How does a tube amp work?

Tube amp terminology: http://www.aikenamps.com/AmpTerms.html

Squidward's Sticky on tube amp topologies - http://www.audiokarma.org/forums/showthread.php?t=163570
- Single-ended, push-pull and output transformerless (OTL) amp circuits explained

Kegger's explanation of how adjustable bias (also called Fixed Bias) amps work - http://www.audiokarma.org/forums/showpost.php?p=5674224&postcount=17

dnewma04's explanation of a simple tube amp circuit - http://www.audiokarma.org/forums/showpost.php?p=1843261&postcount=23

http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm

http://www.audioasylum.com/scripts/d.pl?audio/faq/tubeprimer.html

http://en.wikipedia.org/wiki/Vacuum_tube

Biasing a power tube explained: http://www.hans-egebo.dk/Tutorial/biasing.htm
 
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Continuation of Post #1

This thread starts here and continues here.

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.

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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 :D ).

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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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Hiya Scott,

Edit: an incredibly helpful book on this subject IMO is "Beginner's Guide to Tube Audio Design" by Bruce Rosenblit. I've read it several times, and I still can't really say that I fully understand everything, but the way that it explained the basics was invaluable to me. After doing so I was able to read tube data sheets and actually understand what the parameters there mean, which is really useful.

Ordered it today. Thanks for the recommendation Rob.
 
Would appreciate it is someone would enlighten me on this question/topic. I've not been able to get my head around it so far. I've read that the source signal, which is AC voltage, rides on top of th DC voltage signal once it enters the control grid of the preamp tubes. Does this AC voltage riding on top of DC voltage carry through all the way to the power transformer input?

What do the output transformers in a tube receiver, tube integrated amp or tube power amp?Output transformers take the DC voltage signal from the output tube anode and step down the high voltage DC 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.

One thing I don't understand is how the output tube DC signal to the output transformer is changed over to an AC signal at the output transformers output taps. Maybe the output transformer input signal is not DC after all, but rather the amplified AC signal from the source. Would appreciate someone chiming in and clarifying this for me.​
 
One thing I don't understand is how the output tube DC signal to the output transformer is changed over to an AC signal at the output transformers output taps. Maybe the output transformer input signal is not DC after all, but rather the amplified AC signal from the source. Would appreciate someone chiming in and clarifying this for me.[/INDENT]

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).

That DC offset is one of the main reasons why a push-pull "balanced" output has significant power and efficiency advantages to a single-ended design. In a single-ended design, that DC offset actually "magnetizes" the transformer core, to an extent. That limits the amount of signal that the transformer can process, before the magnetization reaches the limit of the core- at which time, the transformer "saturates", and the output becomes truncated at whatever max level the core can transfer.

OTOH, a push-pull amp has OPPOSING windings for the two output tubes. This causes the DC offset from one output tube, to CANCEL OUT the DC offset from the other. This has great advantages in allowing MUCH more signal power (AC) to be transferred through the same size transformer core, making higher output power amps cheaper and easier to build with PP output, compared to SE output. As long as the two output tubes are biased (essentially, this is the "DC offset" of the signal) the same, the DC components cancel out in the output transformer. This is why it's critical to have matched output tubes, or have adjustable bias, in a PP tube amp... so that these currents CAN be matched, and cancel each other out...

Regards,
Gordon.
 
To piggyback onto Gordon's excellent post an maybe break it down to a simpler form.

---

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.
 
My questions are so simple they are too stupid for experienced people to write about. For example what are the dashed lines within a tube schematic symbol? The check mark shaped lines? The solid lines?

Post a picture and be more specific as to the certain parts and we can certainly help.
(I say post the picture so it is one you want explained)
 
2 v DC _________________________________
1v DC ___________________________________
zero V DC___________________________________

An AC signal can "ride" on any of the DC voltage levels as an AC "component".
 
Some more questions for the tube amp experts.

Many tube amp schematics show that only part of the power transformer output goes through a rectifier to change it from AC to DC power. What parts of a tube amp use AC power and what parts use DC power?

Is the signal supplied from audio sources to the inputs of a tube amp an AC or DC voltage signal?

Can the heaters on vacuum tubes use either AC or DC power? If yes, why would one be preferred over the other?

What do phase inverter tubes do in a tube receiver or tube integrated amp or tube power amp? Or, maybe the questions are, Why does the phase need to be inverted? Or, the phase of what is being inverted?

Breaking this down to a "generic" tube amp.. (and majorly simplifying)

The heaters of the tubes use AC voltage to heat them, commonly 5v for rectifiers and 6.3v for the audio tubes.

The rest of the amp will use the dc voltage supplied by the rectifiers from the power tranny and will tend to
be applied to the plates of the tubes(driver tubes with a plate load resistor, outputs through an transformer)

The music signal from the source is AC.

