How does a Tube Amp work Part 1

[HAMPAMP TUBE AMP SERVICES]

How does a Tube Amp work pt.1
This is a series of threads that I hope will help explain the workings of standard Tube Guitar Amps to those
who have no idea what's going on. I will try to make it simple and easy to understand but sometimes in an
effort to simplify I can complicate things even worse, so hopefully some of the other TECHs and otherwise
knowledgeable people on this forum will help out (Are you there Marty?). I'll be using a schematic found
on Dr Tube's website (I'll try to put up a link but not sure how to do that) drawn by Joe Piazza. It can be found
under the JMP listing entitled : 1987 Plexi http://www.drtube.com/schematics/marshall/1987pljp.gif. First
we will look at the Power Supply then we will move on to the input jacks and go through the amp until we
reach the speaker jacks. Enjoy the ride but it may be bumpy at times.


Power Supply
The job of the power supply is to take AC 120 volts from your wall outlet and change it to AC/DC voltages required
to operate your amp. Before we start, lets define some terms that will be used in the text. Note: If you are located
anywhere other than the United States, your AC system may be 220volts 50Hz

Voltage: Electrical pressure or the speed at which electricity flows unit of measure= volts or V

Current: The amount of electricity that flows past a certain point unit of measure = amperes or A

Wattage: The amount of Voltage multiplied buy the amount of Current. Also known as Power unit of
measure = watts or w

Resistance: The opposition to the flow of current unit of measure = ohms

AC current: Alternating current, This is a current that contains peaks that alternate between positive and
negative. Each alternation of positive and negative peaks is called a "cycle". In the standard
household system this alteration occurs 60 times per second and is called a 60 cycle system.
This is also referred to as 60 Hz.(or the Frequency of the cycle)
Your guitar signal is a low voltage AC signal, the strings vibrate at a certain frequency(or Hz).
The "A" string vibrates at 110 cycles per second or 110Hz.

DC current: Direct current, This is a current that flows in one direction, either positive "or" negative with respect
to ground (or zero volts)



