There is a DC and an AC loadline.
The DC line has to be valid or the amp will not work.
The AC covers your audio output performance.
In Resistor-Loaded stages, the DC and AC are normally very similar.
In choke/transformer loaded, the DC is nearly a straight line. Transformer DC resistance is like 5% of working audio AC impedance.
Going back to what you should do: Your supply voltage is too high. Why? and....
Find the WE tables for 300 and pick one.
http://www.tubebooks.org/tubedata/we300a_b.pdf
Some junior engineer sweated over hot glass to compile this data. Why ignore it?
The DC line has to be valid or the amp will not work.
The AC covers your audio output performance.
In Resistor-Loaded stages, the DC and AC are normally very similar.
In choke/transformer loaded, the DC is nearly a straight line. Transformer DC resistance is like 5% of working audio AC impedance.
Going back to what you should do: Your supply voltage is too high. Why? and....
Find the WE tables for 300 and pick one.
http://www.tubebooks.org/tubedata/we300a_b.pdf
Some junior engineer sweated over hot glass to compile this data. Why ignore it?
Prr,
Im not choosing to ignore it. I simply want to measure twice and cut once. The amp sounds good, but I am trying to understand the measurement process so I at least have an educated path.
Your spot on with the supply voltage, at least as far as anyone who knows about tube amp design and this model amp. Because of the high supply, and the original cathode resistor of only 680 ohms, the 300B was getting beat up. Someone went into the amp before I bought it, upped the cathode resistor to some where about 880 oms. There was a writeup talking about going to 1k. While it reduced the current, I only see now that it wasn't completely thought through. Welcome to my world and why im trying to diagnose where it is now.
And ty for the link..
Im not choosing to ignore it. I simply want to measure twice and cut once. The amp sounds good, but I am trying to understand the measurement process so I at least have an educated path.
Your spot on with the supply voltage, at least as far as anyone who knows about tube amp design and this model amp. Because of the high supply, and the original cathode resistor of only 680 ohms, the 300B was getting beat up. Someone went into the amp before I bought it, upped the cathode resistor to some where about 880 oms. There was a writeup talking about going to 1k. While it reduced the current, I only see now that it wasn't completely thought through. Welcome to my world and why im trying to diagnose where it is now.
And ty for the link..
I/ve posted this link to the Valve Mag archives so many times I figured today it might just be simpler to post the articles. I hope Doc and Paul will see it the same way. It's a great article and anything that removes the pain and suffering of technical bewilderment is a boon to mankind. (I read a book once. It had language like that.)
Fantastic! Ty.. but im still going over my preflight checklist and this is 20,000 ft over my head lol.. the Debate rages, is it B+ or voltage between plate and cathode used for the load line.. keep getting responses that refuse to pin it down, or am I thick headed and stubborn?
Plate to cathode. Why? Because the tube could have it's bias voltage applied either to the cathode (cathode bias - in power stages, usually with a resistor "below" the cathode to ground) or the grid (grid or fixed bias - with an applied voltage from a battery or voltage supply).
If you're thick headed, join the club. If you're stubborn, good! Going ahead without understanding is unwise.
If you're thick headed, join the club. If you're stubborn, good! Going ahead without understanding is unwise.
Vka (Voltage from Cathode to Anode) is used for load-line. The voltage lost across a bias resistor and voltage across the transformer is not seen by the tube.
The voltage from Cathode to ground is only seen as a bias voltage for the grid which is referenced to ground.
The voltage drop across the transformer in static bias condition has no (actually very little) effect on tube operation. The "very little" effect is resistive loss in the transformer which translates to a slight reduction in power out.
I suggest yous tart reading here:
On Line Tube Learning for newbies....
To gain an understanding of tube theory.
The voltage from Cathode to ground is only seen as a bias voltage for the grid which is referenced to ground.
The voltage drop across the transformer in static bias condition has no (actually very little) effect on tube operation. The "very little" effect is resistive loss in the transformer which translates to a slight reduction in power out.
I suggest yous tart reading here:
On Line Tube Learning for newbies....
To gain an understanding of tube theory.
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. . . . . and just because it was part of your question, the term "B" in "B+" dates from the early days of tube tech and refers to the voltage provided by a battery to supply the plate. It is now used to refer to the positive voltage output of any type of plate supply.
The actual output voltage doesn't necessarily appear at the tube's plate . . . . .(because there is most often a load in between the plate and the supply, like a resistor, transformer winding or current source) . . . . . so B+ and Plate Voltage (Vp) are not assumed to be equivalent.
The actual output voltage doesn't necessarily appear at the tube's plate . . . . .(because there is most often a load in between the plate and the supply, like a resistor, transformer winding or current source) . . . . . so B+ and Plate Voltage (Vp) are not assumed to be equivalent.
Chris,
I appreciate your honesty!
