200S, help

[clintbass1]

A few nights ago, I notiecd my amp starting to humm.  When I looked at the back, the power tube closest to the output transformer was glowing red hot.  I turned it off and checked the bias of both tubes and they were normal at that time, but I was wondering what could cause this to happen.  About a month ago, I relpaced the aftermarket power transformer with one from Triode electronics, and changed the diode rectifyer back to a GZ34.  It's worked great ever since.  I'm thinking a resistor (they are almost 40 years old!) , but just wanted to doubble check.
Thanks

[JoeArthur]

When plates glow red, especially on one tube only, it is most always a lost of bias on that tube.  The problem is tracking down how the bias is being lost and fixing it.

Swap the two output tubes, switch the amp into operation (i.e. not in  standby) and watch them carefully.  Once one starts to glow the slightest bit red, switch the amp off.

If it's the same tube location, then the problem is with that tube socket and bias winding.  Could be a burnt out resistor, or bad contact with the grid via the socket.

If it's the other tube location, meaning the same tube in the other socket, then the problem is with the tube.  Happened to me once - seems the tube gets warmed up causing the internal parts to contact each other - out of alignment due to age, bad manufacture, or shock of being dropped.