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Old 8th May 2012, 08:02 PM   #1
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Default KT88 PP UL Design Help Needed

I am learning by designing a tube amp. The design is intended to be simple with the fewest parts possible. I am looking to use a NAD T187 as the input preamp so my design picks up at the phase splitter, driver and power stages. I have found an excellent tutorial showing step by step how to design a LTP phase splitter/driver so that portion is in the I understand category. Now for the KT88 PP UL portion. I have printed out some UL Curves but am somewhat uncertain I am using them correctly. The legend on the graph says Vsc0=400.0 %=43.0. I assume this means the OPT has 43% taps but the Vsc0 listing has me confused, if my screen is attached to the 43% tap it is variable and not fixed. What does this term refer to? Now if I pick a B+ of say 560V and a negative bias of 50V and a cathode bias point of 50mA what is my next step? How do I select the appropriate OPT primary resistance?

I do have other questions (NFB, PSU...)and they will be researched one at a time. For now I am trying to understand the Ultralinear operation of a Beam Tetrode.
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Old 8th May 2012, 08:15 PM   #2
SY is offline SY  United States
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You have two choices- you can get KT88 ultralinear curves, construct composite curves, then use loadline techniques to determine optimum load. Or... you can use the values off the KT88 datasheet, where engineers who knew the tube intimately have already worked this out. MOV recommends 7k plate-to-plate for your operating conditions.
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Old 8th May 2012, 09:23 PM   #3
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Having a solution to the problem is good but I want to learn how to get from A to B. So yes option 1. How do I do this?
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Old 8th May 2012, 10:03 PM   #4
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That might be a bit long to type, and even longer to create figures. Fortunately, Norman Crowhurst already did the work, and in a very clear and readable manner.

http://www.recordcrate.net/diy/ClassRoom/ga400ac.pdf
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Old 8th May 2012, 10:43 PM   #5
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Here you will find about everything related to UL:
Tubes
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Old 9th May 2012, 12:38 PM   #6
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Pieter,
I have read this page before but it doesn't answer my questions.
With a pair of output tubes selected for PP UL class AB how do I determine (select) the quiescent current, the typical operating current, the max current, the optimal drive voltage and current to achieve a given output power, the optimal OPT for a given output power. There is a guide on LTP design here: http://www.aikenamps.com/LongTailPairDesign.htm that was perfectly written for my level of understanding. I am hoping to find a similar article that goes over the elements required for Tetrode UL PP class AB output design.

SY I will scour the site you suggested to me today.

JW
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Old 9th May 2012, 02:21 PM   #7
scott17 is offline scott17  United States
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Default KT88 datasheet

The datasheet at the link below answers every question you asked.

Page one lists the max ratings.

Page two shows the operating parameters and values for push-pull UL, either cathode or fixed bias.

http://frank.pocnet.net/sheets/084/k/KT88.pdf

Scott

Last edited by scott17; 9th May 2012 at 02:25 PM. Reason: clarify intent
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Old 9th May 2012, 02:53 PM   #8
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Scott,
Yes thank you for the link that is a great data sheet, more complete than any I have found. This still leaves me with my original questions, however. I want to learn how to calculate the values tabulated. I may be trying to go into too much detail for an amoplifier design but I really want to understand the design from the input to the output. Once I have learned about this stage I will need to learn about feedback and psu design. I realize it is a long road ahead but I'm a bored engineer and this stuff is really keeping my mind busy.

So if you have any other treasures of links that go into more detail in the how department I would love to have them.

JW
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Old 9th May 2012, 02:57 PM   #9
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That entire Crowhurst series (I linked to the p-p load line chapter) is well worth reading. One thing to keep in mind- don't take the loadlines too seriously. Real speakers are by and large not resistive loads, so that nice loadline turns into a somewhat lopsided load ellipse.
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Old 9th May 2012, 03:33 PM   #10
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Here is another link. Under the first column in the table, "Topics:", click on "Theory".

Steve Bench Tube Pages
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