Understanding a twin triode (LONG, lots of questions)

TelZilla · August 16th, 2007, 02:22 PM

OK, I'm a geek, and I'm thinking about building a kit amp, so I'm making an excel spreadsheet (I said I was a geek, I guess that proves it) detailing the info about all the components used, starting with the tubes. I'm trying to understand how these work, in terms of the flow of electrons and function of the separate parts.

So let's take v1 in a 5E3, which we'll say is a 12AX7. Here's what the data sheet says about the pins:
1,6- Plate (anode)
2,7- Control grid
3,8- Cathode
4,5- Filament
9- Filament Center tap

Just for fun, let's talk about the first triode (pins 1-4). I'll just put what I think is going on and you can correct me:

According to aiken (http://www.aikenamps.com/AmpTerms.html), the Plate is the current collecting element. in the 5e3, pin 1 is being fed DC current (PT Center Tap->Filter Caps-> Various Resistors-> pin 1). Pin 1 is also connected to the 0.1 uf coupling cap which connects to the channel volume. One thing I'm confused about is what the flow of electrons is in this part of the circuit. More on this in a while.

Pin 2 is the Control Grid, described by Aiken as "a wire mesh element located between the cathode and plate of an electron tube which controls the flow of electrons between the two elements. The control grid draws no current". I'm a little confused by this, as pin 2 looks like it's getting current (I think it's AC) from the guitar, through the input jack and the 68k resistors.

On to pin 3, the cathode. Aiken again: "the 'current generating' element of an electron tube. The heater heats the cathode to a very high temperature, causing it to emit electrons, which are then collected by the anode, or plate, which has a high positive voltage, which attracts the negatively charged electrons from the cathode. " On the 5e3, pin 3 is connected to pin 8 (the other triode's cathode), and then, through a parallell cap/resistor to ground. So how is this Generating current? Just through the fils?

Then pin 4 is a filament. Mr. Aiken, if you please: "The filament heats the cathode to a very high temperature, which "boils off" electrons, which are then collected by the plate." The fils can be traced back to the pilot light, which is "fed" by the PT @6.3V. So is this the way the cathode is able to generate current? Is this the only way that current from the wall socket is getting into the tube? And isn't this AC? As far as I can tell, there's no filtering between the PT, Pilot light, and the fils of V1.

So I guess my real questions are these:
~ When I pick a note, this alters the charge on the grid, which somehow sends electrons to the plate?
~ How is the current from the wall socket entering the tube? Just through pin 4? Pin 1 as well? If that's true, how can the plate (pin 1) be the current collector?
~What is the flow of electrons at the junction point of the 100k resistor which is fed by the filter caps, the 0.1uf coupling cap, and pin 1? I assume electrons are leaving the tube here, headed for the first volume control. are they flowing from the 100k resistor through the coupling cap, and on to the volume pot?

Sorry this is such a garbled mess. I promised myself I wouldn't build anything until I understood the function and connections of every element, and I'm trying to wrap my head around these concepts.

sjhusting · August 16th, 2007, 02:50 PM

Quote:

So let's take v1 in a 5E3, which we'll say is a 12AX7. Here's what the data sheet says about the pins:
1,6- Plate (anode)
2,7- Control grid
3,8- Cathode
4,5- Filament
9- Filament Center tap

Just for fun, let's talk about the first triode (pins 1-4). I'll just put what I think is going on and you can correct me:

Think of the first triode as pins 1-3. Think of the heater as something different. Normally you jumper pins 4 & 5 together, connect that to one side of the 6.3v, and pin 9 to the other side.

Quote:

According to aiken (http://www.aikenamps.com/AmpTerms.html), the Plate is the current collecting element.

Which is of course correct because electrons flow from negative (cathode) to positive (plate). The plate is at, let's say +420 V, the cathode is at maybe +12 to 20V (in a cathode-biased amp like the 5e3), and the grid is at 0 volts. This means that the grid is (relatively speaking) more negative than the cathode, because it is at a lower potential (i.e., less volts). How much more negative the grid is than the cathode is the "bias" and is a very important number which is dependent on all kinds of things.

Quote:

in the 5e3, pin 1 is being fed DC current (PT Center Tap->Filter Caps-> Various Resistors-> pin 1). Pin 1 is also connected to the 0.1 uf coupling cap which contols the channel volume.

