since I have a few of the TV doorknob type caps, I could put them in series to get another 170pF or so with some jiggling around. They are overkill at the rated 20KV, but I may be able to finagle them onto L6 which shares a common connection. The relay connection may be a little tricky but as long as I remember to engage it first BEFORE keying up, that ought to greatly diminish the potential for sparks, flames and or smoke....