The Hickok 539B Tube Tester is one of the better tube tester out there. The 539B is known as Hickok's "Engineer's" model. These were often used as field units for military engineers testing military equipment back in the 50's and 60's. This particular unit was built in the late 50's and is built like a tank. The craftsmanship of these units has stood the test of time. Features such as metered bias voltage are usually not found on other units, and this makes the 539B very unique. The 539B is nearly identical to the Hickok 539C. The 539C will allow you to test a few very obscure tubes that you'd never even come across or use for guitar or audiophile amps. The 539A IS much different than the B & C. Stay away from those!!! The Hickok 539B units are more resonably priced than the 539C, so I highly recommend the B. Here are some additional features that the units has:

-- Measures true transconductance in "micromhos".
-- Separate line voltage meter makes it easy to keep the line voltage properly adjusted for more accurate test results.
-- Separate bias voltage meter for precise monitoring of proper grid bias voltage.
-- Bias voltage meter doubles as milliammeter for testing V-R tubes.
-- Very large main meter for measuring tube quality. Quality measurements are made in micromhos (no good-bad scale on this tester).
-- Main meter also measures inter-element leakage (shorts) in ohms. This is a much more sensitive test than the neon shorts light (also found on this tester).
-- Sensitive gas content test which important for amplifier tubes.
-- Provisions for monitoring of heater current with external meter.
-- Provisions for adding a self bias (cathode) resistor.
-- Provisions for monitoring of plate current using an external meter.
-- Includes sockets for 4pin, 5pin, 6pin and 7pin , octal, locktol, acorn, sub-miniature, 7pin miniature, 9 pin miniature tubes.
-- Built-in roll chart contains settings for most common tubes.
-- Roll chart supplement books are included for obsolete and foreign tubes.
-- Tests most common audio tubes such as 2A3, 45, 6L6, 6CA7/EL34, KT-88, 6550, 6BQ5/EL84, 12AX7 and many others.
-- Also tests antique tubes back to type 01A and 99.

Manuals, calibration kits, etc. are available at www.vacuumtubes.com. I highly recommend giving Mike Marx a call and discussing any tube tester and/or tube needs you might have. A very cool guy.