How A Valve Works Part 1

Wilder Amplification · 10-19-2009, 03:44 PM

A forum member requested that I write up an article regarding amplifier classes of operation, i.e. Class A, Class AB, Class B, etc etc. As I was thinking about how I would write the article, I came to the conclusion that in order to gain a full understanding of amplifier classes of operation,there first needs to be an understanding of how a valve works. So this article is the first of a series of articles regarding power amp operation.

I have divided this article into two parts to avoid overwhelming you with too much info to digest all at once. Don't try to read and fully understand it all in one setting...there's a lot of material to remember. Read some...take a break...come back and read some more, re-read if you have to. Don't try to digest it all at once as you will find it confusing if you try to and will quickly lose interest.

What is a valve?

More commonly known as a vacuum tube in the US, a valve is basically an electrical "valve" that uses an externally applied voltage to control the electrical current that flows through it. It can be thought of as a voltage controlled pot. Instead of having a mechanical assembly and allowing you to control its resistance by rotating it like a pot does, it uses an applied voltage to control its effective "plate resistance", which controls electron current flow through it.

The first type of valve was called the "diode" valve. Diode is a Latin term, "Di" meaning 2 and "Ode" meaning "elements". Inside of a diode tube you have two active elements excluding the filament, or "heater".

The diode tube has a "cathode" and an "anode". The cathode can be either the heater itself (directly heated cathode type...heater is referred to as the "filament" in these types of diode tubes, i.e. 5U4, 5V4) or it can be a metal sleeve that is wrapped around the heater (indirectly heated type, in which the filament is referred to as the "heater" since its only purpose in these tubes is strictly to "heat" the cathode). Examples of indirectly heated cathode type diodes are the 5AR4/GZ34. Also, most power and preamp tubes are the indirectly heated cathode type.

The cathode is typically connected to the negative side of the power supply, which has an excess of electrons as compared to the positive side. When the cathode is heated, there is a coating on the cathode that will emit electrons from the cathode itself. This is known as "thermal emission of electrons. This creates a "space charge", which is basically a "cloud" of electrons that are being boiled off of the cathode. This cloud of electrons surrounds the cathode in the free space around it.

Electrons are negatively charged. Opposite charges attract while like charges repel, just like magnets. By having an excess of electrons on the negative side of the power supply, this is what gives the negative side of the supply its negative charge.

Just the same, the positive side of the supply has a positive charge. This is due to there being a shortage of negative electrons on the positive side. Since the negative side has more electrons than the positive side does, this creates a charge difference, which we see as a "voltage". This voltage is a force that wants to pull the electrons from the negative side of the supply to the positive side in an effort to neutralize the charges. Again opposite charges attract.

Back to the diode valve. So now our cathode is heated and is connected to the negative side of the supply. This means that our heated cathode has a cloud of electrons that are being boiled off of the heated cathode and waiting to go somewhere.

Let's place a plate (known as the "anode" plate, or more commonly referred to as the "plate") inside the valve and place it in close proximity to the cathode, but not physically touching the cathode. If we connect the positive side of the power supply to this plate, thereby positively charging the plate, the negative electrons that are being boiled off of the heated cathode will flow through the space between the cathode and plate right to the plate...and back to the positive side of the supply.

However, if you try to reverse it so that the electrons are on the plate and the cathode is on the positive side, the cathode will have no electrons to boil off of itself because they're on the plate. Since the plate is not heated, it cannot emit electrons so all current flow stops. Thus, the diode valve acts like an electrical "one way valve", hence the name.

OK, so let's go back to our original circuit. We now have the cathode connected to the negative side of the power supply (which in amplifiers happens to be ground) and the plate connected to the positive side and current is flowing. Well, since a diode valve has a fixed value of plate resistance, the only way we really have to control current flow through the valve is by changing the voltage applied across the cathode and the plate. But what if we wanted to control the plate's voltage by contolling the current flow through the valve? We would have to have a way of controlling the valve's effective plate resistance to have a way to increase/decrease current flow through it.

This is where the valve now becomes a TRIODE (3 element) valve. A 3rd element is placed between the cathode and the anode plate. This element is known as the "grid". This name comes from the fact that the first grids were basically a metal "screen", much like window screen material, that is placed in between the two original elements. Later, the grid became a spiral of very thin wire wrapped around two posts set a distance apart from each otehr and most of the electrons pass right through the space between the spiraled wire. Current flow from the cathode to the plate passes right through the holes in the grid (VERY little of it actually flows onto the grid).

