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.  

ROSE MUSIC: Communication Systems of Australia. Model 35 PA Amplifier (Goldentone).

This one’s a rare find – a Communication Systems of Australia valve PA head, ~35 watts from around 1967 / 68.  It's super clean, fully functional, with all parts within spec.

Anyone familiar with vintage Australian guitar amps will recognise the front panel straight away: it looks just like a Goldentone.  That’s no accident.  These were built by Goldentone’s parent company, Rose Music in Melbourne, and were sold under the CSA, Goldentone and Zephyr badges.

The layout is typical for the period, with mic and pickup (turntable) volume controls and a single tone control. Like a number of Australian amps of the era, the power section uses TV valves – a quirk that seems to be unique to local designs.

• A pair of 6CM5 in the power stage 
• A&R power transformer
• A&R type 2766 Output transformer, marked 100v, 70v and 50v.
• Silicon bridge rectifier - ITT MB4 
• Standard 12AX7 triodes for preamp / phase inverter.
• Mic input is grid leak biased, low impedance.
• Cathodyne phase inverter, with NFB from transformer winding
• Additional input straight into the phase inverter so the amp can be used as a slave (octal plug)
• Simple treble roll-off tone control
• Pots 500k volume, 250k tone

COMMUNICATION SYSTEMS OF AUSTRALIA, MODEL 35

A&R Type 2766 output transformer with a constant-voltage secondary (50/70/100 V taps). I ran basic DC resistance and low-voltage AC ratio checks to fingerprint the winding and estimate the primary load seen by the output valves (2× 6CM5).  Note that I have not included the negative feedback winding yet.

DC resistance

Primary, plate-to-plate: 85 Ω

Secondary:

    COM–50: 7.1 Ω

    COM–70: 9.7 Ω

    COM–100: 14.5 Ω

AC ratio check (14.49 VAC across primary, plate-to-plate)

Measured secondary voltages:

    COM–50: 2.355 VAC

    COM–70: 3.265 VAC

    COM–100: 4.816 VAC

5    0–70 segment: 0.900 VAC

Derived turns ratios (primary : secondary):

    0–50: 6.15 : 1

    0–70: 4.44 : 1

    0–100: 3.01 : 1

    50–70: 16.1 : 1

16 Ω on 50–70 is about 3.8 kΩ p-p reflected.   Datasheet spec is 3.5 kΩ for 2 × 6CM5 at ~300 V B+

There's a spare noval socket on the far right, and the octal behind it is used as an input for the phase inverter.  I have seen another of these that had 2 x Mic inputs, using the additional socket for another preamp.

The black square is the solid-state bridge rectifier marked as MB4 ITT - nice and compact.  The network sitting next to it is the negative bias voltage supply.

Preamp and phase inverter:  Blue, yellow and white hookup wires are colour-coded for plate, grid and cathode.  Green for ground.

CSA 35 SCHEMATIC

C8 is unusual in that it seems to be on a separate winding, compared to the rest of the output transformer.  It may be some form of shield.  Looking at the phase inverter and negative feedback network, they seem to have gone to some effort to reduce high-frequency oscillations / interference.  R12 / C9 is another example.

Voltages

B+ 380

Screens 188

Plates 375

Bias -32.5

CSA 35 CONVERSION NOTES

Despite being a little reluctant to touch the beautiful work done by the person who built it - it needs to function as a guitar amp.   

First things first - at 100k, the input impedance was too low.  Swapped the 100k for a 1meg resistor to bring it up a bit.   An input jack was added.

While the tone control was OK, a simple low-pass filter is a bit boring, and there was a spare knob...   Goldentones often have a James tone controls for bass and treble (sometimes called passive Baxandall), so I installed one to keep to the theme.  I used mustard caps and a low-value tropical fish capacitor to be consistent with the parts used in the amp.  

A bypass cap was added to the gain stage of the cathodyne phase inverter to make up for the loss of gain introduced by the tone controls, and a grid leak resistor was added to where the old tone control was in the circuit.

It sounds great - it's a solid little amp.

FURTHER READING

Oz Valve Amps

Aussie Guitar Gear Heads

The Trainspotter's Guide to Goldentone Amplifiers  (page 58)

COLORSOUND: Treble Booster, Steve Williams (partial trace)

So there are a few bits missing, but if you are familiar with treble boosters, I'm sure you'll work it out.  Being a small and simple circuit, it's a good candidate for breadboarding.

The filter pot is your standard input blend, probably similar to the values that Steve Williams uses in his Pigdog Competition Special boosters, which is followed by a silicon treble booster with a coupling cap and volume pot attached to the end.  So a little different to a Range Master in that sense.

Updated layout:  Thanks Zollinger Analog for pointing out that there is actually a link between pins 1 & 2.   I couldn't see what was right in front of me...  the link is sitting on the other side of the pot.

COLORSOUND TREBLE BOOSTER - STEVE WILLIAMS

Important to note that this is a partial trace - you may have to test this and work a few things out yourself.

With a 5n / 100n cap and a 100k filter pot, the response on the base of the transistor looks like this.  The output looks about the same with the 100n and 100k pot.  A 47n cap looks pretty similar as well.

Being a silicon boost, they tend to be a cleaner wave than a germanium boost, although this does look to get some dirt as more bass is introduced.  It is just a simulation, so real-life results may differ - and honestly, you never want to hear a silicon treble boost that is not overdriving an amp - horrid...  

ELECTRONICS AUSTRALIA: A Fuzz-box for Electric Guitars

I stumbled across this while looking for something else - so had to drop it in here.   It's a pretty weird looking circuit, and there are some really strange ideas about fuzz mentioned - including placing clipping diodes across the speaker terminals of an amp.  I would not try that at home.  

ELECTRONICS AUSTRALIA: Guitar Speakers

Just some historic speaker info from 1967 - although I had to leave a page of advertisements in, as it has a kit for a fuzz box available from Moody.   For those not from Australia, Moody was a guitar amp brand, which was one of the big names back in the day.  I had no idea that they delved into kits.