How A Valve Guitar Amp Works.

[DrNomis_44]

Hi Everyone,

I think we all have a bit of an idea how a guitar works, so I thought that I would start a thread about how a Valve Guitar amp works, I'm going to try and keep it relatively simple to understand, but I'll have to include a simple scientific principle which I think should help.


Here's the simple Scientific principle:


Like charges repel, un-like charges attract.


Okay, so, when we discuss how a Valve Guitar amp works, we need to first discuss how a Valve works, a Valve is an Electronic device which is used to control the flow of Electrons in an Electronic circuit, so what is an Electron?, an Electron is a very small atomic particle which has a Negative charge, in a basic atom, such as a Hydrogen atom, there is one Electron orbiting a nucleus made up of one Proton, a Proton is a very small atomic particle like an Electron but it has a Positive charge, so it attracts the Electron to it because the charges are un-like.


Back in around the late 1800's, Thomas Alva Edison, an American inventor, was trying to find a way to stop his recently invented Incandescent lamps from developing a black film on the insides of the glass envelope, incandescent lamps had an electrode called a Filament which glowed brightly when an electrical current was passed through it, Edison tried introducing a thin metal plate into the glass envelopes in an attempt to stop the blackening, and it was while he was doing some experiments that he noticed an interesting phenomenon, when the plate was made Negative, he noticed that nothing happened, but, when the plate was made Positive, he observed a small current-flow, this discovery led to the invention of the very first Electronic device, called a Diode (two-electrode), this new Electronic device was put to use in a device called a Radio, in 1907, Lee De Forest, another American inventor, created a new Electronic device, called a Triode (three-electrode), by placing a third electrode in between the filament and the metal plate in the glass envelope, Lee De Forest found that he could control the flow of current from the filament to the plate by applying a Negative-charge to the third electrode which he called a Grid (it was literally a grid of fine wire), he found that his new device, the Triode, could amplify, or make louder, weak signals, so it ended up being used in the early Valve Radios from the 1920's onwards, right up until the invention of a device called a Transistor in the mid 40's.

As it turns out, the Electron Valve proved to be a good device for making guitars louder, most guitarists these days seem to like the sound they get from a good Valve amplifier, although there are also some that like the sound of Solid-State amps too.


Anyway, going back to how a Valve Guitar amp works, when an electrical-current is passed through the filament, the filament heats up and glows a dull-orange colour, but something else also happens, the Electrons in the filament boil-off the surface (this process is called Thermionic Emission) much like how water-vapour boils off water when it's heated, you might have noticed this happening while making a hot cup of tea or coffee on a cold day, now because the plate is held at a high positive-charge, the Electrons (negatively charged) are attracted to the plate and a current flows, but if a negative charge is put on the grid, this repels the Electrons back to the filament, effectively stopping the current-flow.


I'll be continuing the discussion tomorrow, might also post some pics, so stay tuned


Note: if this thread needs to be moved to a more appropriate forum-section, please do so mods.


Note that if there are any forum members who wish to contribute to this thread, you're welcome to do so.

[Brendan]

Looks great DrNo... I've moved it to the PitBull Amps section - figured although it's a general thread, it fits nicely in here.

[DrNomis_44]

Looks great DrNo... I've moved it to the PitBull Amps section - figured although it's a general thread, it fits nicely in here.

Cheers mate, in hindsight, the amp section is where I should have put it in the first place, anyway, no harm done so it's all good.

[DrNomis_44]

Following on from my last post, I thought that I would post a pic of some Valves which are commonly used as Preamp Valves in Valve Guitar Amps, so here you go:





On the left is a 12AU7A, it produces the lowest gain of the three Valves you see in the pic, the 12AU7 Datasheet gives a figure for gain-factor of 19, so theoretically, the output signal is 19 times bigger than the signal at the input, next is the Valve in the middle and it is a 12AT7 with a gain-factor of 60, i.e. output signal is 60 times bigger than the input, and the Valve on the right is a 12AX7 with a gain-factor of 100, i.e. output signal is 100 times bigger than the input.


Personally, I haven't actually come across a Guitar Amp that uses a 12AU7A Valve in it's preamp, but I think the closest would probably be an original Fender Bassman (the yellow tweed model from the 50's) it used a 12AY7, which has a similar gain-factor as compared with the 12AU7A (they both have the same pin-out), on the other hand, the 12AT7 was used a lot in Fender amps as a Reverb Driver Valve, the Fender Twin Reverb is a classic example, and finally the 12AX7 was used a lot in the early Marshall Amps due to their high gain-factor of 100, the 12AX7 was also used in the Fender Twin Reverb, but Fender used the 7025, which is the lower-noise version of the 12AX7, you will also see a Valve called an ECC83 used in Marshall amps too, this is the British/European equivalent of the 12AX7, with the same technical specs, the 12AU7, 12AY7, 12AT7, 12AX7, 7025, and the ECC83 all have the same pinouts (the way the internal elements are connected to the Valve Base pins), and is as follows:


If you look at the base of the Valve with the pins facing towards you, you will notice that there is gap where a pin should be, the left-most pin to the gap is pin one, and the pins are numbered in a clockwise direction from pin one, each pin serves the following function:

Pin 1, Plate of Triode 1.

