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.

Introduction

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).

original 648 with tweed-style case

"newer" 648 model with 648A color scheme

648A model

648-S

648-R

648 Tweed-style case

648S with flipover socket panel

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:

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Sencore Mighty Mite solid-state tube testers (TC28 Hybrider, TC162, and TC154) are durable, easy-to-use, and test a wide variety of modern tubes (including modern sweep tubes such as #8950). They were VERY popular among old-timer technicians. For most people , they are a very good choice.

Sencore TC28 Hybrider

Sencore TC162

Sencore TC154



Regardless of model, all the Mighty Mite solid-state models use the same basic circuitry, albeit configured slightly different among each model.  Model TC162 is functionally equivalent to B&K 607 / 667.

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