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What does biasing a power tube mean?
Only power tubes need to be biased by you in a Fixed Bias amp. Small signal tubes also require biasing, but this is done via cathode biasing, also known as self-biasing. In some amps, power tubes are also self-biasing/cathode biasing. It just depends how the circuit in the amp was designed.
Power tubes that have adjustable bias controls are called Fixed Bias amps. Yes, this is confusing and counter-intuitive. Biasing a power tube is like setting the idle on your car. If set too low, the car won’t idle and the engine dies. If set too high, the engine races and maybe overheats. A “correctly” biased power tube will run as it was designed, last longer and sound better. Power tubes biased too “hot” (too much current) will distort easier from being overdriven and will be shorter-lived. Power tubes are sometimes biased a little “cooler” to extend their life.
When you bias a power tube, you are adjusting the idle current to the grid when no source signal is applied and volume is set to minimum. Bias is sometimes measured in millivolts (mV) or milliamps (mA), see Ohm's Law explained below. It just depends where the test points are located to measure the idle current.
If measuring the voltage across a resistor (the most common method), then a digital multimeter (VOM meter) is used (VOM set to the DC, not AC range) and the mV reading is adjusted via one or more Bias Pots until the proper setting is achieved. Ideally, each individual power tube in an amp can have its bias measured and adjusted manually, but many times this is not the case and only one Bias Pot exists for all the tubes or one Bias Pot per tube pair exists.
Ohm’s Law:
Amperage = Voltage / Resistance or I = V/R or V = I*R or R = V/I
Tubes need to be biased at a specific amperage to operate properly, but we are usually measuring the voltage across a resistor, which translates to the proper amperage via Ohms Law.
If my amp is self-biasing, do I need to worry at all about the power tubes being properly biased?
Probably not, as long as all the electrical components in the self-biasing circuit are still within specification……and they probably are in most amps. How would I know if they are out of specification? (
need input on this topic)
What do coupling capacitors do in an a tube receiver or tube integrated amp?
Coupling caps are wired in series between the signal tube anode (plate) and power tube grids and are some of the most important capacitors in an amp because they can affect the sound quality the most of any small value capacitors in a tube amp. Typically, coupling caps are non-polar (doesn't matter which lead is soldered where) and are replaced with high quality paper in oil (PIO) or metalized polypropylene capacitors. Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass. Coupling caps wired in series block DC voltage and allow AC voltage to pass through to the power tube control grid.
What do phase inverter tubes do in a tube receiver or tube integrated amp or tube power amp?
In a push/pull amp type amp, the each phase inverter tube splits the right or left preamp signal into two halves. The two halves have opposite polarity and one half's polarity is flipped in the phase inverter circuit. Each half is then fed to one of the power tubes in the push/pull pair. There will be one phase inverter tube per pair of power tubes.
@Kegger adds: A phase inverter is used in push pull amps to send 1 phase to one output tube and opposite phase to the other output tube so the output transformer can recombine them to make a complete higher output signal an pass on to the secondary section of the output transformer.
What do the output transformers in a tube receiver, tube integrated amp or tube power amp?
Output transformers take the AC voltage signal from the output tube anode and step down the high voltage AC signal to a low voltage AC signal that is fed to the speakers. Output transformers can have 16 ohm, 8 ohm and 4 ohm outputs.
@GordonW further explains: The "signal" at the plate of an output tube is not actually true DC... there is a DC component, but it has an AC signal superimposed on it, due to the modulation of the plate current by the grid. This "varying DC" ACTS like "true" AC, with a DC offset to it, in essence.
One of the main functions of the output transformer is to remove this DC offset (which would make a speaker voice coil "jack out" or "jack inward" from the normal rest position- which would damage the speaker, as well as greatly increase distortion, as the speaker voice coil travel would no longer be symmetric), as well as to change the voltage/current ratios to work with the impedance of the speaker (which is much lower impedance than the output tubes themselves, in most cases- that's why a "step-down" output transformer is usually seen).
@Kegger adds: The AC music signal enters the grid of the input tubes then feeds all the way through the amp, through the coupling caps to the output tubes, then through output tranny.
At the same time DC voltage from the power supply is mixed in mainly at the plates an cathodes of the tubes with the AC music signal, the coupling cap stop the DC voltage from moving through from sections to sections, but lets the AC signal come through it.
And since a transformer won't pass DC, only the AC signal goes through the output to the secondary's to feed the speakers.
How do I protect the Power Transformer (PT) in my amp from sudden current surges and higher AC voltages that exist today (as compared to when these amps were originally built)?
Back in the 1950s and 1960s, AC line voltage in most homes was around 117V. Over the years, AC voltage has crept up to 120-125V in most homes. A thermistor(s) installed in series ib both legs of the AC power to PT input (primary winding) will lower the AC line voltage a little back to the 117V most amps were designed for. Thermistors will also protect your vintage tube amp from sudden current surges caused by capacitor failures downstream of the PT.
Kegger explains how thermistors work in this post:
http://www.audiokarma.org/forums/showpost.php?p=5757723&postcount=48
Polar electrolytic capacitors
Polar electrolytic capacitors, like those typically used in power supplies, must be installed with the correct polarity. Axial electrolytics usually have a "+" sign on the positive end (caps in 1st picture below) or a white band with negative signs pointing to the "-" end (top cap in 2nd picture below). Radial electrolytics (bottom cap in 2nd picture below) typically have a white band with negative signs in it pointing to the "-" negative lead. Pay special attention when replacing polar electrolytics, installing them backwards usually results in an explosion when they are powered up (don't ask me how I know
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What is a decoupling capacitor (e.g., what does it do)?
@dcgillespie explains it in this post
http://www.audiokarma.org/forums/showpost.php?p=5831904&postcount=5
What is the resultant capacitance if I wire two capacitors in parallel?
C total = C1 + C2
For example, let say a 1.5 uF and 2.0 uF capacitors are wired in parallel:
C total = 1.5 uF + 2.0 uF = 3.5 uF, the voltage rating is unchanged
What is the resultant capacitance if I wire two capacitors in series?
1/C total = 1/C1 + 1/C2
For example, let say a 60 uF and 60 uF capacitors are wired in series:
1/C total = 1/60 uF + 1/60 uF = 30 uF (after taking the inverse) and the voltage rating is doubled
What is the resultant resistance if I wire two resistors in parallel?
R total = (R1 * R2) / (R1 + R2)
R total = (10 ohms * 10 ohms) / (10 ohms + 10 ohms) = 5 ohms
What is the resultant resistance if I wire two resistors in series?
R total = R1 + R2
R total = 10 ohms + 10 ohms = 20 ohms
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