Okay, now we are ready to start. If you look at the diagram on the bottom right-hand side is the AC input,
you should be able to identify the symbol that represents the AC plug. The top and bottom wires are marked ' L'
and "N" respectively, the center wire is labeled "E" or earth, another word for ground. This wire is connected to the
chassis.
The first component we come to is a fuse labeled as 2A, this fuse is a safety device intended to "open"
if the current draw on this end of the transformer exceeds 2 amperes. The next component in the line is the
"Mains switch". this switch allows current to flow through the transformer and is the main on/off switch for the
amp. The next component is the "Mains Voltage Selector". The Power Transformer as several taps on this end
which is called the "Primary" these taps allow for a different voltage to be selected and still have the desired
voltage present at the "Secondary".
The Power Transformer is the heart of the tube amp, it supplies the various voltages needed to work the
amp. Without getting too technical a transformer can step up or step down voltage and may have many different
sections to supply several voltages at one time. The input side is called the primary and in this case will accept
110 VAC and step this voltage up to around 300-320 VAC and at the same time on another section it will
step-down to 6.3 VAC. Some amps will have additional sections to supply voltages for IC circuits and separate
Bias circuits. In this case there are only two different sections on the secondary or output of the transformer
(not to be confused with an output transformer, more on that later). At the very bottom of the page is the 6.3
VAC supply for the filaments of the tubes. This section of the transformer has a center tap that goes to ground,
and a pilot light across the supply lines to indicate that the amp is on. this supply is on when the "Mains"
switch is engaged. Each vacuum tube has elements that need heating, the Cathode in particular so that it can
give off electrons to make the tube work. These elements must be heated prior to the unleashing of High-voltage
on the tube or it can make tube life decrease quickly so this section comes on with the "Mains" switch.
Above that on the diagram is a double pole switch labeled "Standby". This section of the transformer supplies
the High-voltage or High-tension (HT) to the circuit. After the switch are two .22uf capacitors, these are there to
suppress spikes in the line when the standby switch is made.
Next in line are two "Diodes" or "rectifiers", these rectifiers serve as a means to convert AC current into DC
current. In this case, the high voltage goes into the rectifiers from the top and bottom legs of the secondary windings,
( the center tap is again grounded), the end of the rectifier that the windings are attached to is called the "anode",
this rectifier will allow the positive half of the AC voltage pass and reject the negative half. Now this gets a little
complicated, but the upper half and lower half of the windings are 180 degrees out of phase with each other,
so that when the upper half is positive the lower half is negative and vice versa. Now since AC voltage has
positive and negative peaks at the rate of 60 Hz and the rectifiers allow only the positive peaks through, the
result is positive going peaks from the rectifier at a rate of 120Hz. This is considered to be DC since it only
contains positive peaks but a pure DC has no peaks, it is constant so we need to make our DC more pure.
This is the job of the "Filter Caps". Think of a filter cap as a storage device, when the positive peaks are
present the capacitors are "charged" like a bucket of water being filled. when the peak goes away leaving a
valley, the cap will discharge and the valley will be filled. this is not a perfect action and several caps are used
to perform this. Even then the DC is not pure, there is still what is called "ripple" present on the DC line. Ripple
is just a small amount of peak and valley left on the DC. This is what creates "hum" in a tube amp. If the caps
are bad, the hum is worsened. The filter caps are the parts on the diagram marked "50uf", They are connected
from the B+ line to ground. There are two of them followed by a second fuse marked "0.5 A", this is the HT fuse
and serves the same purpose as the mains fuse, if the HT current is above .5 amperes the fuse will open.
Next comes a part called the "Filter Choke", Without getting technical,this part also helps to smooth out the
DC voltages as does the 50uf that follows it. Point "A" dropping down just ahead of thr filter choke is the supply line
connected to the center tap of the "Output transformer" to supply V4 and V5 with the plate voltage necessary to
operate those two tubes, somewhere around 390 VDC. Point "B" on the other side of the choke will supply about
380 VDC to the "Screen Grids" of those same tubes. The filter choke has some resistance to DC which is why
there is a 10 V difference between the input and the output of the choke. The plates of V4 and V5 do not need to
be filtered as much as the screens do so the plate voltage is taken off before the choke.
After passing through the choke and past the 3rd 50uf cap , the B+ line turns north to pass through 2 10k
resistors on their way to the preamp circuits, these resistors drop the voltage further due to the fact that the
preamp stages use a lower value of DC voltage than the output tubes.
There is one other section that branches off from the upper secondary winding of the transformer, this is the
"Bias Circuit", it will be explained later when we come to the Output Tubes in our look at the signal path.

Well that is our look at the Power Supply in our 1987, hope you're a little less confused or at least that you are
confused on a higher plane. The next installment we will start at the input jacks and make our way through following
the signal path until I get burned out again. This ends Part 1 of How does a Tube Amp work.

 

[MartyStrat54]

Hell of a thread. Why I might just hand-wire myself a point-to-point amp. The only thing I have to add is that the older Marshall's had a tube rectifier. These rectifiers ran on a heater voltage (5 volts) off of an additional tap on the ole power transformer. Many old amps with tube rectifiers have a number on the tube that begins with the number 5. This designates the voltage requirements of the heater circuit in the tube. A 12AX7 preamp uses 12 volts in the heater circuit. EL34, or 6CA7's as they are called in the States use six volts on the heater circuit within the tube.

The PWR XFMR is a very important part of the amp. If it gets sick, it affects the entire amp immediately.

Can't wait for the next edition. Marty

 

[HAMPAMP TUBE AMP SERVICES]

You are so right about the tube rectifiers, but I'll not touch on those topics at the moment, We'll get through this particular amp and then branch off to other worlds seeking new life, new civilizations, to boldly go where no man has gone before.
Thank you for bringing up the fact that 12AX7 filaments are 12 volts, good topic for yet another thread (a good one for you Marty). Your knowledge on vacuum tubes is untouched by any normal human, but then, we are not ordinary men. When my girlfriend looks into my eyes sees the warm glow of tube filaments. Ha, It's a wonderful world we live in.

