|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
40.136.240.175
In Reply to: RE: balancing the plate volts and plate current posted by Tre' on September 28, 2016 at 07:49:32
I might be calling it by the wrong name. This schem is for the 45 amp, but the layout is the same even though the values are different. The 22K resistors that are tied to pin 2 of the 26's are the ones I am changing, but I am using different values.
Edits: 09/28/16Follow Ups:
A voltage drop resistor is a resistor between the B+ and the decoupling cap.
The plate resistor is feed by the drop resistor but the value of the drop resistor is not part of the plate load because there is a the decoupling cap in place.
The decoupling cap is a AC ground point so the resistor value of the drop resistor is not part of the plate load. That is to say, the tube will not "see" the drop resistor as part of the load impedance that it is driving.
You never show everything in your schematics and that makes it hard to talk about your circuit. I can't point out what I can't see.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
If you can see the changes I made to your schematic you will see that the 1.5k resistors are the voltage drop resistors and the 47uf caps are the decoupling caps and the 22k resistors are the plate resistors.When you want to change the plate voltage you should change the value of the voltage drop resistors, not the value of the plate resistors.
I'm not sure what your note "B+ 2 supplied from terminal strip that has reduced B+" means.
The only thing that should be reducing the B+ is the drop resistors. That is to say, the full B+ voltage should be feeding each drop resistor and those drop resistors should be whatever value they need to be to give the voltage that you want feeding the plate resistors, and by extension, the plate voltage you want.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 09/28/16
Ok, I am confused. What does the plate resistor do if it's not there to adjust the plate voltage? That is what I have been doing with it. The 22k resistor shown in my schem is both plate resistor and drop resistor in one by your definitions. There is nothing else to show.
The b+2 is just taken from the b+ supply to the plate resistors of the 26's where it is reduced to the plate voltage I need for the 26 (this schem does not show it). I get the feeling you are going to tell me I have it all wrong again. : )
Let me know if you need me to and I will post a schem of the 26/845 amp. I will have to clean it up a bit and take a picture.
Edits: 09/28/16 09/28/16 09/28/16 09/28/16
Hi Vinnie2,
I want to thank you for sharing your build experiences. I always learn something new or gain a deeper understanding of how tube circuits work, from your interchanges with more knowledgeable forum members. (Thanks Tre, for your time.) II'm like you, I have a tough time with theory.
This brings me to your B+2. You said "The b+2 is just taken from the b+ supply to the plate resistors of the 26's where it is reduced to the plate voltage I need for the 26 (this schem does not show it)." Could you show me on your schematic where you are tapping your B+2 or describe the node? Thanks.
Regards,
David
That schem is not for the amp I am working on, it is just similar and it was handy. But to answer your question, on that schem it would come from where the B+ goes to the lower half of the output trans on the bottom right of the schem. It then is taken to the plate resistors of the 26's, which according to what Tre' is saying should have another resistor in front of them to reduce the B+ down to the B++ that is needed for the plate voltage. I am going to make that change tomorrow and see how that works out. I think it would help explain the trouble i was having trying to the plate volts and plate current to design levels. We'll see what happens.
The more knowledgeable members of the forum will always have my gratitude for their willingness to share their knowledge with those of us who struggle through the valleys while they leap from mountain top to mountain top.
Edits: 09/28/16
22k is plate load resistor from which we derive our static load line
read the Crowhurst article and post questions
Roger on that, will do. Thanks.
"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: 09/28/16
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.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: