I recently found a February 1940 Jackson tube tester modernization bulletin. This bulletin explains the modernization options available for Jackson tube tester models 627, 627-C, 630, 633, 634, 634-C, 634-D, 635, 637, 680. It also includes the wiring diagram to install a Loctal socket into models 627, 630, 633, 635, 680.

Very interesting bulletin for anyone collecting or repairing vintage Jackson tube testers. I have scanned this 4-page document into a PDF for your benefit.

[ Download the Jackson 1940 Modernization Bulletin PDF ]

To summarize the bulletin:

• Models 634 and 637 sold in 1940 were already modernized at the factory
• Models 634 and 637 sold in their first year (1939) can be easily modernized by the owner (see my article on Jackson 634 for modernization data to install 9-pin miniature socket into these models).
• Models 627, 630, 633, 635, and 680 could be sent back to the Jackson factory and modernized for $5.00 (model 633) or $6.50 (models 627, 630, and 635, which dated from 1938), or $12.50 (model 680). All five of these models had to modernized at the factory because they required installation of a supplementary high-voltage filament transformer, among other parts.
• Models 427-A, 435-A, 535, and 580 were too obsolete to be modernized because “no provision for the high voltage filament types can be made. Miniature types cannot be tested.” Jackson noted that these models were “as much as five years old”, which dates them between 1935 and 1940. Jackson recommended upgrading to the new 1940 models of 634 and 637.

If you are only interested in the Loctal socket modernization wiring diagram, see the accompanying thumbnail photo (click photo to enlarge).

Loctal socket modernization

This article will discuss repair and calibration of the Jackson 561 tube tester. High voltages are present, repairs should only be attempted by a qualified technician. Copyrighted, all rights reserved.

Introduction

The Jackson 561 appears to be a rare (uncommon) model. Very little documentation exists, and I have seen only a small number of them for sale.

The model 561 is a combination of the Jackson 634 and the 648. Photo below shows Jackson 561 (right); Jackson 648A (left).

Comparison: Jackson 648A vs 561

If the 634 and 648 mated, the 561 would be its offspring. It shares the following characteristics from the 648: (a) the same meter, (b) variable leakage control, (c) color scheme, and (d) case design. From the 634, it shares (a) the same test method (which Jackson calls “Dynamic Output”), (b) Shorts Test control, (c) lack of a Noise test, and (d) function control layout. In fact, the 634 tube setup chart can be used (with slight modification) with the 561, as explained later.

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If you appreciate vintage tube test gear, you will enjoy this 1940 vintage Jackson 634 Tube Tester. It is an excellent choice for anyone looking for a quality, well-built, tester that can test the VERY OLD tubes, such as #50, 01A, 1V, #10, 2A3, #15, #19, HY113, HY115, HY125, #26, #45, 71-A, #83, etc.

This article is also relevant to Jackson 637 tester, which is same as model 634 tester with additional integrated multimeter functionality (volts and ohms testing).

(Jackson 634 testing Cunningham CX-350 Globe #50 Triode tube)

What is really “cool” about this tester is its very small size. Measuring only 8.5 x 8.5 x 6 inches and weighing 9 lbs, this unit packs a big punch in a small size!

This tester has been in my personal collection since 1990, and it is the oldest tester in my collection (except for a Supreme 550 Radio Tester from 1936, which also has some basic tube testing capability).

Jackson 634 - 637 modernization bulletin. Click to enlarge.

This tester is so old that it did NOT come with a 9-pin-miniature socket installed — that was too modern!

(Article UPDATE, Nov 3, 2008): I found an original Jackson modernization bulletin, and the Jackson 634 and 637 testers were released in 1939. All 1939 models were sold without the 9-pin-miniature socket installed. The 1940 models were already modernized at the factory.

I have installed a new 9-pin-miniature socket, and will teach you how to “modernize” your 1939 vintage Jackson 634 also. (Jackson sockets are wired unique, and do not correspond to the 1-to-1, 2-to-2, wiring that many testers employ).

Service information and many additional photos are as follows…

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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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