[Mondial]

Have you looked at the audio level on the grid of the 42 with a scope? It sure seems like an audio problem, but there is not much there to go wrong. I would try and measure the P-P audio level on the 42 grid and compare it to what you have on the plate. About 26V P-P should drive a 42 to full output, so if you have good drive but low output you might then check the output transformer or speaker for problems.

I doubt the problem would be the 36 detector as you can get regeneration, but you can switch it with the converter 36 to check and see if it makes a difference. Maybe a bad coupling cap or detector plate load resistor?