[morzh]

Paul

try the alignment (it does not take long) and see whether the problem cures itself.

Now as for 6A8 tubes, I had a nightmare experience with buying them when they came three times from three different vendors all very weak in the triode section (oscillator tested fine). There is some plague with them.

Which does not mean that they do not work.
Your change with the 1st tube could be due to the different inter-electrode capacitance which screws the alignment you have.

When you perform the alignment use your generator to produce the exact frequency you are looking for (1440 is it?) and see if you receive.