 

[HAMPAMP TUBE AMP SERVICES]

Part 2 is on it's way

 

[knk2691]

Great Thread, I'm really looking forward to part 2

 

[Adwex]

Me too.

 

[Reginald]

How does a Tube Amp work pt.1
This is a series of threads that I hope will help explain the workings of standard Tube Guitar Amps to those
who have no idea what's going on. I will try to make it simple and easy to understand but sometimes in an
effort to simplify I can complicate things even worse, so hopefully some of the other TECHs and otherwise
knowledgeable people on this forum will help out (Are you there Marty?). I'll be using a schematic found
on Dr Tube's website (I'll try to put up a link but not sure how to do that) drawn by Joe Piazza. It can be found
under the JMP listing entitled : 1987 Plexi http://www.drtube.com/schematics/marshall/1987pljp.gif. First
we will look at the Power Supply then we will move on to the input jacks and go through the amp until we
reach the speaker jacks. Enjoy the ride but it may be bumpy at times.


Power Supply
The job of the power supply is to take AC 120 volts from your wall outlet and change it to AC/DC voltages required
to operate your amp. Before we start, lets define some terms that will be used in the text. Note: If you are located
anywhere other than the United States, your AC system may be 220volts 50Hz

Voltage: Electrical pressure or the speed at which electricity flows unit of measure= volts or V

Current: The amount of electricity that flows past a certain point unit of measure = amperes or A

Wattage: The amount of Voltage multiplied buy the amount of Current. Also known as Power unit of
measure = watts or w

Resistance: The opposition to the flow of current unit of measure = ohms

AC current: Alternating current, This is a current that contains peaks that alternate between positive and
negative. Each alternation of positive and negative peaks is called a "cycle". In the standard
household system this alteration occurs 60 times per second and is called a 60 cycle system.
This is also referred to as 60 Hz.(or the Frequency of the cycle)
Your guitar signal is a low voltage AC signal, the strings vibrate at a certain frequency(or Hz).
The "A" string vibrates at 110 cycles per second or 110Hz.

DC current: Direct current, This is a current that flows in one direction, either positive "or" negative with respect
to ground (or zero volts)



Okay, now we are ready to start. If you look at the diagram on the bottom right-hand side is the AC input,
you should be able to identify the symbol that represents the AC plug. The top and bottom wires are marked ' L'
and "N" respectively, the center wire is labeled "E" or earth, another word for ground. This wire is connected to the
chassis.
The first component we come to is a fuse labeled as 2A, this fuse is a safety device intended to "open"
if the current draw on this end of the transformer exceeds 2 amperes. The next component in the line is the
"Mains switch". this switch allows current to flow through the transformer and is the main on/off switch for the
amp. The next component is the "Mains Voltage Selector". The Power Transformer as several taps on this end
which is called the "Primary" these taps allow for a different voltage to be selected and still have the desired
voltage present at the "Secondary".
The Power Transformer is the heart of the tube amp, it supplies the various voltages needed to work the
amp. Without getting too technical a transformer can step up or step down voltage and may have many different
sections to supply several voltages at one time. The input side is called the primary and in this case will accept
110 VAC and step this voltage up to around 300-320 VAC and at the same time on another section it will
step-down to 6.3 VAC. Some amps will have additional sections to supply voltages for IC circuits and separate
Bias circuits. In this case there are only two different sections on the secondary or output of the transformer
(not to be confused with an output transformer, more on that later). At the very bottom of the page is the 6.3
VAC supply for the filaments of the tubes. This section of the transformer has a center tap that goes to ground,
and a pilot light across the supply lines to indicate that the amp is on. this supply is on when the "Mains"
switch is engaged. Each vacuum tube has elements that need heating, the Cathode in particular so that it can
give off electrons to make the tube work. These elements must be heated prior to the unleashing of High-voltage
on the tube or it can make tube life decrease quickly so this section comes on with the "Mains" switch.
Above that on the diagram is a double pole switch labeled "Standby". This section of the transformer supplies
the High-voltage or High-tension (HT) to the circuit. After the switch are two .22uf capacitors, these are there to
suppress spikes in the line when the standby switch is made.
Next in line are two "Diodes" or "rectifiers", these rectifiers serve as a means to convert AC current into DC
current. In this case, the high voltage goes into the rectifiers from the top and bottom legs of the secondary windings,
( the center tap is again grounded), the end of the rectifier that the windings are attached to is called the "anode",
this rectifier will allow the positive half of the AC voltage pass and reject the negative half. Now this gets a little
complicated, but the upper half and lower half of the windings are 180 degrees out of phase with each other,
so that when the upper half is positive the lower half is negative and vice versa. Now since AC voltage has
positive and negative peaks at the rate of 60 Hz and the rectifiers allow only the positive peaks through, the
result is positive going peaks from the rectifier at a rate of 120Hz. This is considered to be DC since it only
contains positive peaks but a pure DC has no peaks, it is constant so we need to make our DC more pure.
This is the job of the "Filter Caps". Think of a filter cap as a storage device, when the positive peaks are
present the capacitors are "charged" like a bucket of water being filled. when the peak goes away leaving a
valley, the cap will discharge and the valley will be filled. this is not a perfect action and several caps are used
to perform this. Even then the DC is not pure, there is still what is called "ripple" present on the DC line. Ripple
is just a small amount of peak and valley left on the DC. This is what creates "hum" in a tube amp. If the caps
are bad, the hum is worsened. The filter caps are the parts on the diagram marked "50uf", They are connected
from the B+ line to ground. There are two of them followed by a second fuse marked "0.5 A", this is the HT fuse
and serves the same purpose as the mains fuse, if the HT current is above .5 amperes the fuse will open.
Next comes a part called the "Filter Choke", Without getting technical,this part also helps to smooth out the
DC voltages as does the 50uf that follows it. Point "A" dropping down just ahead of thr filter choke is the supply line
connected to the center tap of the "Output transformer" to supply V4 and V5 with the plate voltage necessary to
operate those two tubes, somewhere around 390 VDC. Point "B" on the other side of the choke will supply about
380 VDC to the "Screen Grids" of those same tubes. The filter choke has some resistance to DC which is why
there is a 10 V difference between the input and the output of the choke. The plates of V4 and V5 do not need to
be filtered as much as the screens do so the plate voltage is taken off before the choke.
After passing through the choke and past the 3rd 50uf cap , the B+ line turns north to pass through 2 10k
resistors on their way to the preamp circuits, these resistors drop the voltage further due to the fact that the
preamp stages use a lower value of DC voltage than the output tubes.
There is one other section that branches off from the upper secondary winding of the transformer, this is the
"Bias Circuit", it will be explained later when we come to the Output Tubes in our look at the signal path.

Well that is our look at the Power Supply in our 1987, hope you're a little less confused or at least that you are
confused on a higher plane. The next installment we will start at the input jacks and make our way through following
the signal path until I get burned out again. This ends Part 1 of How does a Tube Amp work.

I'm sorry...I don't understand as may to get for short reply,without to transport the whole your thread.i'm sorry very well
Here's question of mine...I had yet asked theese questions at others peole but I'd like to have also yours

1)Supposing I wish to set a old Marshall at 16 Ohms load using two old 4x12 1960 speakers cabs in full stack.
Inside each cabinet ther's four 16 Ohms 30W green-back speakers..
so,I will need two 32 Ohms speakers cabs,you know..
so,What do I re-wiring(re-solder) the speakers wires connection into the cabs?
Should I wire them in series or in parallel or in series/parallel?
is it possible,what's the way?
I own this old Marshall and I wish it work much less hard as possible,you know,
but I'd like use two 4x12 speakers cabs in full stack.

2) I live in Italy and there's the 220V voltage power line...but in old Marshall you can choice among many voltage setting on the amp ,so I wondering if I could set it at 240/250V instead 220/230.....in order to obtain a less warm up from transformers and extend,prolung the life of the amp.....is it a foolish thing to do or it may a right solution?

3) about the lenght of cables connected speakers cabs with head.
if I use it in full stack...often I use a 1 meter cable for cab A and a 2(or 3 meter lenght sometimes)meters cable for cab B ....could this different lenght
to be damage the amp or its transformers....or 'to unbalance' them?

