What is GM in tube testing?
Depending on your tester, you will either be testing for Emission or Gm (Mutual Conductance, or sometimes called Transconductance). Either test method does a good job at evaluating whether the tube is likely to work or not. This is really all that a tube tester was ever designed to do.
How does a tube tester work?
The mutual conductance tester tests the tube dynamically by applying bias and an AC voltage to the control grid, and measuring the current obtained on the plate, while maintaining the correct DC voltages on the plate and screen grid. This setup measures the transconductance of the tube, indicated in micromhos.
How do I choose a tube tester?
If you want greater accuracy get a tester that has fixed Gm ranges (switch selectable) like 3000, 6000, and 15000. Not units like the Hickok 600 series, 800 series, or 6000 series, which use a variable control to set the ranges. The accuracy and repeatability of the test results are not as good on these testers.
How do you match tubes without tube tester?
Oh, you could use a preamp or power amplifier to match tubes, taking measurement of plate current, bias current but you really have to know what you are doing as you are working with 100 to 500 volts and enough current to kill, but this is a very accurate way to check tubes as they are “in” the circuit intended for …
What is mutual conductance?
Definition of mutual conductance
: the quotient of a change in plate current in an electron tube by the change in grid voltage producing it, the plate voltage remaining unchanged.
How are GM vacuum tubes measured?
The formula for transconductance or Gm of a vacuum tube is as follows. Gm=delta Ia ÷ delta Vg. This is the difference in plate current divided by the difference in grid voltage that produced it. So in simpler terms it would be plate current divided by grid voltage.
How can you tell if a tube is bad?
Crackling, squeals and feedback, excessive noise and muddiness or low output are all evidence of tube problems. Power tubes. The two main symptoms of a power tube problem are a blown fuse or a tube that begins to glow cherry red.
Can I test a tube with a multimeter?
Open or shorted filaments can be detected using a multimeter, and should be done before plugging the tubes in. Set the multimeter to measure resistance and connect the meter’s test leads to the filament pins. (You can consult on-line tube manuals for the pin configuration.
What happens if you leave a tube amp on too long?
There is always risk of damage wherever heat is involved, but the biggest issue with leaving a tube amp on all the time is if a tube blows. This can turn the amp into a dangerous hazard. It could even start on fire, which is a pretty big deal, especially if no one is around to call the fire department.
Can you test tubes with a multimeter?
How close should tubes be matched?
We match tubes to be within 5% of measured plate current (that is, the tubes won’t differ from each other by more than 5%, or 1 milliamp in 20). Despite what many people may say, this is about as accurate as power tubes can be matched.
How do you calculate mutual conductance?
gm=mutual conductance (usually mA/V) ra=anode resistance (kΩ, Ω). The amplification factor is defined by: The amplification factor (μ) of a valve is the ratio of the change in anode voltage ΔVa to the change in grid voltage ΔVg, with anode current held constant.
What is the principle of mutual induction?
When changing current in one coil induces an EMF in the other, the phenomenon is called mutual induction. The strength of the EMF induced depends on the mutual inductance of the pair of coils. The S.I. unit of mutual inductance is Henry, the same as that of self-inductance.
What is GM in vacuum tubes?
Since this “conductance” is from the input to the output circuits of the tube, and goes “across” the tube, it is called “transconductance” or “mutual conductance”. Since “G” is the symbol used in electronics for conductance, this explains the use of the symbol “Gm”.
Can you test vacuum tubes with a multimeter?
How do you test old tubes?
Put the tube into your guitar amplifier, tester, or other tube-using machine. Turn on the machine to activate the tubes, and look for an orange, red, or purple glow. If the heated filament inside the tube glows orange like a setting sun, it’s usually a sign that the tube is healthy. The filament may be hard to see.
What causes tubes to go microphonic?
What causes tube microphonics? Microphonic vibrations are most often caused by internal component electronics like transformers, relays, motors, and external vibrations like sound waves.
What can you not do with a tube amp?
Never, never, never run the amp with no speaker plugged in. This can cause major damage. Do not flip the power switch off, then back on rapidly. This can cause power supply damage.
How long should a tube amp warm up?
20 to 30 minutes
As a rule of thumb, your tube amp needs to be warmed up for 20 to 30 minutes at least before you can start playing your guitar. Most of the time, tube amps have warm up specifications included in their instructions for use.
How do I know if my tubes are bad?
Loss of Power and other Odd Noises. Sometimes you will hear strange sounds, loss of power, or heavily distort sound. These are signs that a tube is failing. Often power loss that seems like the amp is performing at half power or less will be one or more bad power tubes, or even a dying phase inverter tube.
Do matched tubes make any difference?
This is done so the tubes can work together in your amp giving you optimal sound quality and tube longevity. Matched tubes also receive extra quality checks to reduce the possibility of getting a faulty power tube that may have slipped by the manufacturer.
Do 12AX7 need to be matched?
Using 12AX7s as an example, matching these preamp tubes is a good idea, but by no means essential. These tubes, along with the 6DJ8 variety, are known as linestage tubes. When they are matched, this will ensure that you get even gain from both tubes.
Where is mutual induction used?
Mutual Inductance is the basic operating principal of the transformer, motors, generators and any other electrical component that interacts with another magnetic field.
Why is mutual induction important?
Significance of Mutual Induction
Electric motors also use their principal and are a very important piece of equipment. A transformer’s mutual inductance, also known as the coefficient of coupling, is a measurement of the efficiency with which power is transmitted from the primary to secondary coils.
Are old vacuum tubes worth anything?
There is a large market today for newer tubes used in vintage music amplifiers and vintage tube stereo equipment. Sought after by Audiophiles, these include the common U.S. made 6L6 ($15-$25) and 12AX7 ($8-$15). If it’s a Telefunken brand you can easily double or even triple the price.