Thing is, i know zip.. nada.. im simple trying to reconcile why books on tube amp design say differently.
Could it be we are talking about two different load lines??
I now noticed it referred to Dc plate load line at robrobinette.com..
We start our exercise with the DC plate load line. When you see just "load line" that's what is meant, the "DC plate load line". The bottom point of the line is the plate supply voltage (B+3) of 250 volts. We use the supply voltage because when current flow is cutoff by the tube there well be no voltage drop across the plate load resistor so plate voltage will equal supply voltage.
Again, he says plate load resistor. You ( or someone else) said earlier that is the difference (more or less) i contend that regardless of plate load or cathode resistor, when the current is cut off, both methods of bias will show B+ at the plate. We are talking about potential when we say voltage. If overall circuit voltage (B+) is the same in two different amps, one cathode resistor and the other with a plate load resistor of equal values, we could agree that amperage would be the same. If we can agree, then we have to agree the potential or difference of an open circuit in both are the same..
Again i can barley speak the language, but do know there is a difference between the two voltages.
Are we comparing two separate types of measurements?
I appreciate your honesty!
Thing is, i know zip.. nada.. im simple trying to reconcile why books on tube amp design say differently.
Could it be we are talking about two different load lines??
I now noticed it referred to Dc plate load line at robrobinette.com..
We start our exercise with the DC plate load line. When you see just "load line" that's what is meant, the "DC plate load line". The bottom point of the line is the plate supply voltage (B+3) of 250 volts. We use the supply voltage because when current flow is cutoff by the tube there well be no voltage drop across the plate load resistor so plate voltage will equal supply voltage.
Again, he says plate load resistor. You ( or someone else) said earlier that is the difference (more or less) i contend that regardless of plate load or cathode resistor, when the current is cut off, both methods of bias will show B+ at the plate. We are talking about potential when we say voltage. If overall circuit voltage (B+) is the same in two different amps, one cathode resistor and the other with a plate load resistor of equal values, we could agree that amperage would be the same. If we can agree, then we have to agree the potential or difference of an open circuit in both are the same..
Again i can barley speak the language, but do know there is a difference between the two voltages.
Are we comparing two separate types of measurements?
See my last response regarding cathode based or with plate load and the following...
In a simple DC circuit,
+v , load, | open circuit |, -v
Or
+v, |open circuit|, load, -v
The potential is exactly the same across the open circuit...
Is this incorrect?
. And, i will beat this to a pulp and continue until i understand it lol. I have to.. my "Sheldon" is in full swing, and gets that way when i can't connect the dots lol..
I honestly super appreciate everyones replies..
Lastly, if we measure voltahe across a resistor, we measure the voltage drop of that resistor, Thats not the same as whats applied to it. I guess the question could be reworded as "does the dc load line use B+ or voltage drop of a tube?"
Perhaps what you are missing is that with transformer loading, the voltage at the plate can be much higher than B+ voltage when the current is cut off. This is due to the inductive nature of the transformer.
As an example, with B+ of say 400V, and resistor loading of say 5k, a current of say 40 mA and assuming cathode at 0V (grid is negative), you will drop 200V across the resistor, and you end up with 200V on the plate. Your operating point for the load line is 200V and 40 mA. When the tube is cut off, that point on the loadline will be 400V and 0 mA. If the plate could swing to 0V (it can't...), that point on the loadline will be 0V and 80 mA.
Now consider a B+ of only 200V and transformer AC loading of 5K (with negligable DC resistance), same current of 40 mA and 0 volts on the cathode. Your operating point for the loadline will be just under 200V and 40 mA. When the tube is cut off, that point on the loadline will be 400V and 0 mA. If the plate could swing to 0V, that point on the loadline will be 0V and 80 mA. So you have the same voltage swing (0 to 400V) as the resistor loading example, but you get that with half of the B+.
As an example, with B+ of say 400V, and resistor loading of say 5k, a current of say 40 mA and assuming cathode at 0V (grid is negative), you will drop 200V across the resistor, and you end up with 200V on the plate. Your operating point for the load line is 200V and 40 mA. When the tube is cut off, that point on the loadline will be 400V and 0 mA. If the plate could swing to 0V (it can't...), that point on the loadline will be 0V and 80 mA.
Now consider a B+ of only 200V and transformer AC loading of 5K (with negligable DC resistance), same current of 40 mA and 0 volts on the cathode. Your operating point for the loadline will be just under 200V and 40 mA. When the tube is cut off, that point on the loadline will be 400V and 0 mA. If the plate could swing to 0V, that point on the loadline will be 0V and 80 mA. So you have the same voltage swing (0 to 400V) as the resistor loading example, but you get that with half of the B+.
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Tikiroo,
Exactly lol..
