This article will discuss repair and calibration of the Jackson 648-series tube tester. It will also discuss the procedure to eliminate the 1S5 tube from models 648 and 648A. High voltages are present, repairs should only be attempted by a qualified technician. Copyrighted, all rights reserved.
The Jackson 648 was available in several versions, including 648, 648A, 648P, 648R, 648S, 648-1, 648-1T. The basic circuitry is fundamentally the same in all versions, with only minor changes (socket layout/configuration, adjustable grid leakage circuit in 648-1T).
I am a big fan of the Jackson 648. It is definitely one of my favorite tube testers. While it does not employ a mutual conductance test circuit, it does use an advanced emission circuit that applies separate element voltages to each tube element, and separate load circuits are also used. “These voltages and loads have been carefully selected for each tube to meet most ideally the normal operating condition of the tube.” (Source: 1950 Jackson catalog).
In my experience, the Jackson circuitry is very effective, and often will surprise you with accuracy that approaches Hickok’s coveted “mutual conductance” test method.
Example: testing a weak 6N6-MG (metal envelope) triode tube:
(as tested with calibrated Jackson 648A)
(above: the same 6N6 triode, as tested on the Good-Bad scale of my calibrated Hickok 6000 mutual conductance tube tester)
Here is another comparison: