How does a tube amp work?

[jackevorkian]

Before someone gets their panties in a wad and tells me to do a search - I already have. I am just looking for the "in an nut shell" answer.

When the signal leaves my pedal board and hits the amp input jack, what is the sequence of tubes, etc that it passes thru before being heard thru the speaker? And, what is the function of the different tubes ie: preamp, rectifier, power?

Lastly, what are the "in an nutshell" differences with tube configuration...6V6, 6L6, ELXX, 12AX7 and what phase do they relate to
(are they preamp, power or rect tubes?)

Some of this info I think I know fairly well, but I don't really have a handle on the big picture...and it would be very helpful when reading different reviews to be a little more fluent. Thanks in advance!

[unklmickey]

Before someone gets their panties in a wad ...


...Lastly, what are the "in an nutshell" differences with tube configuration...6V6, 6L6, ELXX, 12AX7 and what phase do they relate to
(are they preamp, power or rect tubes?)...


firstly, i don't wear "panties". well not usually anyway.......unless she's cute and sorta kinky.:rotflmao

in a nutshell:

12AX7, 12AT7, 12AU7, etc.......... first tubes the signal sees as it enters the amplifier (pre-amp). the take the tiny signal from the guitar (about 1/10 of a volt, and amplifiy it to tens of volts.

6v6, 6L6, EL34, EL84, KT88, etc......... output (power) tubes take the signal that is now of sizeable voltage, and drive the output transformer, the last link in the signal chain.

5U4, 5AR4/GZ34, 5Y3, etc............rectifiers. these tubes are not in the signal chain. they take the AC from the power transformer, and act at one-way valves, converting the AC to DC.


hope that helped,

unk

[brad347]

Really quickly:

The basis of tube audio amplification can be illustrated with description of a simple "triode" tube. The triode is so called because it has three electrodes, or "parts." The cathode, anode (or plate), and the control grid.

Electrons are attracted from the cathode to the plate. They can flow in one direction only. There are high voltages going to the plate. It is a very large electron flow. A signal is introduced onto the control grid. The grid lives between the cathode and plate. A small electrical signal on the grid 'modulates,' or causes changes in, the large signal going from the cathode to the plate. The small voltage "controls" the larger voltage.

Imagine a light projector as a cathode. Then imagine a big huge screen is the plate. Imagine taking your hand and making a shadow puppet between the light and the screen. It causes the image/shadow of your hand to become HUGE on the screen, but your hand hasn't changed sizes. Your hand is acting as the control grid. This is a drastic oversimplification, but illustrates the basic principles of tube amplification. When an "image" of a louder signal is created, we have what is called a "gain" in the signal. "Gain" does not mean distortion, it means exactly what you think it would... a "gain" or increase of volts from one stage to the next.

The preamp tubes in your amp amplify the signal in this way a little bit.
The power tubes amplify it a lot.

These tubes need high DC voltages on their plates to operate properly. The wall outputs AC power. The power transformer raises or lowers the voltage to the voltage needed. Your rectifier tube "chops off" half of this AC to make it a pulsating DC. Then the 'gaps' between those pulses are 'smoothed out' by your filter caps and choke to make something like battery power.

You may have additional gain stages that 'cascade' into one another to achieve distortion. You may have a gain stage for "driving" the reverb springs, and another to make up for the loss encountered in the highly inefficient reverb system.

You may have one tube that is set up to 'oscillate' in order to control a vibrato or tremolo.

Your "phase inverter" serves to split the signal in half so that each half can be amplified separately. In a class A/B amp with two power tubes, each tube amplifies one half of the signal. The two halves are of opposite polarity, in terms of audio. The phase inverter circuit and the tube in it is what divides and flips the polarity of one of the halves. This is done for maximum efficiency, and they are re-combined into a single signal at the output transformer.

Hope this helps some! :)

[rmconner80]

Think of a basic theoretical tube as a "triode". It comprises a control grid, a plate (anode), and a cathode. A 12AX7 is excactly that - a triode (actually a twin triode - meaning it has two independent triodes in it)

The fourth element is the heater. The heater gets a low AC voltage (in guitar amps, usually 6.3V from the filament circuit) and heats the cathode either directly or indirectly. The cathode is an element generally covered with some metal or other substance which, when heated, emits large quantities of electrons. Over time, cathodes slowly wear out their ability to emit electrons - hence one aspect of tube wear.

The anode or plate is held at a positive DC voltage (the B+ circuit). This means that the negatively charged electrons emitted by the heated cathode travel at light speed through a small internal space in the tube towards the plate.

Were it not for the control grid, this tube would be conducting these electrons near 100% idle current, all the time.

Instead - we use the control grid. The grid is an element between the cathode and the plate. It serves to interupt, or to enhance, the flow of electrons from cathode to plate.

This is where your guitar signal comes into play. It is an AC voltage, and it is applied to the grid of a tube. The control grid is arranged physically inside the tube in such a manner that the tiny mV AC signals of a guitar can have a big effect on the larger voltage gain of a tube.

A tube's bias refers to the idle current conducted between cathode and plate when no signal is applied. Even 12AX7s are biased.

So basically, your tiny guitar signal goes in at the grid and then is amplified by controlling the flow of electrons from cathode to plate. The voltage gain or AC signal is now much larger that it was before, when it "comes out" at the plate. This AC signal is basically "riding on top" of the DC power supply, in a manner of speaking (remember the B+ is also "attached" to the plate).

Theoretically, this same pattern is followed in the next triode, and so forth; at each stage the signal gets bigger and bigger. 12AX7s are quite limited in the amount of grid voltage swing they can get. Frequently they get too much, and they distort, clipping off the ends of the AC sine wave. This loud distorted signal can be attenuated or shaped by tone controls, and then amplified again cleanly as a distorted signal in the next stage, or if the magnitude of the signal is high enough, can distort the next tube as well.

The power tubes (6V6, EL34, 6L6, etc) work in much the same way, with a couple of important exceptions. These power tubes are not triodes, they are pentodes or tetrodes, meaning they have additional elements within the tube, not just a control grid, a plate, and a cathode. Usually this means the addition of extra grid elements, such as a screen grid, to assist in the amplification, and also idle current of the tube.

Rectifiers are a bit different and serve to create pulsing DC from AC. This supplies the B+ power supply. The pulse is mitigated by filter caps, which shunt AC to ground, while discharging DC back into the power supply when they are not charged - i.e. at low points in the pulse. Therefore, you get a relatively stable or relatively primitive (depending who you talk to) DC power supply in your amp.

[bob-i]

Sounds go in.... get louder... come out.... :lol:

[Old Tele man]

...a LITTLE signal voltage (0.1V) from guitar goes into PREAMP tube

...PREAMP tube amplifies the LITTLE voltage about 100-times to MED voltage.

...MED voltage goes to TONESTACK which "shapes/filters" frequencies but also "shrinks" MED voltage down to LOW-MED voltage.

...LOW-MED voltage goes to VOLTAGE amp (BIG-preamp) which amplifies signal back up to about 1V.

...1V signal goes to PHASE-INVERTER which "splits" signal into UPPER & LOWER "halves"...and depending upon PI circuit AMPLIFIES (~10-40 X) signals.

...the separate UPPER & LOWER signals each "drive" one half of the PUSH-PULL output.

...the OUTPUT transformer "reassembles" the two "half" signals from the PUSH-PULL tubes (visualize "milking a cow" here--left, right, left, right...) and matches "high" TUBES impedances to "low" SPEAKER impedance.

...and, that's an amplifier!

...basically, its a reverse-version of a POWER SUPPLY, in that it takes DC-power and makes AC-signals/power; where as POWER SUPPLY takes AC and makes DC-power from it.

[avwalker]

I thought it was all just smoke and mirrors....with a dash of pixie dust

:jo

[KeithC]

As I told my foreign language teacher in high school..."this Spanish is Greek to me.." ;)

But, I did glean some good knowledge in this thread. Still Greek to me but I think I understand it better thanks to the thoughtful explanations.

Once again, TGP rocks!

[Mickey_C]

Nutshell answer:

A tube amplifier is a modulated power supply driving an electro-mechanical transducer fluctuating the air-pressure.

[Mickey_C]

Hey - I like that, even if I do say so myself.

;-)

[unklmickey]

I thought it was all just smoke and mirrors....with a dash of pixie dust

:jo


Shhhhhhhhhhhhhhhhh! it IS. just don't tell nobody. that's supposed to be a secret!

[Old Tele man]

...but the real secret is the use of "Thermionic Pixie" dust...called electrons.

[JimmyR]

I don't think we should question why tube amps work. If we investigate too much we might find that they are actually impossible and then all our amps won't work any more.

Hey - I should start a religion or something!

[ClinchFX]

PFM - Pure F***ing Magic.:D

Alternatively, electronic components work on the smoke trapped inside. When something goes wrong and the smoke gets out, they don't work anymore.:messedup

Peter.

[Old Tele man]

...that "smoke" is actually the thermionic pixie dust manifested in the visible frequency range...when it's manifested in the audible range its called MUSIC!

[epluribus]

So how did all those little people get inside my television?

ps: Pittman's The Tube Amp Book or Hunter's Guitar Amp Handbook explain these things very nicely. The amps, not the people I mean...

[warren0728]

So how did all those little people get inside my television?

haven't you seen "Willy Wonka and the Chocolate Factory" your answer is in the movie! :dude

ww

[Thomas D]

Go here:

http://www.ax84.com/?pg=associateprojects&project_id=p1

Download the theory document (PDF file.) It will take you step by step through each component of the amp.

[vibroverbus]

magic "cathode" fairies actually, trapped inside each vacuum tube. the so called 'heaters' are really electrodes that shock and agitate the fairies until they begin to throw off magic particles (AKA thermionic pixie dust). the music signal is then applied to a miniaturized dream-catcher known as a 'control grid' which uses supernatural forces to control the pixie dust from escaping via a multi-dimensional portal known as the 'anode' or 'plate' which in turn transmogrifies the pixie dust back into a larger electrical signal which can then be carried to the next stage.