This cap doesn't "control channel volume," it blocks the high-voltage DC from showing up at whatever follows. Caps block DC, pass AC (like, for example, a guitar signal).

Quote:

One thing I'm confused about is what the flow of electrons is in this part of the circuit. More on this in a while.

cathode->plate. Except in the phase inverter.

Quote:

Pin 2 is the Control Grid, described by Aiken as "a wire mesh element located between the cathode and plate of an electron tube which controls the flow of electrons between the two elements. The control grid draws no current". I'm a little confused by this, as pin 2 looks like it's getting current (I think it's AC) from the guitar, through the input jack and the 68k resistors.

The signal input to the grid (your guitar, or the previous stage) makes the grid more or less negative compared to the cathode. Negative repels negative, but the grid isn't (normally) negative enough to stop everything (that point is called "cutoff"). But the variations in more-or-less-negative of the grid affect (think of it as) "how many" electrons make it past the grid to the plate. Less negative = less repelling = more electrons to the plate; more negative = more repelling = fewer electrons to the plate. This is a very simple description of what happens, and I'm sure people will be happy to argue with me about it. But it's all you need to know right now.

Quote:

On to pin 3, the cathode. Aiken again: "the 'current generating' element of an electron tube. The heater heats the cathode to a very high temperature, causing it to emit electrons, which are then collected by the anode, or plate, which has a high positive voltage, which attracts the negatively charged electrons from the cathode. " On the 5e3, pin 3 is connected to pin 8 (the other triode's cathode), and then, through a parallell cap/resistor to ground. So how is this Generating current? Just through the fils?

You just explained it. Current is the flow of electrons. The filaments heat up the cathode so electrons break free and travel to the plate. That's current. The cathode generates current when it is heated by the filaments. This is called "thermionic emission" and is how tubes work. It's also known as "the Edison effect," after it's discover (but not exploiter), Thomas Alva, the light bulb guy.

Quote:

Then pin 4 is a filament.

Well, it's one end of the filament.

Quote:

Mr. Aiken, if you please: "The filament heats the cathode to a very high temperature, which "boils off" electrons, which are then collected by the plate." The fils can be traced back to the pilot light, which is "fed" by the PT @6.3V.

The pilot light is irrelevant. This is just an easy way to have a source of ~6V for the light bulb. Some amps use 120V instead, and get it from somewhere else.

Quote:

So is this the way the cathode is able to generate current? Is this the only way that current from the wall socket is getting into the tube? And isn't this AC? As far as I can tell, there's no filtering between the PT, Pilot light, and the fils of V1.

So I guess my real questions are these:
~ When I pick a note, this alters the charge on the grid, which somehow sends electrons to the plate?
~ How is the current from the wall socket entering the tube? Just through pin 4? Pin 1 as well? If that's true, how can the plate (pin 1) be the current collector?

Oh, this would take more time than I have. Go to Pete Millett's site (http://www.pmillett.com/tecnical_books_online.htm) and read Basic Theory and Applications of Electron Tubes, a great book which will explain everything.

Quote:

~What is the flow of electrons at the junction point of the 100k resistor which is fed by the filter caps, the 0.1uf coupling cap, and pin 1? I assume electrons are leaving the tube here, headed for the first volume control. are they flowing from the 100k resistor through the coupling cap, and on to the volume pot?

This is the plate resistor. It's function is to create a voltage drop. See the book above for a better explanation, or read the tech info at www.aikenamps.com. Basically, you need something which will create a bigger AC signal at the next stage than the AC input coming into the previous stage (i.e., "amplification") and you do that by sticking a resistor in there which reacts to the change in current (see above) by producing a varying voltage drop. This is the input into the next stage. Read the theory document at AX84.com; that is a great doc that explains everything very well, better than I can.

Quote:

Sorry this is such a garbled mess. I promised myself I wouldn't build anything until I understood the function and connections of every element, and I'm trying to wrap my head around these concepts.

We all go through this. Read the Army book. You need to understand this a bit better before you start mucking about with high voltage.

steven

TelZilla · August 16th, 2007, 03:05 PM

Steven-
Thanks for the replies. I will check out the army book. Interesting that a "fairly modern" book about tubes is from 1952. This is old technology for sure, but if it ain't broke...

Wow, this stuff is soooooo interesting. I totally dig it.