Pin 2, Control-Grid of Triode 1.

Pin 3, Cathode of Triode 1.

Pin 4, Heater Filament.

Pin 5, Heater Filament.

Pin 6, Plate of Triode 2.

Pin 7, Control-Grid of Triode 2.

Pin 8, Cathode of Triode 2.

Pin 9, Heater Centre-Tap Common.


Incidentally, because these Valves have two-identical Triodes within the glass envelope, they are all known as Twin-Triode Valves.


Here's a pic of a larger Twin-Triode Valve, called a 6SN7GT, it uses what's called an Octal-Base (8-Pins), from looking at the data sheet, it has a gain-factor of 70 for each Triode, i.e. the output is 70 times larger than the input, they can be used as a preamp Valve or as a Power-Valve, I have actually built a working Valve amp that used one of these as a Power-Valve so it does work:

[DrNomis_44]

Here, we will take a look at some commonly used Power Valves:


Firstly, here's a 6V6GT Valve which is used in some Fender amps, you could think of it as a lower-powered version of the 6L6 which is also used in Fender Amps:





Next is an EL34 Valve, these are commonly used in most Marshall amps:




And finally here's a KT66 Valve, these are apparently very similar to the 6L6, the KT66 was used in the Marshall Bluesbreaker Combo Amp, made famous by Eric Clapton during his tenure as a guitarist in John Mayall's Bluesbreaker band:




Unfortunately, I don't happen to have an EL84 Valve in amongst my stash of Valves, so if anyone can chime-in and post a pic of one, feel free to do so.

[DrNomis_44]

Here's an interesting little bit of trivia for you, did you know that the first Marshall Valve Amp, the JTM45, was based-on a certain Fender amp from the 50's?, yes, the JTM45 was based-on the Tweed Fender Bassman 4 X 10 Combo Amp which was originally intended as a Bass Guitar amp, but guitarists soon found out that it worked equally well as a guitar amp, as the story goes, Jim Marshall, who founded the Marshall company, asked his techs, Ken Bran and Dudley Craven to design a new amp after listening to some suggestions from local guitarists who frequented his shop, Ken Bran and Dudley Craven chose to base the new amp on the Fender Bassman because guitarists said that it came closest to the new sound of Rock they were hearing in their heads, and the rest is history.

[Muzza]

The Fender Bassman was is also very popular for blues harp.

I'll post a pic of an EL84 tommorow, unless someone beats me to it. I THINK one of my Orange amps has EL84's.

[DrNomis_44]

The Fender Bassman was is also very popular for blues harp.

I'll post a pic of an EL84 tommorow, unless someone beats me to it. I THINK one of my Orange amps has EL84's.

Cheers mate, yep, I would have to agree with you on that, the Fender Bassman in my opinion, has to be one of the most versatile amps of all time, I think that's probably due to the simplicity of the circuit, I certainly wouldn't mind building one from scratch, I guess it's versatility is one of the reasons why original 50's Bassmans command such high prices.

[DrNomis_44]

A couple of other types of Power Valves that you might see in some guitar amps are the 5881, and 6550, I seem to remember seeing a set of six matched 6550 Valves used in a Bass Amp once, I think it was the venerable Ampeg SVT, or I could be mistaken, Marshall used 6550 Valves in their Marshall Major 200 Watt amps.

One interesting observation is that, as guitar amplification evolved from the relatively low-powered models from the 1940, the Power Valves not only increased in power output, amp designers were using more of them in the power-amp sections of the amps, in the 40's most guitar amps used either a single Power Valve, or a couple of them, these early amps produced anywhere from 1 watt to about 5 or 10 watts at the most, during the 50's and 60's more powerful amps were designed and built due to the demands of guitarists wanting to be able to be heard clearly over the noise the drummer and crowds were making, Vox are a good example with their Vox AC15 and AC30 amps, the AC15 used two EL84 Valves to generate 15 watts of audio power, in response to requests for a more powerful amp, Dick Denney, who designed the AC15, designed the AC30 which used four EL84 Valves to generate 30 watts of audio power, the Vox AC30 circuit is much like the AC15 in other respects, the Vox AC30 was used by famous bands, like The Beatles, The Shadows, and The Rolling Stones to name a few, Queen's Brian May used a total of 18 Vox AC30 amps while on tour, the Vox AC30 has a very musical chiming sound to it.


Another thing to mention is that in the early days when Rock music was evolving, bands didn't have access to the high-powered P.A. systems that we have today, so they had to use high-powered guitar amps to be able to be heard, as a consequence, or maybe because bands could be heard better, the music became louder and heavier, apparently loud music tends to generate feelings of excitement in humans.

[stan]

great thread DrNo, good read