The key word above in this topic is "Extreme"...
https://www.dropbox.com/s/2svymck947yq651/ST-70%20InputStage1.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
https://www.dropbox.com/s/2svymck947yq651/ST-70%20InputStage1.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
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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
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
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
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
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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/k46swkao8jgnnhu/ST-70%20InputStage2.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.
https://www.dropbox.com/s/k46swkao8jgnnhu/ST-70%20InputStage2.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.
https://www.dropbox.com/s/8yc8t32f14279v2/ST-70%20InputStage3.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
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
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.
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/w91b3vs236rqbti/ST-70%20InputStage4.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
This Sound values 1991 schematic of an "ST-70" is way different than my 1961 Dynaco ST-70, more complicated. the original can be downloaded from freeinfosoc or dynaco-doctor.com or someplace. In particular R31-32-33 and C21-22 are not in the original. I've wondered about the input grid direct coupled to the RCA jack center maybe blowing up laptops or CD players if the tube shorts out internally, but a 50 V rated C21 is not really going to improve safety in such a case. A 600 v rated non-polarized C21 would seem to be a cheap improvement. I use my ST-70 with an op amp disco mixer as preamp sometimes, no problems with interference from the CB radio nut driving by, or the lamp dimmer. I guess that is what C22 is for.
The Sound Values B+ power supply has replaced the 5AR4 tube rectifier with a pair of silicon ones with no soft start circuit which might be a bit shocking on the power output tubes. The bigger B+ caps might improve steady state power a bit, although on music (not bass guitar) I find the 30-20-20-20 Dynaco cap not a problem.
The complicated A4 A3 supply has something to do with the LED's, which I suppose might just blink the LED's on when the bias current was right on the output tubes, or something. I don't find the original bias set circuit annoying, you need a meter anyway to check the B+ voltage at power at least as often as you need to readjust output tube idle current. I've run the B+ output tube voltage (420 V) to an unused pin of the front panel socket, to check it since the B+ cap needs changing so frequently and the 5AR4 more often than than the output tubes, to keep power up to rated value. I've changed the 30-20-20-20 cap 4 times in 43 years for low power. (not 17.5 VAC out on the 8 ohm speaker). I think I replaced the 5AR4 thrice for low power, the output tubes twice.
The Sound Values B+ power supply has replaced the 5AR4 tube rectifier with a pair of silicon ones with no soft start circuit which might be a bit shocking on the power output tubes. The bigger B+ caps might improve steady state power a bit, although on music (not bass guitar) I find the 30-20-20-20 Dynaco cap not a problem.
The complicated A4 A3 supply has something to do with the LED's, which I suppose might just blink the LED's on when the bias current was right on the output tubes, or something. I don't find the original bias set circuit annoying, you need a meter anyway to check the B+ voltage at power at least as often as you need to readjust output tube idle current. I've run the B+ output tube voltage (420 V) to an unused pin of the front panel socket, to check it since the B+ cap needs changing so frequently and the 5AR4 more often than than the output tubes, to keep power up to rated value. I've changed the 30-20-20-20 cap 4 times in 43 years for low power. (not 17.5 VAC out on the 8 ohm speaker). I think I replaced the 5AR4 thrice for low power, the output tubes twice.
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