11 Gauge · August 16th, 2007, 03:43 PM

Despite all the pins on a tube, it is really cool at just how simple they really are - plate/screen/grid/cathode/filament is just about the whole shooting match!

When I'm designing a preamp, I like to sketch out some triode stages. Then with some math, I plug in the numbers I want for the 3 main spots that are tweaked on any gain stage...

...After you do this awhile, you can sort of 'sketch out' a preamp in the way that a composer writes sheet music. You can take into account how much the tube will conduct, how you will alter the bias, and when you get really comfortable with it, you can start tweaking the interstage impedance, compensate for 'insertion loss' created by tone stacks, and use the 'Miller effect' to your advantage (one of my favorites).

...You can also eventually glance at schematics and see what gives some amps some of their signature characteristics. Conversely, you can sometimes spot some Achilles' heel issues with some amps as well.

I'm a total amp geek, completely addicted by amp design. I'm currently working on a potential better solution (than the traditional ones) for reducing AC filament hum. Rather than switch to DC fils (which causes a mess of heat at the bridge rectifier), and rather than shield the surrounding components, I'm trying to figure out a way of shielding the filament leads themselves. Alternatively, I'm considering using a high amp 6 volt battery that will be recharged when the amp is not being used. While it's possible to make an amp very quiet with proper lead dress alone, the geek in me is not content with the idea of having an AC transmitter in the guts of my tube amps...

jjmantele · August 16th, 2007, 09:35 PM

For me the most important "revelation" was that a tube puts out pulsating DC. When this is presented to a cap, the other side of the cap "puts out" only the "pulses" which is AC (signal.) When it's run thru a transformer, the other winding on the tranny only "sees" and puts out the "pulses" which are AC (signal.)

The whole electrons-flying-thru-space-when-heated concept is pretty cosmic. I read that Edison noticed this could happen inside his light bulbs but he wasn't looking to invent amps. At least not that way.

sjhusting · August 17th, 2007, 01:24 AM

yeah, otherwise the idea that a voltage amplifier stage produces a varying DC voltage drop, which is then blocked by the coupling cap doesn't make much sense.

I got into this after I started playing again after a long break. I had sold all my beautiful old Fender BF amps when I moved to Europe

and when I finally bought a new tube amp (I thought I could just go out and buy a used SF Twin for 400 bucks

) I just wanted to finally know how these things work. This whole electrons-flying-through-a-vacuum business hit me as a beautiful, satisfying, elegant thing

steven

JohnnyCrash · August 17th, 2007, 03:00 AM

I don't have much to add... it is very late afterall and a lot was already covered :)

I'll throw in a few things too...

Quote:

Originally Posted by TelZilla

The fils can be traced back to the pilot light, which is "fed" by the PT @6.3V. So is this the way the cathode is able to generate current?

Some amps don't use 6.3v lamps. A light emiting diode (LED) can also be used. Think of the square Marshall lamps when you think LEDs. Some LEDs are 6v so you can run them off of the 6.3v heater taps the PT is spitting out... others are 120v so you can run em straight off of the primary.

I sometimes use these instead of typical amp jewel lamps.

Quote:

Originally Posted by TelZilla

Pin 2 is the Control Grid, described by Aiken as "a wire mesh element located between the cathode and plate of an electron tube which controls the flow of electrons between the two elements. The control grid draws no current". I'm a little confused by this, as pin 2 looks like it's getting current (I think it's AC) from the guitar, through the input jack and the 68k resistors.

Quote:

Originally Posted by TelZilla

So I guess my real questions are these:
~ When I pick a note, this alters the charge on the grid, which somehow sends electrons to the plate?

Its pretty much all there... you'll get to Screen Grids a little later.

Europeans call Vaccum Tubes "Thermionic Valves." This is exactly what it is... a valve. Like your kitchen sink or garden hose. The amount you twist forward or backward on the sink controls how much water FLOW you get.

The grid is sort of the gate keeper... or garden hose valve :)

In the old days batteries were used to supply the different voltages needed by the tube's electrodes. The first set, the "A Battery" gave the heaters/filaments voltage. They were usually 2, 4, or 6 volts - so a lot of tubes' heaters were designed and built around these voltage ratings (6 still sounds awfully familiar today hehehe). The second set were usually for the Plates' voltages. These second "B Batteries" (usually a ton of little batteries coupled together) are why we still call the higher voltage feed "B+".

