EHT Units wear out!

The original Quad ESL (1955-57) used the most advanced IN1001 diodes of the time.  Sounds amusing in today's world, but that was the case.  They are often buried in a block of bees wax and the diodes do fail over time.  They were being stretched to their limits at the top of the step-up ladder in any case.  You will find an EHT rebuild section n this site, with further details.  Go for it !