HICKOK: Model 800, Tube & Transistor Tester

As I’ve been working on valve amps for a while now, I decided it was finally time to get a proper tube tester.  You can absolutely get by without one - for a long time, I managed without.  But I use a lot of old valves that often come with the amp, or are pulled from vintage equipment - it sometimes felt like I was flying blind.  Some valves were clearly performing better than others, but without a tester I had no reliable way to work out what was going on.

There’s a huge range of testers out there. I even considered building one myself at one point.  Broadly speaking, they fall into two main categories:

Emission testers:  These are the simpler kind, and they measure cathode current.  You pop the tube in, adjust a few parameters, and a single meter tells you "bad  ?  good".   They’re handy for quickly screening dead or weak tubes, but don’t tell you much beyond that.  Some also test for shorts, which is very handy.

Mutual conductance (Gm) testers:  These are a step up from an emission tester.  Instead of just telling you if a tube works, they give you a measurement of its transconductance (how well it amplifies), which is a much better indicator of how the tube will actually perform in an amp.  They also often have the "good / bad" reading as well.   

HICKOK 800

Why the Hickok?

I went with a Hickok 800, one of the classic mutual-conductance testers. It’s not the absolute top of the range (those honours go to the lab-grade Hickoks and AVOs), but it’s a very capable and well-regarded unit. It also fit within my budget, had been recently serviced, and came with a reference valve with accurately measured values for calibration.  An added bonus was that it had been modified to allow cathode-current measurement using an external multimeter — a great feature for matching pairs.

Using the Hickok, there are three things that I test for.  Leakage, transconductance and gas.  

Leakage tests for resistance between the various elements in the valve.  If a short is found, the valve goes straight in the bin - no further tests required.  I valve with shorts is bad news, it can damage equipment.  

Gas - All vacuum tubes contain a small amount of residual gas, but if that gas becomes excessive - due to age, seal failure, or internal damage - it can cause the tube to conduct current even when it shouldn’t.

The Hickok checks whether the valve is pulling grid current when the grid should be at zero or negative potential. In a healthy valve, the control grid doesn’t draw measurable current. But if there’s too much gas inside, it gets ionized by the electron stream and starts to act like a leaky diode, drawing current from the grid circuit.  That’s a red flag.

Mutual conductance (often abbreviated Gm) is a measure of how effectively a tube controls plate current with its grid voltage. It’s expressed in micromhos (µmhos), which is the same as µA/V (or milliamps per volt).

A higher Gm means the tube is more responsive — i.e. a small grid voltage swing causes a larger change in plate current. This directly correlates to gain and overall performance in an amplifier.

The Hickock has a chart listing the average GM of new valves, under the test conditions that the Hickock uses.  As an example, a 12AX7 lists 1,250 GM so if a valve under test reads at 500, it's clearly in a bad way.  

The manual also suggests testing under adverse conditions, which involves dropping the heater voltage down a notch and seeing if it still performs well.  If it does, this is a sign that it has plenty of life left.

THE CONTROL PANEL

Clearly, there are a lot of buttons and knobs to contend with.  But it's really not that complicated.

The first place to start is with the roller chart, or the manual.  This is where you find test settings - and you will want to double-check these, as it's easy to miss one.   Most valve testers have some form of manual that lists settings for the same purpose - what's the pinout and test conditions.

LEAKAGE 

This is used to test for shorts or resistance between valve elements & to switch the meter into test mode.  

This is the first test to do on any valve.  The knob is turned from 5 down to 1, while watching the meter for movement.  Note the leakage scale on meter measured in kilohms and megaohms.  For good valves, the meter barely moves, if at all.   

After testing for leakage, move the leakage knob to the TUBE TEST position.

FILAMENT

This sets the heater voltage, which for pretty much everything I use, is either 5, 6.3 or 12.6v.  

The SELECTORS set the pinout of the valve, for heaters, grid, plate, screen, cathode and suppressor.  The buttons above are for various tests that can be performed.  

The LINE ADJUST knob is a mains voltage trimmer - line voltage from the wall can vary, and by pressing the line voltage button, the meter will show if it needs adjustment (note the line test mark on the meter).  The weird looking socket next to it is for straightening bent pins.  

BIAS & ENGLISH

The BIAS sets grid conditions; the ENGLISH knob actually sets meter sensitivity or range.  It's called the English, as it can be set to rate the valve on the "bad  ?  good" scale.   

I prefer to set the range to use the Micromhos scale, which is in three ranges.  0 to 3,000.  0 to 6,000  and 0 to 15,000.   There are red dots on the English knob to show where to set it for this purpose.  

Using the ranges mentioned above, a valves GM can be directly compared to known values.