Imagine how heavy a Twin would be if we still used batteries??!!

TelZilla · August 17th, 2007, 07:37 AM

Hey sjhusting, thanks for the tip on the army book. It's a great resource.

The fog is clearing...slowly.

sjhusting · August 17th, 2007, 09:11 AM

yeah, it really explains things well. I don't know why it's not recommended more often. You probably don't need anything after chapter 9, though, unless you're planning to work on transmitters. Chapter 9 is interesting if you're interested in tremolo.

steven

emu! · August 17th, 2007, 11:26 AM

Also a self-professed geek here. I built my first amp using the Dave Hunter book. It is written with the musician in mind. (no algebra). Since then, I have built two more amps and a tube stomp box. It's very addictive!

The Jack Darr electronics book is also very good. Look for it on the internet and print it out.

August 16th, 2007, 02:22 PM	#1 (permalink)
TelZilla Tele-Afflicted Join Date: Jan 2007 Location: Cleveburg, USA Age: 38 Posts: 1,762 6 Photos	Understanding a twin triode (LONG, lots of questions) OK, I'm a geek, and I'm thinking about building a kit amp, so I'm making an excel spreadsheet (I said I was a geek, I guess that proves it) detailing the info about all the components used, starting with the tubes. I'm trying to understand how these work, in terms of the flow of electrons and function of the separate parts. So let's take v1 in a 5E3, which we'll say is a 12AX7. Here's what the data sheet says about the pins: 1,6- Plate (anode) 2,7- Control grid 3,8- Cathode 4,5- Filament 9- Filament Center tap Just for fun, let's talk about the first triode (pins 1-4). I'll just put what I think is going on and you can correct me: According to aiken (http://www.aikenamps.com/AmpTerms.html), the Plate is the current collecting element. in the 5e3, pin 1 is being fed DC current (PT Center Tap->Filter Caps-> Various Resistors-> pin 1). Pin 1 is also connected to the 0.1 uf coupling cap which connects to the channel volume. One thing I'm confused about is what the flow of electrons is in this part of the circuit. More on this in a while. Pin 2 is the Control Grid, described by Aiken as "a wire mesh element located between the cathode and plate of an electron tube which controls the flow of electrons between the two elements. The control grid draws no current". I'm a little confused by this, as pin 2 looks like it's getting current (I think it's AC) from the guitar, through the input jack and the 68k resistors. On to pin 3, the cathode. Aiken again: "the 'current generating' element of an electron tube. The heater heats the cathode to a very high temperature, causing it to emit electrons, which are then collected by the anode, or plate, which has a high positive voltage, which attracts the negatively charged electrons from the cathode. " On the 5e3, pin 3 is connected to pin 8 (the other triode's cathode), and then, through a parallell cap/resistor to ground. So how is this Generating current? Just through the fils? Then pin 4 is a filament. Mr. Aiken, if you please: "The filament heats the cathode to a very high temperature, which "boils off" electrons, which are then collected by the plate." The fils can be traced back to the pilot light, which is "fed" by the PT @6.3V. So is this the way the cathode is able to generate current? Is this the only way that current from the wall socket is getting into the tube? And isn't this AC? As far as I can tell, there's no filtering between the PT, Pilot light, and the fils of V1. So I guess my real questions are these: ~ When I pick a note, this alters the charge on the grid, which somehow sends electrons to the plate? ~ How is the current from the wall socket entering the tube? Just through pin 4? Pin 1 as well? If that's true, how can the plate (pin 1) be the current collector? ~What is the flow of electrons at the junction point of the 100k resistor which is fed by the filter caps, the 0.1uf coupling cap, and pin 1? I assume electrons are leaving the tube here, headed for the first volume control. are they flowing from the 100k resistor through the coupling cap, and on to the volume pot? Sorry this is such a garbled mess. I promised myself I wouldn't build anything until I understood the function and connections of every element, and I'm trying to wrap my head around these concepts. __________________ Thanks,Nick Go, cat, go! Last edited by TelZilla : August 16th, 2007 at 02:56 PM.

