servicing a Sylvania 620 Tube Tester

I recently refurbished a Sylvania model 620 tube tester. The manual was dated 1955, so that is the approx vintage.

Sylvania describes the test method as “Sylvania Composite Transconductance and Emission.” (Source: Sylvania 620 service manual, page 5). This method is not the same as Hickok’s Mutual Conductance.

This is an extremely well-built tester. Construction of the cabinet is heavy gauge steel, “built like a tank.” The leather handle needed replaced, and the cabinet needed completely stripped and repainted. Brand new rubber feet and a new 3-wire grounded power cord were installed.

This is an excellent tester for anyone collecting antique radios, early audio amps (1930s-1950s vintage), Jukeboxes, etc. It tests:

• 4-pin tubes (including #50, #10, #45, 2A3, #80, etc.)
• 5-pin tubes (#807, etc.)
• 6-pin tubes (magic eye tubes 6E5, 6U5, etc.)
• 7-pin large tubes (6A6, etc.)
• 8-pin Octal tubes (including 6146, 6L6GC, 6550, 7591, 5U4-GB, 2050, KT66/6L6, KT88/6550, 6v6, 6SN7gt, etc.)
• 9-pin miniature tubes (including 6BQ5 /EL84, 12AX7, ECC83, 12AT7, 12AU7, 6973, 7199, 6CA4, etc.)
• 7-Pin miniature tubes (6AU6, 6J6, etc.),
• 8-pin Loctal tubes (7N7, 7A6, 7Y4, etc.)

Everything from an old #50 Triode to a #6550 Power Pentode!

Electrically, I rebuilt the entire unit. It uses one 6C4 tube as part of the Line and Leakage circuit. All resistors were checked for accuracy and replaced where necessary.

Calibration was an interesting issue. The manual does a good job of explaining basic calibration, but did not explain how to calibrate the 600 ma test. The basic calibration involves using a Variac to regulate exactly 117.5 input voltage while performing all adjustments. Here is a simplified explanation:

First, you calibrate the Line , then the Leakage circuit, using a combination of two different potentiometers (R106 and R115 on the schematic).

Second, using a 7Y4 calibration tube, you will adjust the D potentiometer knob to set the tube quality result.

Third, you need to calibrate the 600ma test, and this is where the manual completely fails you. The manual does not discuss this calibration. Sylvania used an early solid-state dual diode (two diodes in one physical package) in the circuit. I analyzed the circuit and wrote instructions to calibrate the 600ma test. You will need a digital milliammeter to perform the calibration. Complete instructions are included with the Sylvania 620 manual that I offer for sale.

When you are done with your restoration, you will have a high quality tester, with durable build quality throughout, designed to last a lifetime.

regards,

Bob Putnak

eBay ID = rjputnak