ST-70 Output Stage, Explained For Extreme Beginners

The key word above in this topic is "Extreme"...

ST70 Output Stage Schematic

in the above link you can click to zoom in or download it.

1) What are all the parts that comprise the "output stage"?  For question number one here, I'm not looking for a comprehensive parts list.  For me it would be easier if you could answer this question by refering to the attached schematic and use the red line in it.

2) what function do those parts in the output stage do?  again, not a comprehensive write up just in general summary format

3) in the attached schematic did I draw my red line correctly? I'm trying to include within the red line only the parts that are considered as parts of the "output stage"
        if I drew the red line correctly, the output stage only consists of the following; two EL34 valves, and three resistors (R8, R24 & R25).  That's it?  
        just five parts? no capacitors nothing else is part of the output stage in this schematic?

So what does the output stage in this schematic require as inputs?  In other words, what are the parameters of the audio signal immediately before they enter into the output stage?  And what are the parameters of the audio signal at the exit point of the output stage?

I know I need to learn many more complex things but at this phase of my learning, I need to first learn the basics of the output stage expressed to me without any formulas and without any technical jargon (well maybe a little jargon, like I know voltage, ohms and capacitance, and amperage and audio signal, etcetera).  And I know there are two output stage sections one for left and other for right.

Kind thanks
Eddie

How about turning to Internet , heret two good places to look at:
http://lenardaudio.com/education/14_valve_amps_4.html

http://www.valvewizard.co.uk/pp.html

peterh wrote:How about turning to Internet , heret two good places to look at:
http://lenardaudio.com/education/14_valve_amps_4.html

http://www.valvewizard.co.uk/pp.html

thanks so much PeterH, I have definitely done a tremendous amount of searching and reading through the "internet" but many of the explanations I have read still leave me with the questions I have asked in this topic above.  I also bought 4 books on Valve amps and checked out 7 books on basic electronics from the library and none of those eleven books have cleared up the questions I asked above.

So I was just reaching out in hopes that at least one person in this forum could clear up my question somehow.  I will no doubt read the two websites you suggested as well, thank you PeterH !

EDIT
actually, I have looked at those two websites already, I can't look at the Valvewizzard website because it is a little slanted towards guitar amps and I don't know which sentence pertains more toward guitars or not. It says that some of the things he talks about help to achieve the distortion desired in a guitar amp. I will look at the Lenard site again...I recall that it is a very good site which has indeed helped me a lot, but again, some stuff there is unclear to me as an "EXTREME" beginner.

I can tell you what every part is for and what it does but don't have the time right now. The output stage starts at the c15 and 16 capacitors. The first tube section is a pentode tube and amplifies the signal. The next section a triode does no amplification but is used to split the signal into 2 signals that are out of phase with each other. r26 resistor is the neg. feedback resistor.

As an Extreme beginner do you understand Ohm's Law, capacitors/ inductors, and transformer basics?  Without these fundamentals electronics is difficult to grasp.

The two links provided by PeterH are excellent given some basic knowledge!

Here is a good resource that starts with the basics and progresses to tubes and amplifiers.

http://jacquesricher.com/NEETS/

Or, maybe this:

sailor wrote:... The output stage starts at the c15 and 16 capacitors. The first tube section is a pentode tube and amplifies the signal.  The next section a triode does no amplification but is used to split the signal into 2 signals that are out of phase with each other. r26 resistor is the neg. feedback resistor.

So is the red line in the following image closer to capturing "ONLY" the "output stage" ?

Output Stage ST70

I understand that all parts are interdependent and that you can't really isolate or "red-Line" only the output stage. But it's like a human body. Which parts comprise the respiratory system? Even the heart is critical for the respiratory system to work properly and so is the digestive system and even the kidneys...I get that concept, but I would greatly appreciate it if anyone could help me to isolate the "output stage" as much as possible please.

I would have to include the output transformer also, as the output tubes can't drive a speaker without it.

ok, so this should be it right?

ST70 Output Stage

by the way folks, I know it seems like im spinning my wheels around aimlessly and hopelessly lost...but I've reading for hours each day and I feel like I made a major break through in the study session that ended 10:45pm Eastern Flrorida time. I'm feeling really good right now about my chances of getting there someday. So thank you all so very much because each of you helped me more than you realize.

great people....a BrotherHood of The Craft

EddieGnz1 wrote:ok, so this should be it right?

ST70 Output Stage

by the way folks, I know it seems like im spinning my wheels around aimlessly and hopelessly lost...but I've reading for hours each day and I feel like I made a major break through in the study session that ended 10:45pm Eastern Flrorida time.  I'm feeling really good right now about my chances of getting there someday.  So thank you all so very much because each of you helped me more than you realize.

great people....a BrotherHood of The Craft

No.
The output stage is nearly all of the schematic you have shown. It includes
phase inverter, it includes feedback loop and it includes output transformer.

One way of looking at it is that any changes here will affect the output in some way.

BTW "output stage" is a broad description rather then a technical term. Somewhat simular to "power train" in a car.

how about now, does the following image encompass just the isolated "output stage"? What about R27? should R27 be in the redLine circle or excluded?..."in the circle of trust...or out of the circle of trust"  

OutputStage4

thanks

The caps 15 and 16 are there to stop all DC voltage from the driver stage from entering the output stage. So you can consider them the door between the driver and output stages. The line should actually be just after the caps as the caps. are usually considered as part of the driver stage. Although peterh is correct I think what you are trying to do is separate each individual system so you can analyze and understand each system and what it does. First yes the output transformers must be included in the output stage. The bios voltage supply directly above the output stage provides the necessary Neg. bios to the grid. It can be considered as part of the power supply or the output stage. The Neg. feedback loop I consider as its own system as it reversely links the output to the input of the driver stage. But that is my opinion. The power supply and filament supply I consider it the engine that runs everything but does not directly pass the the audio signal. [That statement is going to get me some adverse comments.] I consider it as a separate system.
One of the hardest things to understand about tubes is bios to the grid. The Bios sets the operating point [starting point]. The load line is set by this starting point and available voltage to the tube. Changing the bios voltage moves that starting point and load line. There are 2 ways of setting the grid voltage. The first is directly which is what the ST70 uses for the output stage. The other is cathode bios which is the most common way of biosing a driver tube. Cathode bios is controlled by the cathode resistor. Although it is called cathode bios the net effect is the same. The resistor sets the neg voltage to the grid. Go to the site below and change the value of the cathode resistor and watch the red load line move up and down. The lower graft with the blue and green lines show the neg. bios voltage and starting point. The actual starting point is up to the designer of the circuit to decide where he wants that red line to be and pick the correct parts to accomplish his goal. Where does the load line come from? As an audio signal starts to pass threw the tube it causes the voltage to swing from the starting point. As voltage goes up amperage goes down and voltage goes lower amperage increases per Ohms law. This site has a lot of calculators you should look at all of them.
http://www.ampbooks.com/home/amplifier-calculators/12AX7/

Eddie as far as your question about the resistors. Everything directly above your red line including the 2 resistors mentioned are part but not all of the neg. gird bios power supply which I described it's function in detail above.

Before you ask. R17 is the cathode resistor for the pentode. R 18 is part of the neg. feedback loop. The ratio between R18 and R26 sets the % of neg. feedback per a formula that you can look up. The small capacitor c17 does net affect the amount of feedback but is there, if I remember correctly, to stop adverse oscillation from the transformer.