 

[TPR]

Reginald, I would recommend first doing a search on the forum for answers to your questions. If you don't find what you're looking for, start new threads under the appropriate categories (Cabinets and Speakers for the speaker wiring questions, Marshall Amps for the voltage question). That will probably get more eyeballs on them, and would be more appropriate than "hijacking" this thread. :cool2:

Hampamp, great post. Thanks for it.

 

[T-Bird]

Hi.

Thanks Hampamp, great idea. Hhopefully these threads will get more attention than the army tube paper did .

And hopefully these threads will reduce the constant need for rectifying some misconceptions and myths people have.

Regards
Sam

 

[Reginald]

okay friends

 

[HAMPAMP TUBE AMP SERVICES]

Thank you all for your kind words and interest in this thread, I had hoped to post a part 2 before now but I've been down for a while due to computer problems. I'm in the process of moving from Myrtle Beach back to my hometown of Asheville, N.C. I will not have the time to write up a part 2 until the move is complete but I hope to have it by early December if not sooner so keep checking in. Man, it's good to be back

Reginald,
You can't get a 32 ohm load out of four 16 ohm speakers unless you added another 16 ohm load to each cabinet (in series), but that is not good practice. If you just like having 2 cabinets (who wouldn't) you can disconnect two of the speakers in each cabinet and wire the remaining two in parallel. But your amp isn't going to work any harder by using both cabinets and setting the selector to 8 ohms, that is why you have the selector, to match the load to the amp so don't worry about driving it like it is designed for. Just use both cabinets and enjoy the ear shattering vintage Marshall tone.
As for the voltage selector for the mains, there too I would just stick with the 220v setting, If you use the 250v setting it will change the voltages in the amp required for correct operation. The 6.3volts for the filaments would change as well as the bias voltage, if either one of those were to change drastically it could destroy your amp. Don't risk it.
The cable issue shouldn't be a problem as long as they are good quality cables, you don't want to use small gauge wire (I try to use at least 18 gauge for most of mine) but unless your cables are bad then you shouldn't worry too much about them. I wouldn't use a 15 meter cord just because it is a waste but as long as it were a quality cable and a heavier gauge then there should be no problem. I often use a 20 foot cable to run to an extension cabinet on the other side of the stage and I've experienced no problems at all.

 

[andyman95023]

Awesome info and I too am looking forward to part 2 and hopefully something geared towards my 77 JMP 2204 w/6550's so I can determine its health

Thanks
Andyman95023

 

[Reginald]

Thank you all for your kind words and interest in this thread, I had hoped to post a part 2 before now but I've been down for a while due to computer problems. I'm in the process of moving from Myrtle Beach back to my hometown of Asheville, N.C. I will not have the time to write up a part 2 until the move is complete but I hope to have it by early December if not sooner so keep checking in. Man, it's good to be back

Reginald,
You can't get a 32 ohm load out of four 16 ohm speakers unless you added another 16 ohm load to each cabinet (in series), but that is not good practice. If you just like having 2 cabinets (who wouldn't) you can disconnect two of the speakers in each cabinet and wire the remaining two in parallel. But your amp isn't going to work any harder by using both cabinets and setting the selector to 8 ohms, that is why you have the selector, to match the load to the amp so don't worry about driving it like it is designed for. Just use both cabinets and enjoy the ear shattering vintage Marshall tone.
As for the voltage selector for the mains, there too I would just stick with the 220v setting, If you use the 250v setting it will change the voltages in the amp required for correct operation. The 6.3volts for the filaments would change as well as the bias voltage, if either one of those were to change drastically it could destroy your amp. Don't risk it.
The cable issue shouldn't be a problem as long as they are good quality cables, you don't want to use small gauge wire (I try to use at least 18 gauge for most of mine) but unless your cables are bad then you shouldn't worry too much about them. I wouldn't use a 15 meter cord just because it is a waste but as long as it were a quality cable and a heavier gauge then there should be no problem. I often use a 20 foot cable to run to an extension cabinet on the other side of the stage and I've experienced no problems at all.