Heaters can use AC or DC voltage, most tube amps use just AC as the signal is high enough that noise isn't a concern.
(DC voltage is used when wanting very low noise, say a preamp or phono stage, typically not the amp)

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.
 
Thanks Gordon and Kegger.....excellent answers to my question!!

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).

That DC offset is one of the main reasons why a push-pull "balanced" output has significant power and efficiency advantages to a single-ended design. In a single-ended design, that DC offset actually "magnetizes" the transformer core, to an extent. That limits the amount of signal that the transformer can process, before the magnetization reaches the limit of the core- at which time, the transformer "saturates", and the output becomes truncated at whatever max level the core can transfer.

OTOH, a push-pull amp has OPPOSING windings for the two output tubes. This causes the DC offset from one output tube, to CANCEL OUT the DC offset from the other. This has great advantages in allowing MUCH more signal power (AC) to be transferred through the same size transformer core, making higher output power amps cheaper and easier to build with PP output, compared to SE output. As long as the two output tubes are biased (essentially, this is the "DC offset" of the signal) the same, the DC components cancel out in the output transformer. This is why it's critical to have matched output tubes, or have adjustable bias, in a PP tube amp... so that these currents CAN be matched, and cancel each other out...

Regards,
Gordon.
 
It'll be important to narrow the scope of what you want to accomplish with this thread to keep it manageable.
For Crowhurst knowledge, I'd ask dcgillespie.
 
I would like to accomplish three things with this thread:

1. gain a basic understanding of how each major component in a tube amp works,
2. gain a basic understanding of how these components work together to amplify a source signal and feed it to a set of speakers,
3. understand basic maintenance tasks like biasing a power tube or replacing a coupling capacitor and why each is important.

Building a tube amp or modifying a circuit is outside the scope of the thread.

I already have a much better understanding of the first three items based on the posts already contributed!! :banana:
 
ok i will be awkward and ask why does it work ? rather than how does it work .. now then that's food for thought .:D:D:smoke::scratch2:
no offence intended ..its my humour at its worst ..
nice thread by the way and very informative:thmbsp::thmbsp:
 
OK guys, here's another capacitor question.

In earlier posts we learned that coupling caps block DC voltage and allow AC voltage to pass from the preamp tube anode (plate) to the power tube control grid.

So, if capacitors block DC voltage, how can they be used as DC power supply filter caps that smooth out pulsing DC power? I do know power filter caps are usually polar electrolytics and coupling caps are usually non-polar film or paper-in-oil caps. Does part of the answer lie in this difference?
 
The operative word is "filter". they are filter caps that filter AC variations/pulsations in the dc to be sent to ground, thus filtering/smoothing the DC, but letting it to pass on without much affect..The freq they need to filter is usually 120hz. A piece of kake for some large value cap say 40- 20000 uF or more, as needed. All depends on how much storage you need and how much current you're running.

Electronics 101... DC power supplies
 
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OK guys, here's another capacitor question.

In earlier posts we learned that coupling caps block DC voltage and allow AC voltage to pass from the preamp tube anode (plate) to the power tube control grid.

So, if capacitors block DC voltage, how can they be used as DC power supply filter caps that smooth out pulsing DC power? I do know power filter caps are usually polar electrolytics and coupling caps are usually non-polar film or paper-in-oil caps. Does part of the answer lie in this difference?

The Main difference is HOW there used, in coupling there in series to block the dc but pass ac,
in an power supply filtering there in "parallel" from positive to ground so as the dc voltage rides
across the positive with the ac ripple in there they attempt to fill in the holes in the not perfect
dc waveform from the rectifiers as they discharge and shunt the ac.

Look at a power supply an see how the caps are arranged, then look at them as a coupling cap
in an amp and you can see they are used in a different way, also non polarized caps like poly or
oil can be used in power supplies as they can take both AC an DC voltage, and for coupling you
want that non polarized cap so it can handle the same, polarized cap Only in the power supplies.
(polarized meaning electrolytic here)
 
Kegger, your explanation in these two paragraphs is just the kind of thing that is so hard to find elsewhere. Or maybe you explain it better! So, thanks.

Rather than worrying about being too systematic, this thread could be a place for Q&A on how tube amps work, plus cross posting of helpful explanations from posts to other threads?
 
The Main difference is HOW there used, in coupling there in series to block the dc but pass ac,
in an power supply filtering there in "parallel" from positive to ground so as the dc voltage rides
across the positive with the ac ripple in there they attempt to fill in the holes in the not perfect
dc waveform from the rectifiers as they discharge and shunt the ac.

Look at a power supply an see how the caps are arranged, then look at them as a coupling cap
in an amp and you can see they are used in a different way, also non polarized caps like poly or
oil can be used in power supplies as they can take both AC an DC voltage, and for coupling you
want that non polarized cap so it can handle the same, polarized cap Only in the power supplies.
(polarized meaning electrolytic here)

Great explanation Kegger. Thanks for your on-going contributions to this thread!!
 
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