Beethoven Glowing Orange

A few days ago I as mecasting a performance of Beethoven's Ninth Symphony and was monitoring the audio using my reconditioned H. H. Scott LK-48 amplifier tapped into my audio chain.  All was well until about half way into the first movement I noticed a bit of distortion in the right channel.

Inspection from the top of the amplifier revealed one of the four output tubes glowing orange...not just the filament but the whole tube excluding the glass envelope.  The plate was orange hot.

There were options:  Panic, run away, keep going, call the fire department,  cook hot dogs, read a book by the light, or chicken out and turn the amplifier off.  I chose the last option.

I grew up with tubes and knew this was a problem with the tube bias.  A negative voltage needs to be present on the control grid to prevent excessive plate current so I benched the amplifier and measured the grid voltage.  It seemed OK at -14.7 volts but after a while it drifted from -15 to -14 volts...not normal.

The negative bias supply in this amplifier supplies not only the bias needs but also the filaments of the low signal amplification stages.  Monitoring this DC -48V supply revealed that it was not stable.  Close inspection indicated one bad solder joint and one plastic insulated lead at the tube socket that apparently, after almost 55 years of operation, had an insulation failure where it crossed two of the connections to the tube socket.  Fixed both and the orange glow is gone.

As I type this I am enjoying Beethoven once again through this amplifier with only the beautiful deep red glow of the filaments of the tubes with pitch black plates.  Amazingly, the previously orange plate tube was not damaged.  Glad I didn't run away or it might have been.

Moral of this story is pay attention when something doesn't sound right.  Another lesson is tubes often tell you when they are not happy.

Neil