Hey all,
Can someone explain how to draw load lines for a pentode for a push-pull output stage and how to use the load line for amp design?
I found a couple articles online, but it would be nice to have someone to ask questions of through out the explaination.
Thanks in advance.
Can someone explain how to draw load lines for a pentode for a push-pull output stage and how to use the load line for amp design?
I found a couple articles online, but it would be nice to have someone to ask questions of through out the explaination.
Thanks in advance.
Steve Bench can.
Miles also gave a very good round-up of the entire design process.
Everyone will tell you about Morgan Jones. Go get a copy.
Miles also gave a very good round-up of the entire design process.
Everyone will tell you about Morgan Jones. Go get a copy.
I would recommend going to Pete Millett's page, download the Radiotron Designer's Handbook 4th Ed, and specifically read through chapter 13.
I find this book very refreshing as it was written at a time predating calculators, so even though the theory is described in detail, estimation and good design rules are stressed as the main tools for design. It was simply too cumbersome to plug numbers into all those equations to work out values in detail.
The distortion ruler at the start of chapter 13 is a great example.
I find this book very refreshing as it was written at a time predating calculators, so even though the theory is described in detail, estimation and good design rules are stressed as the main tools for design. It was simply too cumbersome to plug numbers into all those equations to work out values in detail.
The distortion ruler at the start of chapter 13 is a great example.
Thanks for the links. I'm going to have to download Pete's entire library. I'm sure there's going to be more questions.
Ok, so I want to use the 6BN11 dual Pentode. I've been reading through Steve Bench's page on load lines. I got the maximum power dissipation curve plotted and I picked a quiescent point. Now I'm getting a little confused on plotting the second point to draw the load line.
Steve says to pick a voltage close to double the voltage of the quiescent point and zero milliamperes to draw the load line. And he gives an equation to figure out the primary impedance of the output transformer. And he says that you can choose a transformer and use that impedance for the load line.
So, what if the end voltage from the chosen transformer is less than twice the quiescent voltage? do I need to pick a different quiescent point or is the load line ok with the chosen point?
Steve says to pick a voltage close to double the voltage of the quiescent point and zero milliamperes to draw the load line. And he gives an equation to figure out the primary impedance of the output transformer. And he says that you can choose a transformer and use that impedance for the load line.
So, what if the end voltage from the chosen transformer is less than twice the quiescent voltage? do I need to pick a different quiescent point or is the load line ok with the chosen point?
Remember the elemntary school and function basics at your maths class ? 🙂 There are two ways to draw the line, one is by drawing a line through two points (which is what you're trying to do) and the other is drawing a line based on the chosen slope and though selected point (which is what you should be doing).
Load impedance = slope. Load impedance = reflected speaker impedance + (usually neglected) transformer load.
Now that you think about it it all makes sense ... but your next question arises: how do I figure out the right load impedance for my tube of choice in the first place ? Afterall most popular tubes (such as EL84, EL34, KT66, etc.) have their load impedance specified in the datasheet so idjits who are into "modding" their gear can pretend they understand tubes and electronics too but perhaps your tube doesn't 😱 Well, you've got the maximum suggested dissipation curve plotted and you want to stay below it in quiescent point, around quiescent point and probably at least throughout up to 50% of the output swing if not more (small excursions across into "red zone" are OK, because tube only stays there less than 1/2 of time due to push-pull operation and cutting off in each halfperiod of the signal, and thanks to thermal inertia of the anode that evens out over time and doesn't cause instantaneous failure as in sandy components). Therefore it makes sense to choose your load and quiescent point so that loadline breaks in such a way that it just about touches the dissipation curve in two points or only crosses it when in class B and towards the outermost end of the excursion.
The 2 : 1 slope relation between class B and A remains, and once taken into account a suitable load can be chosen. Only then can you purchase/order your output transformer 🙂
Oh and I just sense another question coming up: how do I know where precisely the loadline actually breaks ? Well, it breaks when one tube cuts off. If your quiescent point is at Vg = -10V and the output tube cuts off at -15V, the point where the loadline breaks must obviously be where the class A loadline and Vg = -5V curve intersect.
Comprende ? 😉 Oh and the other method (the one you described) is also perfectly valid, you'll figure it out as soon as you grasp this one.
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This link only discusses class A triodes, although I think it is important to understand this first before trying wrap your head around push-pull.
For pentodes, the advice seems to be to draw the load line from the 'knee' in the pentode curve to the quiescent voltage point at zero current (figures 13.42/13.43 in RDH4). In AB push-pull, even though each tube is drawing some current at the quiescent point, the net effect is zero current through the transformer.
Once you have an idea of the desired impedance (don't forget the factor of 4) then you can go looking at what values are available for real transformers and try to pick something close. If you want to emphasize power or low distortion, you may fudge a bit one way or the other.
This simple spreadsheet that I made many years ago might help you.
http://www.revintage.se/PPABAMP.xls
Can be used for pentodes, triodes, A or AB.
http://www.revintage.se/PPABAMP.xls
Can be used for pentodes, triodes, A or AB.
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