By applying a voltage force to this grid, we can now increase or decrease current flow through the tube. If a positive voltage is applied to the grid, this causes more current to flow from the cathode to the plate because the positive charge on the grid pulls more negative electrons from the cathode. Again, most of these electrons pass right through the grid and end up at the plate, so current flow to the plate (i.e. plate current flow) INCREASES. Applying a positive voltage to the grid basically decreases the valve's effective plate resistance by adding a second positive voltage to the grid to assist in pulling more electrons from the cathode.

Now let's apply a negative voltage to the grid. Remember like charges repel? Now the negative electrons from the cathode are being repelled by the negative voltage on the grid. This creates a "resistive field" between the cathode and the grid that the electrons must pass through, which effectively increases the resistance between the plate and the cathode, known as the valve's "plate resistance". As a result of the plate resistance increase, current flow to the plate DECREASES.

A small voltage change on this grid creates a large current flow change through the valve from cathode to plate, which in turn creates a voltage change on the plate that is larger in amplitude than the voltage change on the grid. Example, a 1 volt change on the grid might create a 10 volt change in plate voltage. We now have amplification.

Now we get into PENTODE valves. A PENTODE (5 element) valve is a valve with...yep you guessed it...5 active elements. We now have 2 more grids...the "screen grid" and the "suppressor grid". The original grid is now referred to as the "control grid". We'll get into why in a bit.

The screen grid is a 2nd grid placed between the control grid and the plate. It typically has a positive voltage applied to it that is just a few volts lower than the plate. Inside the valve you have electrodes (triode = cathode, grid and plate) that are placed in close proximity to each other. This creates a capacitance that exists between the electrodes, much like how a capacitor has two metal elements in close proximity seperated by a dielectric. In the case of a valve, the free space in the valve makes a dielectric.

The purpose of the screen grid is to reduce this capacitance that exists between each element, known as "interelectrode capacitance". This increases the efficiency of the valve.

So now we have an amplifying tube. However, as current flow to the plate increases, so does the temperature of the plate. Eventually the plate may reach a temperature hot enough for the plate to emit electrons. This will cause some of the electrons that flow to the plate to be emitted from the plate. This is known as "secondary emission of electrons". These electrons would normally get sucked up by the positive charge on the screen grid, causing some power loss. This is where the "suppressor grid" comes in.

The suppressor grid is connected to the negative side of the power supply. Its job is to "push" these stray electrons emitted from the plate back onto the plate. The negative charge on it from being connected to the negative side of the supply creates a repelling force that repels electrons (electrons being negative...like charges repel) back onto the plate. This reduces power loss within the valve by not allowing the stray electrons emitted through secondary emission to be sucked up by the screen grid, hence they are forced to flow to the plate, and through the output transformer.

Stay tuned for Part 2 to this article.

HOT TUBES 70 · 10-19-2009, 11:21 PM

HOLY FUCK MAN !!
thats awesome ........

tubes eh........

thrawn86 · 10-20-2009, 12:24 AM

This is awesome, Jon. Thank you so much once again!
PS....don't burn your eyes or fingers out writing all this stuff!!!

HOT TUBES 70 · 10-20-2009, 12:46 AM

yes , thanks for taking the time to do this , this forum has the best cats around !!

MajorNut1967 · 10-20-2009, 02:08 AM

Yes again thanks! I too old and grumpy to explain it and you did a fine job!
+1

Wilder Amplification · 10-20-2009, 03:41 AM

Thanx for all the kind words guys. I enjoy typing these articles up and being able to break it down "Barney style" since lots have tried to understand other people's books and the theory just overwhelms them, or the writer didn't use the correct things to relate things to. Something tells me by the time I'm done I'll be able to write a damn book. lol

Things are about to start getting interesting...amplifier operation like you've never seen it explained before.

Gonna take me a few days as I'll need to draw up some diagrams to help you guys better visualize what's happening. But this is gonna be good.

I wonder if there's any way to get the moderators to either make my tech articles a sticky, or create a subforum dedicated to tech articles.

