ST-70 Input Stage, Explained For Extreme Beginners - diyAudio
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Old 16th November 2013, 01:27 AM   #1
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Default ST-70 Input Stage, Explained For Extreme Beginners

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

https://www.dropbox.com/s/2svymck947...nputStage1.jpg

you can zoom into the above schematic by just clicking on the schematic after it opens, or you can download the image file.

In regards to the above schematic for this Dynaco ST-70

1) What are all the parts that comprise the "Input stage" (aka "gain 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 referring to the attached schematic and use the red line in it.

2) what function do those parts in the "input 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 "input stage"
** if I drew the red line correctly, the "input stage" consists of the
following; eleven (11) resistors, four (4) capacitors, and one 6GH8
valve. Is that right?


So what does the "input stage" in this schematic require as inputs? In other words, what are the parameters of the audio signal immediately before it enters into the input stage? And what are the parameters of the audio signal at the exit point of the "input 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 "input 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 "input stage" sections one for left and other for right (for this ST-70 schematic)

Kind thanks
Eddie

Last edited by Eddiegnz1; 16th November 2013 at 01:31 AM.
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Old 16th November 2013, 01:57 AM   #2
chip647 is offline chip647  United States
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You circled the input stage and the phase splitter stage.
The first tube section adds gain, the second splits the signal into two opposite phase signals to dtive the output tubes.
There is a pentode section and a triode section in the one 6GH8 tube. The pentode or first section is the gain stage and the triode section is the splitter.
The resistors above the tube provide the plate load, those below provide the bias.
The caps bypassing the cathode resistors remove feedback from the cathode.
You need to go by a Morgan Jones book
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Old 16th November 2013, 02:19 AM   #3
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Quote:
Originally Posted by chip647 View Post
You need to go by a Morgan Jones book
I think he already has one.

jeff
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Old 16th November 2013, 12:38 PM   #4
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also , the ratio of R17 / R18 determines (along with a lot of other factors) the global feedback ratio. The bigger R17 is, the less feedback.

For a full explanation of feedback calculations, refer to Morgan Jones "Bevois Valley" amplifier.

EDIT

Schematics cannot be really chopped up with a cleaver. These things work in relation to what is before and after them

Last edited by costis_n; 16th November 2013 at 12:40 PM.
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Old 16th November 2013, 01:52 PM   #5
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does this revised red line get closer to being more of the input stage and less of the other parts by excluding parts R40 & R41?

https://www.dropbox.com/s/k46swkao8j...nputStage2.jpg

I realize that my incoherent questions seem invalid to the experts out there but please trust me, when you attempt to answer my stupid questions it helps me more than you realize...so thank you for trying to humor me as I try to get out of the weeds in my efforts to get oriented.

And yes, I understand the concept of not being able to chop it up with a cleaver. It's like a human body, which part is the respiratory system...even the heart is critical for the respiratory system to work properly and so is the nervous system and even the liver...I get that concept, but just try to isolate the respiratory system as best as possible please.
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Old 16th November 2013, 06:32 PM   #6
wicked1 is offline wicked1  United States
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Re-read post #2. You still have the phase splitter section in the red circle.
The signal basically goes through 3 stages.. The input voltage amp, the phase splitter (both happen to be in the same bottle for this amp), and then the output stage.
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Old 17th November 2013, 02:52 AM   #7
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https://www.dropbox.com/s/8yc8t32f14...nputStage3.jpg

is that what you mean, does the red line in the image above get me closer or did i go in the wrong direction?

By the way, each day I spend hours on studying based on all of your recommendations and believe it or not, I feel I had some good break throughs today during my study session that just ended at 10:45pm Eastern Florida time. I'm starting to feel really good about the posibility that I will be able to get there some day. I feel great about this....thank you all so much, you have no idea how much you've helped me.

you're good people and knowledgeable....a brotherhood of the craft

Eddie
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Old 17th November 2013, 01:39 PM   #8
wicked1 is offline wicked1  United States
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Yep, that looks correct to me. I'm not sure what c24 and r39 are for.

There are simpler schematics to learn from...

And, hang in there.. You'll eventually get it. It took me several years of my limited free time before it all started making sense.
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Old 17th November 2013, 01:41 PM   #9
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by Eddiegnz1 View Post
is that what you mean, does the red line in the image above get me closer or did i go in the wrong direction?
Yes, correct, although there was no need to include the "V4-B 6GH8" bit, since that is the label for the second stage / phase inverter triode.

Also, C24 and R39 would probably be considered to be part of the first stage. These components stabilize the global negative feedback, to prevent oscillation.
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Old 17th November 2013, 04:58 PM   #10
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Quote:
Originally Posted by wicked1 View Post
...

There are simpler schematics to learn from...

...
Below should be the final red line that marks the "input stage" of the ST-70. Or at least what over half the members here would agree is the best place to draw the line. If you don't agree, please help me improve the placement of the red line in the image below.

https://www.dropbox.com/s/w91b3vs236...nputStage4.jpg

And when you say "there are simpler schematics to learn from", are you saying that there are other ways to draw out the schematic of the ST-70 such that it's easier to learn from? or that there are other non-ST70 tube amps that have easier schematics to learn from?

thanks again
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