Im confused by your response. Im not sure if your stating plate to cathode voltage or B+ should be the voltage taken into consideration when drawing the dc load line. It sounds like you believe the higher of the two(b+) should be used and if so we are in agreement.
i this all started because furtur up i drew a schematic with B+ of 468v and 390v from plate to cathode. Folks said my dc plate load line should use the 390 ( which is the voltage dropbof the tube with zero signal)
Good times all..
Exactly lol..
Im confused by your response. Im not sure if your stating plate to cathode voltage or B+ should be the voltage taken into consideration when drawing the dc load line. It sounds like you believe the higher of the two(b+) should be used and if so we are in agreement.
i this all started because furtur up i drew a schematic with B+ of 468v and 390v from plate to cathode. Folks said my dc plate load line should use the 390 ( which is the voltage dropbof the tube with zero signal)
Good times all..
AudioFanMan, the thing is that "DC load line" in an utterly pointless concept. Just forget about it, it does nothing.
You're trying to understand things backwards, and obviously it doesn't work.
You need two things to design the tube stage.
The first is the operating point (OP) - the tube's quiescent plate current and plate-to-cathode voltage (yes, 390V, not the B+). It's your job to find the optimal OP - you don't calculate it with the B+ and/or cathode resistor which are already present, you choose it. Let's call those Ip0 and Upc0 respectively. Let's also say they are 74mA and 390V. These two parameters and the plate curves also determine the bias voltage (about -78V).
The second is the AC load line - the dynamic characteristics of the tube's AC output.
This line passes through two points: the OP and the point [I=0; U=Upc0+Ip0*Rl] on the plate curves, where Rl is the load impedance (doesn't matter if the load is resistive or inductive). In your case with, say, with 5k transformer it will be at 390V+74mA*5k=760V (yes, it is absent on the graph - extend the voltage axis on the paper or computer screen).
Then and only then you calculate the cathode resistor (if you choose to use self-bias, of course) by dividing bias voltage (use positive value, not negative) it by quiescent current (78V/74mA=1.05kOhm).
Then and only then you calculate the B+ (390V+78V=468V for self-bias or just 390V for fixed bias - those are the values for the inductive load, for resistive one you'll also have to add the static voltage drop on the plate resistor, Ip0*Rl) and design the PSU.
You're trying to understand things backwards, and obviously it doesn't work.
You need two things to design the tube stage.
The first is the operating point (OP) - the tube's quiescent plate current and plate-to-cathode voltage (yes, 390V, not the B+). It's your job to find the optimal OP - you don't calculate it with the B+ and/or cathode resistor which are already present, you choose it. Let's call those Ip0 and Upc0 respectively. Let's also say they are 74mA and 390V. These two parameters and the plate curves also determine the bias voltage (about -78V).
The second is the AC load line - the dynamic characteristics of the tube's AC output.
This line passes through two points: the OP and the point [I=0; U=Upc0+Ip0*Rl] on the plate curves, where Rl is the load impedance (doesn't matter if the load is resistive or inductive). In your case with, say, with 5k transformer it will be at 390V+74mA*5k=760V (yes, it is absent on the graph - extend the voltage axis on the paper or computer screen).
Then and only then you calculate the cathode resistor (if you choose to use self-bias, of course) by dividing bias voltage (use positive value, not negative) it by quiescent current (78V/74mA=1.05kOhm).
Then and only then you calculate the B+ (390V+78V=468V for self-bias or just 390V for fixed bias - those are the values for the inductive load, for resistive one you'll also have to add the static voltage drop on the plate resistor, Ip0*Rl) and design the PSU.
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TG, please, dont hold back, tell me how you feel lolol. I would like your post if there was a button hahah..
I will re-read what you wrote, try to apply to what i have and see where it leads me.
I will re-read what you wrote, try to apply to what i have and see where it leads me.
I'm not holding back by any means.
I do think you're trying to reverse-engineer the thing before you've learned how to forward-engineer it. It won't work, neither for you, nor for anyone else on the planet.
I do think if you are able to get through my post you'll answer most (if not all) of your previous questions yourself.
That's it.
I do think you're trying to reverse-engineer the thing before you've learned how to forward-engineer it. It won't work, neither for you, nor for anyone else on the planet.
I do think if you are able to get through my post you'll answer most (if not all) of your previous questions yourself.
That's it.
Why? . . . . . Because no current can flow through an open circuit and when no current flows there is no voltage drop.
Get a basic book on electricity,(Look up Van Valkenburgh, Nooger,& Neville.) then electronics.
Old military texts are especially good as they are simple, one principle at a time explanations with little questionaires at the end of each section to make sure you actually took in what you read.
Happy trails!
Get a basic book on electricity,(Look up Van Valkenburgh, Nooger,& Neville.) then electronics.
Old military texts are especially good as they are simple, one principle at a time explanations with little questionaires at the end of each section to make sure you actually took in what you read.
Happy trails!