August 16th, 2007, 03:05 PM	#3 (permalink)
TelZilla Tele-Afflicted Join Date: Jan 2007 Location: Cleveburg, USA Age: 38 Posts: 1,762 6 Photos	Steven- Thanks for the replies. I will check out the army book. Interesting that a "fairly modern" book about tubes is from 1952. This is old technology for sure, but if it ain't broke... Wow, this stuff is soooooo interesting. I totally dig it. __________________ Thanks,Nick Go, cat, go!

August 16th, 2007, 03:43 PM	#4 (permalink)
11 Gauge Friend of Leo's Join Date: Mar 2003 Location: The East Coast Posts: 2,833 10 Photos	Despite all the pins on a tube, it is really cool at just how simple they really are - plate/screen/grid/cathode/filament is just about the whole shooting match! When I'm designing a preamp, I like to sketch out some triode stages. Then with some math, I plug in the numbers I want for the 3 main spots that are tweaked on any gain stage... ...After you do this awhile, you can sort of 'sketch out' a preamp in the way that a composer writes sheet music. You can take into account how much the tube will conduct, how you will alter the bias, and when you get really comfortable with it, you can start tweaking the interstage impedance, compensate for 'insertion loss' created by tone stacks, and use the 'Miller effect' to your advantage (one of my favorites). ...You can also eventually glance at schematics and see what gives some amps some of their signature characteristics. Conversely, you can sometimes spot some Achilles' heel issues with some amps as well. I'm a total amp geek, completely addicted by amp design. I'm currently working on a potential better solution (than the traditional ones) for reducing AC filament hum. Rather than switch to DC fils (which causes a mess of heat at the bridge rectifier), and rather than shield the surrounding components, I'm trying to figure out a way of shielding the filament leads themselves. Alternatively, I'm considering using a high amp 6 volt battery that will be recharged when the amp is not being used. While it's possible to make an amp very quiet with proper lead dress alone, the geek in me is not content with the idea of having an AC transmitter in the guts of my tube amps... __________________

August 16th, 2007, 09:35 PM	#5 (permalink)
jjmantele Tele-Holic Join Date: Aug 2003 Location: New Jersey Posts: 843	For me the most important "revelation" was that a tube puts out pulsating DC. When this is presented to a cap, the other side of the cap "puts out" only the "pulses" which is AC (signal.) When it's run thru a transformer, the other winding on the tranny only "sees" and puts out the "pulses" which are AC (signal.) The whole electrons-flying-thru-space-when-heated concept is pretty cosmic. I read that Edison noticed this could happen inside his light bulbs but he wasn't looking to invent amps. At least not that way. __________________ JJman If it says "Vintage" on it -it isn't.

August 17th, 2007, 07:37 AM	#8 (permalink)
TelZilla Tele-Afflicted Join Date: Jan 2007 Location: Cleveburg, USA Age: 38 Posts: 1,762 6 Photos	Hey sjhusting, thanks for the tip on the army book. It's a great resource. The fog is clearing...slowly. __________________ Thanks,Nick Go, cat, go!

August 17th, 2007, 01:24 AM	#6 (permalink)
sjhusting Tele-Holic Join Date: Jul 2003 Location: Germany Posts: 576 11 Photos	yeah, otherwise the idea that a voltage amplifier stage produces a varying DC voltage drop, which is then blocked by the coupling cap doesn't make much sense. I got into this after I started playing again after a long break. I had sold all my beautiful old Fender BF amps when I moved to Europe and when I finally bought a new tube amp (I thought I could just go out and buy a used SF Twin for 400 bucks ) I just wanted to finally know how these things work. This whole electrons-flying-through-a-vacuum business hit me as a beautiful, satisfying, elegant thing steven

August 17th, 2007, 09:11 AM	#9 (permalink)
sjhusting Tele-Holic Join Date: Jul 2003 Location: Germany Posts: 576 11 Photos	yeah, it really explains things well. I don't know why it's not recommended more often. You probably don't need anything after chapter 9, though, unless you're planning to work on transmitters. Chapter 9 is interesting if you're interested in tremolo. steven

August 17th, 2007, 11:26 AM	#10 (permalink)
emu! Tele-Holic Join Date: Aug 2007 Location: Runnin' from the law Posts: 924	Also a self-professed geek here. I built my first amp using the Dave Hunter book. It is written with the musician in mind. (no algebra). Since then, I have built two more amps and a tube stomp box. It's very addictive! The Jack Darr electronics book is also very good. Look for it on the internet and print it out.