[morzh]

Terry and I worked through this on the phone yesterday (that man has the patience of a saint).

That he does

I think he is right, if you went through it; it could very well be what he thinks it is, which is a hot tube defect.

I wonder if this can be tried (only in case you have no spare 24): 20 is a TRF and even without the first tube if the antenna or the finger are connected to the second tube the same station (strong local) could be received. Or so I think. If this is the case, you could substitute the 24 tubes in the same socket (say, the second RF tube) until the defect shows up/disappears, and figure out which tube causes that.