Push-Pull 7C5 Amplifier in Vintage Style

7C5PP-2015

Here it is, finally! Oh, and I hope you like green! This is a complete rebuild of my very first amplifier. Using everything I’ve learned since that time, I began this project with the goal of creating an artistic and functional version of that design. The tubes, output transformers, and a few internal parts remain from the original build. A new power transformer was used, as well as the new chassis, sockets, and other internal parts. Being mostly the same circuit and parts, it sounds exactly the same.

The schematic shows what few changes were made:

  • The 7C5 screen grids are supplied from the plate B+ through 100 ohm resistors instead of a separate filter section. This is more protective of the 7C5s, whereas the prior connection gave no noticeable improvement.
  • The power supply filter capacitors are reduced to 50 & 50 µF, from 100 & 100 µF. 100 µF is too large a value to follow a standard 5U4 rectifier, and can cause the tube to arc-over internally.
  • The 7C5 cathodes are bypassed to ground instead of to B+, which is known as “Ultrapath”. Other than more expensive higher voltage capacitors, “Ultrapath” resulted in no discernible difference.

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