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In Reply to: RE: Let me try again posted by vinnie2 on September 28, 2016 at 13:56:49
"What does the plate resistor do if it's not there to adjust the plate voltage?"We need to go back to the basics of how tubes work.
With a voltage potential between the cathode and the plate, DC current flows from the cathode to the plate.
A plate resistor sits between the plate of the tube and the power supply.
There will be a voltage drop across the plate resistor in accordance with the idle current. (Ohm's Law...voltage = current times resistance)
When the music signal is placed on the control grid the the tube, the DC current through the tube increases and decreases.
The positive half of the input signal on the grid of the tube causes the DC current through the tube to increase and the negative half of the input signal on the grid of the tube causes the DC current through the tube to decrease.
When the DC current through the tube increases the voltage drop across the plate resistor increases (the voltage at the plate decreases).
When the DC current through the tube decreases the voltage drop across the plate resistor decreases (the voltage at the plate increases).
The decreasing and increasing plate voltage is the AC output signal voltage of the stage.
The total impedance that the tube is "playing" into determines the load line.
The load line defines just how the tube will behave as the signal is applied to the grid. (gain, distortion, etc...)
If you change the value of the plate resistor, you change the load line and that changes the behavior of the tube stage.
If you need/want to change the plate voltage you should changed the supply voltage to the supply side of the plate resistor.
This is done by changing the value of the voltage drop resistors not the value of the plate resistor. (as you decrease the plate voltage you will need to decrease the bias voltage to keep the idle current the same. This is done by decreasing the cathode resistor value. Once you have learned this stuff you will be able to see this clearly by just looking at the plate curves.)
The plate resistor (in parallel with the grid resistor for the next stage) is seen by the tube's plate as the load for the tube.
The voltage drop resistor is not seen by the plate of the load for the tube because of the decoupling cap.
The decoupling cap is a dead short to ground for AC and the DC voltage drop resistor is on the other side of the decoupling cap so the voltage drop resistor is not seen by the tube as being part of the tube's load impedance.
That is to say, there will not be a AC voltage drop across the voltage drop resistor because of the decoupling cap but there will be a AC voltage drop across the plate resistor.
Vinnie, I have to say it again. Get a good book and read it and learn this stuff. Tubed audio electronics are not complicated at all. You just have to want to understand it and you have to be willing to put in the time studying it.
IMO you have taken enough of the inmates time with simple questions that you should be able to answer yourself. It's time for you to learn this stuff using your own time and energy.
A good place to start is Ohm's Law. Study Ohm's Law until you understand it forwards and backwards. Just understand that alone would answer many of your questions.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/28/16Follow Ups:
Tre
I really appreciate your explanation, and I will file it with my other information, for future reference. However, I must say once again that a carpenter does not need to be a structural engineer or an architect to build a house, he just needs a good set of plans. I am that carpenter, and I try to find complete sets of plans to work with, but sometimes I have to wing it.
As far as taking inmates time, that is what I thought this forum was for. Are you telling me it's only for the architects and no carpenters allowed?
I would rather spend my time building, and I have told you on numerous occasions that the theory does not come easy to me nor is it my primary interest. If I pick some up as i go along fine, but I do not wish to spend most of my audio time trying to learn the theory when I could be building instead.
I would hope there are at least a few others out there like myself and that at least some of the inmates are ok with us being carpenters and will help with the architecture when they can. For Pete's sake Tre', I have built or bread boarded over 30 amps and preamps in the last 15 years or so. How many other inmates of the "tube diy" forum can claim that? I have had lots of help from the forum sure, but I have done a lot of it on my own as well when I could find good schematics.
Edits: 09/28/16 09/28/16
Nothing wrong with building a proven/tested circuit.
The problem happens when you deviated or modified the circuit.
Then you have to know the tube theory or be able to test for distortion/spectral data to know if circuit changes are taking you in the correct direction or not.
I am having the same issues with the RH84 type circuit I tried to build.
If I was just changing things and listening for better or worse results, it would be a lost cause to ever getting it correct.
By all means, keep posting your projects.
We are all here to learn and help each other if possible.
Thanks for the support. That's sort of the way I look at it too, and I try to help if I can.
"Nothing wrong with building a proven/tested circuit.
The problem happens when you deviated or modified the circuit.
Then you have to know the tube theory..."
That will inevitably happen.
The best defense is a good offense. It's time to take the time to learn.
The time spent will save even more time in the long run.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
You are welcome to your opinion Tre', but not everyone shares it.
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