Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ritchie200 on September 22, 2011, 10:23:16 pm
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Reading up on UL operation and I have a question. All the articles I read state that the 6L6/KT88/KT66/etc family "likes" about a 42% plate v tap. The 6V6 family "likes" about a 23% plate v tap. Ok, I'll take their word for it. However, nothing that I have read explains WHY! Why that percentage and why the difference in tube families? Other than it is figured on a percentage of the impedance - there is no explanation. If optimal is 40%, what happens if it is 60%? Or 20%? Can anyone "esplain" to me in plain english what the heck is going on?
Thank you in advance!
Jim
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Reading up on UL operation and I have a question. All the articles I read state that the 6L6/KT88/KT66/etc family "likes" about a 42% plate v tap. The 6V6 family "likes" about a 23% plate v tap. Ok, I'll take their word for it. However, nothing that I have read explains WHY! Why that percentage and why the difference in tube families? Other than it is figured on a percentage of the impedance - there is no explanation. If optimal is 40%, what happens if it is 60%? Or 20%? Can anyone "esplain" to me in plain english what the heck is going on?
Thank you in advance!
Jim
http://en.wikipedia.org/wiki/Ultra-Linear (http://en.wikipedia.org/wiki/Ultra-Linear)
Apparently gives you characteristics of pentode and triode operation. As far as how they arrived at the numbers, I'm guessing trial and error. I wouldn't be surprised if no one has managed to analyze it physically.
Chris
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As I understand it (and I'm usually wrong), it's got a lot to do with the physical construction of the tube. Beam Power tubes tend to have a more durable screen and therefore can handle higher voltages better. True pentodes and tetrodes are happier with a lower screen voltage and therefore the UL tap should be further away from the plate taps. Tubes such as the 6CD6 call for a screen voltage to be roughly 1/2 of the plate voltage. They might not even work in a UL configuration. Hopefully someone in the know will answer this question properly.
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Without burning the mid-night oil, that's what I'm thinking. Take the 6550: http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6550A.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/135/6/6550A.pdf)
Especially good for hi-fi. In Class AB1 you get only 0.6% Harmonic Distortion with 400V plate; 270V screen at idle. UL is a good way to get there. And for mass production, UL also eliminates the cost of filter circuitry, including a choke, for the screens. May as well put all that iron in one package.
So I'm thinking that design choice in selection of: a) the power tube; and b) the class of bias -- will determine what %-age of plate voltage the UL tap should be. The goal seems to minimize Harmonic Distortion, and to fine-tune quiescent plate/screen voltage to do so. But this will not apply if Harmonic Distortion is not an issue, or if it is desirable; then screen & plate voltages can be equal at idle.
However, I would consult the literature.
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try the link below. i have not read the whole article... it may or may not contain the information you seek, however, broskie does explain things well.
http://www.tubecad.com/january2000/page3.html (http://www.tubecad.com/january2000/page3.html)
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> WHY! Why that percentage
It's BS. The percent isn't that fussy.
There's the two extreme "UL" conditions:
0% == Pure Pentode, high power output, no damping
100% == Pure Triode, lower power output, good damping
In-between you have a range of in-between possibilities.
There is a study, somewhere, showing that for specific tubes and goals, 25%-60% is in-between triode and pentode, with a slight preference for "43%". But it isn't that fussy. And if you pick different conditions and goals, you'd come up with some other "optimum", still not critical.
6V6 at traditional voltages (250V-300V) is marginal for current. Heavy UL probably trims power a lot. A lesser tap might do better than you'd pick for a fat tube like EL34.
Since the only OTs you can actually buy are 40% or 43%, and that's as good as any other value, it isn't worth asking "WHY?"
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> Tubes such as the 6CD6 call for a screen voltage to be roughly 1/2 of the plate voltage. They might not even work in a UL configuration.
If you must do it, you use an additional winding, wound about 40% of the plate winding and fed the lower G2 voltage.
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Why? Because it's there! :icon_biggrin: Thanks PRR, that makes sense. Yeah, all the iron I've seen is 40% so it's a moot point if I was building - I was just trying to figure out how they got there. And thanks to the rest of you guys for weighing in - I've been to all those sites!
Jim
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I read (though I can't remember where) that the UL tap that results in the greatest power/distortion ratio is closely related to the distance between screen and cathode; valves that have the screen closer to the anode naturally want a UL tap closer to the anode, and vice versa.
Plausible, but I don't know whether to believe it.
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If you must do it, you use an additional winding, wound about 40% of the plate winding and fed the lower G2 voltage.
PRR do you mean like this ?
Kagliostro
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> related to the distance between screen and cathode; ...Plausible, but I don't know whether to believe it.
The form of the problem suggests a ratio, not a pure "distance".
It's late. I'm not thinking. But a triode has good (certain assumptions) balance of power and gain when Mu is 4 or 5. 6L6 EL34 and kin have Mu of 10. 42% of 10 is 4.2, a good Mu. But I can't convince myself that 42% of Mu is the right answer.
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> related to the distance between screen and cathode; ...Plausible, but I don't know whether to believe it.
The form of the problem suggests a ratio, not a pure "distance".
You know perfectly well what I mean. If you want to be pedantic, "100% times the screen-cathode distance / anode-cathode distance".