[Tubesforme]

I have a model 21, which uses a model 20 chassis in it. I have a problem that is driving me nuts. I just put my third interstage transformer in this chassis. I use this radio daily and after it has been playing for several hours, withing a few weeks of replacing the interstage transformer, the volume will drop. After some time the volume comes back up to normal. This continues for several weeks until 1/2 of the secondary of the interstage transformer opens up. When I restored this chassis 1/2 of the secondary was open so I replaced it with one I had purchased from AES. This worked and the radio played for several weeks. Then the volume would suddenly drop and without doing anything, suddenly the full volume would return. What I found unusual was whenever I attempted to check the bias voltage on the grids of the 71A's, the volume would come up by itself, until the secondary would open up completely. Checking the plate voltage on the driver tube didn't affect anything. I have changed out the transformer three times as well as I finally changed the 27 driver tube and output tubes. I thought my problem was solved, but it suddenly has come back again. I know I have seen a lot of interstage transformers where the primary was open, but never the secondary side. Any ideas? Thanks guys.