Hey everyone,
Well, let me put this project into a bit of perspective, because if I blurt out what I want to do, I WILL be called crazy. Keep in mind, you'll probably call me that anyway, but at least you know what you're dealing with.
Basically, I'm on the high side of 20 years old, and since even before getting my license, I've wanted to build a 4-1000A linear amplifier, due to a multitude of reasons - namely, the fact that such an amplifier is so difficult to design if I could do it would prove de facto to myself that I know a thing or two. Over time, it became a way, to do just that - to prove my skills to myself.
Also, one need not discount the wonder of it all. Gigantic, 9x5" tube that glows like a lightbulb and can produce those enormous power levels. For someone who is young (and a kid in many ways, notwithstanding), such a tube piqued my interest very early on, and I've run through many articles, many drawings, and many thoughts of how I would do it as soon as I had the resources. You can boil it down to a little bit of the youthful mentality of "bigger the tube, bigger the glory".
Past few days, I've begun wondering about the idea of building a two-tube PP amp and running it at reduced power to stay under legal limit and stay well below the maximum ratings of 4.2kW at ~5kV. Considering most HF transceivers I've seen have around 100W finals at full power, this would be more than achievable.
Before I go on with what questions I have, keep a few things in mind:
1) The idea of building a PP linear at this point is just a bunch of musings. If it finally sets on me that it's a ridiculous amount of power or if I find that the construction isn't practical/feasible, I won't let it leave the pen-and-paper stage.
2) Building a 4-1000A amp at all is still a ways off for me - I'm a college student, and that kind of money doesn't like being in my wallet.
Alright, now for the questions.
I've designed many a push-pull, but only in the audio realm. I know the topology of the tubes well enough, but only up to the point of the output. Obviously, these devices have to be put through impedance tuning, and to the extent of my knowledge the standard matching networks aren't designed to be balanced as is the direct output of a PP pair. I've thought of using a balun rated for 5kW max, but the only ones I've found are for antenna use and designed to accept 50 ohm coax. Ideally, the length aspect will be minimal - would this work - taking the tube pair and converting it to an unbalanced topology for the matching network - or would the 50-ohm normalization of the balun interacting with the high-impedance final output cause back-reflection enough to fry the tubes?
Also, I had interest in building a custom bucking regulator to develop the filament power (7.5V @ 21A per tube). Most designs I see use a centre-tapped transformer with the tap grounded to provide a reference. Would a bucking regulator arrangement with the negative leg grounded function similarly?
I have design experience in high-power buck/boost design, just not the use of those systems for RF systems. It should be noted that I would shield the main circuit to prevent against stray RFI interactions.
I'm sure more questions will come up later, if any of you can be bothered to help a kid with an absurd dream project I'd love to talk more.
73,
Eli KK6FVT
Well, let me put this project into a bit of perspective, because if I blurt out what I want to do, I WILL be called crazy. Keep in mind, you'll probably call me that anyway, but at least you know what you're dealing with.
Basically, I'm on the high side of 20 years old, and since even before getting my license, I've wanted to build a 4-1000A linear amplifier, due to a multitude of reasons - namely, the fact that such an amplifier is so difficult to design if I could do it would prove de facto to myself that I know a thing or two. Over time, it became a way, to do just that - to prove my skills to myself.
Also, one need not discount the wonder of it all. Gigantic, 9x5" tube that glows like a lightbulb and can produce those enormous power levels. For someone who is young (and a kid in many ways, notwithstanding), such a tube piqued my interest very early on, and I've run through many articles, many drawings, and many thoughts of how I would do it as soon as I had the resources. You can boil it down to a little bit of the youthful mentality of "bigger the tube, bigger the glory".
Past few days, I've begun wondering about the idea of building a two-tube PP amp and running it at reduced power to stay under legal limit and stay well below the maximum ratings of 4.2kW at ~5kV. Considering most HF transceivers I've seen have around 100W finals at full power, this would be more than achievable.
Before I go on with what questions I have, keep a few things in mind:
1) The idea of building a PP linear at this point is just a bunch of musings. If it finally sets on me that it's a ridiculous amount of power or if I find that the construction isn't practical/feasible, I won't let it leave the pen-and-paper stage.
2) Building a 4-1000A amp at all is still a ways off for me - I'm a college student, and that kind of money doesn't like being in my wallet.
Alright, now for the questions.
I've designed many a push-pull, but only in the audio realm. I know the topology of the tubes well enough, but only up to the point of the output. Obviously, these devices have to be put through impedance tuning, and to the extent of my knowledge the standard matching networks aren't designed to be balanced as is the direct output of a PP pair. I've thought of using a balun rated for 5kW max, but the only ones I've found are for antenna use and designed to accept 50 ohm coax. Ideally, the length aspect will be minimal - would this work - taking the tube pair and converting it to an unbalanced topology for the matching network - or would the 50-ohm normalization of the balun interacting with the high-impedance final output cause back-reflection enough to fry the tubes?
Also, I had interest in building a custom bucking regulator to develop the filament power (7.5V @ 21A per tube). Most designs I see use a centre-tapped transformer with the tap grounded to provide a reference. Would a bucking regulator arrangement with the negative leg grounded function similarly?
I have design experience in high-power buck/boost design, just not the use of those systems for RF systems. It should be noted that I would shield the main circuit to prevent against stray RFI interactions.
I'm sure more questions will come up later, if any of you can be bothered to help a kid with an absurd dream project I'd love to talk more.
73,
Eli KK6FVT