Hi,very thanks for theese informations..
let me ask you another a coupe foolish questions

supposing about two 4x12 16ohm speak. cabs identical about speakersw but one is wiring as like that of that guy's model into the photo of other thread in this forum (speakers cabinets the thread named 'speakers wiring' into the page n 1 ....I'm not able to transport it here)and another is of mine at 16ohms wiring in serial/parallel....or of mine is in parallel and the one of him is in serial/parallel?I don't know....but both are 16ohms,right?
but the questuin is this :does the amp work much less hard with a wire connection like that one of mine or works less much hard with one similar at that wiring connection of that's one of him?

 

[HAMPAMP TUBE AMP SERVICES]

I'll try and remember to add some information about the 2203 and 2204 amps, there are a few differences but basically the same circuits, the input is wired different and a master volume is added. The explanations for the circuits and the jobs they do will apply to 6550's as well as EL-34's, you just have different resistor values for 6550's and I will touch on that also. I will try to explain circuits for various other Marshall amps too but I'm starting with the vintage series as this is the most basic design. Also, the circuit descriptions apply for 100watt amps too, you just have two more tubes, a different OT and PT and a little more filtering. These are going to be basic circuit operations to help those who do not have a clue what is going on to understand a little better, I'm not going to go into great detail but will elaborate on topics as time goes on. Right now I just want to help those that do not understand, it sure will help when a tech tells you he found " a leaky coupling cap ", it's good to know what he is talking about.

 

[Reginald]

I'll try and remember to add some information about the 2203 and 2204 amps, there are a few differences but basically the same circuits, the input is wired different and a master volume is added. The explanations for the circuits and the jobs they do will apply to 6550's as well as EL-34's, you just have different resistor values for 6550's and I will touch on that also. I will try to explain circuits for various other Marshall amps too but I'm starting with the vintage series as this is the most basic design. Also, the circuit descriptions apply for 100watt amps too, you just have two more tubes, a different OT and PT and a little more filtering. These are going to be basic circuit operations to help those who do not have a clue what is going on to understand a little better, I'm not going to go into great detail but will elaborate on topics as time goes on. Right now I just want to help those that do not understand, it sure will help when a tech tells you he found " a leaky coupling cap ", it's good to know what he is talking about.

i,very thanks for theese informations..
let me ask you another a coupe foolish questions

supposing about two 4x12 16ohm speak. cabs identical about speakersw but one is wiring as like that of that guy's model into the photo of other thread in this forum (speakers cabinets the thread named 'speakers wiring' into the page n 1 ....I'm not able to transport it here)and another is of mine at 16ohms wiring in serial/parallel....or of mine is in parallel and the one of him is in serial/parallel?I don't know....but both are 16ohms,right?
but the questuin is this :does the amp work much less hard with a wire connection like that one of mine or works less much hard with one similar at that wiring connection of that's one of him?

 

[thrawn86]

Incredible stuff...thank you for posting this. I have a vauge idea of how all this works, and this is filling me in greatly!!!

 

[donobagel]

I really enjoyed your post on how a tube amp works. I had a question though. On Dr Tube there are two other schematics for this amp. Why do the other two schematics show a connection to the 120V on the primary side? Thanks. -James

 

[Jonathan Wilder]

I really enjoyed your post on how a tube amp works. I had a question though. On Dr Tube there are two other schematics for this amp. Why do the other two schematics show a connection to the 120V on the primary side? Thanks. -James

Wall voltage used to be 110VAC at one point in time many years ago. Nowadays it's 120VAC in the US.

However, that is a voltage selector that's on the back of the amp and it should be set accordingly depending on which country you're in. North America is the only continent that runs 120VAC anyway I think.

 

[jjack]

Hi all
Thanks Hampamp lots of helpful info!! Just wondering if their is a part II ??

 

[Jonathan Wilder]

Hi all
Thanks Hampamp lots of helpful info!! Just wondering if their is a part II ??

Hampamp is no longer on the site for the time being. I'm currently in the process of writing up a continuation of what he's already written here as per his request.

Hopefully soon I'll have some more to post up.