Ronald Reeland - Eico 666 Function Trace Scans


The Eico 666 and 667 tube tester manuals provide a very helpful circuit description and partial schematic to assist in understanding the various functions of the instrument. However, I thought completely detailed schematics of each test function would contribute to an improved comprehension of the of the tube tester. A detailed schematic would also be more useful to track down a malfunction in a particular circuit.


To that end, I created a series of ten current path tracings upon the standard Eico 666 schematic. They encompass the following:

  1. Cathode to heater leakage test

  2. Plate-to-all leakage test

  3. Line adjust circuit

  4. Merit test, pentode

  5. Merit test, duo-triode

  6. Merit test, light-duty diodes

  7. Merit test of cold cathode gas rectifier (0Z4)

  8. Merit test, full-wave rectifier

  9. Calibration of “LINE CAL” potentiometer, R18

  10. Calibration of “LEAKAGE CAL” potentiometer, R7



I used various color marking pens to differentiate between filament/heater currents, cathode currents, grid currents, screen, and plate currents.

I wanted to use yellow or orange on screen circuits, but they do not copy or scan vividly enough.


I did not include screen-to-all or grid-to-all leakage tests as they are identical to plate-to-all leakage test, but different tube elements. Similarly, I only traced one section of a duo-triode merit test and only one section of a full-wave rectifier merit test. Lastly, I did not trace the circuit for 0B2 and similar voltage regulation tubes as the circuit is the same as 0Z4 gas rectifier except R20 current limiting resistor is omitted.


I added notes of explanation to each schematic, but the information is necessarily limited due to space restrictions.

The Eico 666/667 tube testers are not overly complicated, but I was surprised at just how many switches are in each current path.

Ronald Reeland - April 2011