Bad tube or tubes.
Poorly-chose substitute tubes. Some folks will substitute a 6AU6 tube for one of the three 6BJ6 tubes V1, V3 and V4. This tends to bring up the receiver's noise level. Boneheads would treat this as a cheap preamp mod.
Trouble is, that tube isn't meant to be controlled by the AGC voltage. Won't turn itself down properly for strong signals. Causes the receiver to overload easily.
Bum tube or tubes, wrong tube or tubes.
Or, it could be something else.
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