VHT Tube Tester 2 User Manual

VHT TUBE TESTER 2

MONITOR YOUR OUTPUT TUBE PERFORMANCE

The VHT Tube Tester 2 provides two completely independent current-measuring tube tester
sections. Simply plug your tubes into the adapters and then plug the included 8-pin octal-based
adapters into your amp.
The two high-quality analog milliamp meters measure the tubeʼs cathode current. You can use
these measurements to observe your tubeʼs real-time dynamic performance, see how well theyʼre
matched, adjust the bias, monitor your tubeʼs performance as they age, check for unstable
conditions, and more.
Additional test-pin jacks are provided for precise digital multi-meter measurements and analog
meter calibration.
The VHT Tube Tester 2 is an affordable bench and road tool that lets you see how your output
tubes are performing in real time, and whether itʼs time for replacement or a bias adjustment, etc.
Get the most from you tubes — and amplifier — with a VHT Tube Tester 2.

SET UP

Setup is easy, simply unplug your output tubes and carefully plug them into the Tube Tester 2ʼs
adapters, then plug the adapters into your tube sockets.
Pay close attention to the tubeʼs aligning guide pin, and be sure the adapters are aligned correctly
with your tube sockets. (An incorrectly installed tube or adapter can cause serious damage to
your amp and/or tubes.)
Some tubes may be a tight fit in the adapter. Tube pin diameter varies slightly between different
tube manufacturers, and sometimes there will be some excess solder on the pin. For an easier fit,
you can file-down excess solder with an ordinary fingernail file.
After the tubes and adapters have been properly secured, connect the adapterʼs banana plugs to
the meter unit.
Test-pin jacks: For precise digital measurements and analog meter calibration, plug a DMM
(digital multimeter) into the front panel test-pin jacks and set the DMM to its millivolt range; the
test-pin jacks measure the voltage drop across a 1-ohm precision cathode resistor, 1 millivolt = 1
milliamp cathode current.
You can also plug the adapterʼs banana jacks directly into a DMM (set for milliamps).

OPERATION

While other tube testers test your tubes externally, the truest and most relevant test is how your
tubes perform in your amplifier.
The Tube Tester 2ʼs high-quality analog meters provide a unique window into your tubeʼs
performance. Analog meters provide a much better view of your tubeʼs dynamic response than
digital meters (which are still useful for precise measurement of stable idle currents and voltages,
and also for meter calibration).
After the tubes and adapters have been installed properly, power-up the amp as usual.

OPERATION, CONTINUED

Observe the meters as the tubes warm up and slowly begin to draw current.
The meters will show a gradual rise in current until the tube idle current begins to stabilize.
(If the meters deflect in the negative direction, reverse the banana plugʼs polarity.)
If one of the tubes, or both, continue to draw current without stabilizing, this is a clue that the tube
is defective (prone to “runaway”); it needs to be replaced.

Do the two tubes rise at about the same rate, or does one tube rise faster than the other?

Tubes with similar characteristics will warm-up at similar rates. An unusually fast warm-up rate
can indicate a problem; it could be a tube problem or perhaps an amp problem.

When the idle current stabilizes, how closely are they matched?

The analog meters show 4 milliamps per division; a close/acceptable match is generally
considered to be something less than a 10% difference between the two tubes.
With practice, you will learn to quickly recognize common tube problems such as runaway “red
plating” as well as unstable conditions that cause unreliable operation.

MICROPHONICS TEST

Tap the tubes and listen for microphonic rattles and rings while also watching the meter for
unstable readings. (A doctorʼs rubber knee reflex hammer can be purchased cheaply on eBay, or
a pencil with added eraser is also good, and a good alternative to the old-fashioned tried-and-true
“fingernail flick.”) Unstable meter readings indicate a tube with loose internal components; these
tubes should be discarded.

BIASING

After youʼve confirmed that your tubes are stable and the idle current is reasonably matched, you
can use the Tube Tester 2 to adjust the idle current level (assuming your amp has an adjustable
“fixed-bias” type circuit).
Simply put, “proper biasing” is any bias setting that sounds good to you without destroying the
tubes. It is, after all, about tone, and tone is of course very subjective. Some people prefer a hot
bias (high current), while some people prefer a colder bias (lower current). Fine-tune by ear, and
season to taste, while always being aware of the tubeʼs practical limits (its real-life maximum plate
dissipation).
An output tube has a maximum plate dissipation rating; these generalized ratings were developed
many decades ago, and they can be found in old tube manuals and reproduced online.
But tube manufacturing is different now; some currently manufactured tubes can easily exceed
the old maximum ratings, while others donʼt measure up. So the old maximum plate dissipation
ratings should be regarded as good guidelines rather than absolute limits. Your results may vary.

CALULATING PLATE DISSIPATION

To accurately calculate a tubeʼs plate dissipation, you need to know the tubeʼs plate voltage. This
is the voltage between the tubeʼs plate and its cathode (pin 3 and pin 8). You can measure this
with a DMM, but youʼll need to remove the chassis to access the tubeʼs socket.
If this is not convenient, you can estimate; most amps with large octal (8-pin base) tubes (like 6L6
or EL34, for example) operate at around 400 to 475 volts on the plate, while smaller tubes like
EL84s and 6V6 tubes will typically operate at lower plate voltages, around 350 volts or less if
theyʼre cathode biased.

PLATE DISSIPATION FORMULA

Plate voltage x current = plate dissipation (watts)
Example: 450 volts plate voltage x 40 mA (meter reading) = 18 watts plate dissipation; this would
be considered safe for an EL34 but dangerous for a 6V6.
Some cathode biased amps are known to routinely exceed the recommended maximum plate
dissipation, so donʼt be surprised if your meter readings indicate plate dissipation approaching or
exceeding the textbook maximum rating.