VHT Tube Tester 2 - Manual (Page 2)

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.