I'm also working on a Wiki page for my website as well, known as ValveWiki. I've kinda taken a break from the Wiki, but typing these articles I think has motivated me to get back on that project.

PaoloJM · 10-20-2009, 05:47 AM

Wow, that's a heck of a lot of educated, well thought out and accurate writing there!!

I could never have the patience required to do anything like that.
Looking forward to your next installment.

Wilder Amplification · 10-23-2009, 01:42 AM

Quote:

Originally Posted by PaoloJM

Wow, that's a heck of a lot of educated, well thought out and accurate writing there!!

I could never have the patience required to do anything like that.
Looking forward to your next installment.

Glad you enjoyed the article! Next installment is posted.This next one explains general series circuits. In order for my Part 2 of this article to make sense the way I plan to explain it, a basic understanding of series circuits must be obtained. Figured that will keep the forum busy while I work on typing up part 2.

Sir Don · 10-24-2009, 08:14 PM

Thanks for this great info.

KickStart · 11-01-2009, 08:02 AM

Make is sticky please.

Wilder Amplification · 11-01-2009, 01:09 PM

Really glad that you guys are enjoying my tech articles. Should have How A Valve Works Part 2 up by sometime Tuesday so stay tuned.

If you haven't done so already, be sure and read up on "Intro To Series Circuits" as it will help you to understand the material that will be in Part 2.

10-19-2009, 11:21 PM	#2 (permalink)
HOT TUBES 70 Senior Member Join Date: Oct 2008 Location: SASKATOON Posts: 1,675	Re: How A Valve Works Part 1 HOLY FUCK MAN !! thats awesome ........ tubes eh........ __________________ MARSHALL 6100LM MARSHALL 1960A W/-G12H 30's CUSTOM BUILT 2X12 W/-V30's "MARSHALL" - only the best will do !

10-20-2009, 12:24 AM	#3 (permalink)
thrawn86 Senior Member Join Date: Aug 2009 Location: Far Nor Cal USA Posts: 426	Re: How A Valve Works Part 1 This is awesome, Jon. Thank you so much once again! PS....don't burn your eyes or fingers out writing all this stuff!!! __________________ Drivin' faster in my car... 02 DSL 401-Ei's 02 1960b 02 Gibson LP Studio Silver 03 PRS Santana SE Holy Grail reverb Originally Posted by J. Robert Oppenheimer "We knew the world would not be the same."

10-20-2009, 12:46 AM	#4 (permalink)
HOT TUBES 70 Senior Member Join Date: Oct 2008 Location: SASKATOON Posts: 1,675	Re: How A Valve Works Part 1 yes , thanks for taking the time to do this , this forum has the best cats around !! __________________ MARSHALL 6100LM MARSHALL 1960A W/-G12H 30's CUSTOM BUILT 2X12 W/-V30's "MARSHALL" - only the best will do !

10-20-2009, 02:08 AM	#5 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 710	Re: How A Valve Works Part 1 Yes again thanks! I too old and grumpy to explain it and you did a fine job! +1 __________________ MJN1967 aka BA (Bad Attitude) 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

10-20-2009, 05:47 AM	#7 (permalink)
PaoloJM Senior Member Join Date: Jun 2008 Location: Cork, Ireland Posts: 534	Re: How A Valve Works Part 1 Wow, that's a heck of a lot of educated, well thought out and accurate writing there!! I could never have the patience required to do anything like that. Looking forward to your next installment. __________________ www.youtube.com/paolojm

10-24-2009, 08:14 PM	#9 (permalink)
Sir Don Senior Member Join Date: Aug 2009 Location: Adelaide, South Australia Posts: 139	Re: How A Valve Works Part 1 Thanks for this great info. __________________ My gear: 2204, 2210, 2555, 9005, 1959, 5005, 4103, 2100, TSL100 www.myspace.com/rockthebosss

11-01-2009, 08:02 AM	#10 (permalink)
KickStart Member Join Date: Sep 2009 Location: Ga Posts: 37	Re: How A Valve Works Part 1 Make is sticky please. __________________ Turn that shit up... I can still hear myself think! Marshall JCM 800 Lead Series 2203 Dean Markley Over Lord, Cry Baby Wah Pork Loin ART SGX 2000/X-15 Midi Foot Controller Model 4 Charvel

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