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Old 19th March 2012, 04:16 AM   #1
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Default Operating point for pp KT88

For pentode operation, what is the grid voltage for 420V plate 70~75mA per tube,fixed bias?
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Old 19th March 2012, 09:52 AM   #2
DF96 is offline DF96  England
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Silly question this, but does the KT88 datasheet cast any light on this?
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Old 19th March 2012, 11:18 AM   #3
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Quote:
Originally Posted by DF96 View Post
Silly question this, but does the KT88 datasheet cast any light on this?
Nope. Those datasheet I found only show ultra linear connection. What I proposed actually uses the same plate and screen voltage at 420V which is similar to those ultralinear. But looking at the differences between UL curve and Pentode curve, I think the biasing will be different. Pentode should be lower (less negative).

I was hoping to get response from people who convert UL kt88 to pentode which 420V is pretty common. But seem like its not, oh well. The most similar design is a Macintosh MC75 or MC275 where it is a pentode in operation. Read that mc275 bias at 50mA per kt88 at -45V. Is that right?
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Old 19th March 2012, 11:29 AM   #4
DF96 is offline DF96  England
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Dynamic characteristics will be different between UL and pentode connections, but static will be the same. Under quiescent conditions the valve does not know the difference, it just sees DC voltages on each electrode.

You can interpolate/extrapolate from a given bias point by using the g2 mu figure - not sure if the data sheet gives that or not. For example (figures made up) if the g2 mu is 10, and you have data for 450V screen, -50V grid then for 420V screen you would need around -47V on the grid. 30V screen change is equivalent to 30V/10=3V grid change.
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Old 19th March 2012, 05:28 PM   #5
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Quote:
Originally Posted by DF96 View Post
Dynamic characteristics will be different between UL and pentode connections, but static will be the same. Under quiescent conditions the valve does not know the difference, it just sees DC voltages on each electrode.
I understand that the plate current is very much determined by the screen-cathode voltage. But those curves between tetrode, UL and triode show differences enough to confuse me. From what you mentioned, then if the screen-cathode voltage remain constant, the bias should hold between tetrode, UL and triode mode of operation. Is that correct? Then how do we interpret the curves then?
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Old 19th March 2012, 06:17 PM   #6
DF96 is offline DF96  England
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There are two bias issues, which sometimes get confused. The first is choosing the quiescent current. To do this you either need to use the published curves for the mode you have chosen, or follow someone else's advice. You have already chosen your current.

The second issue is choosing the grid bias to get the current you have chosen. For this you can use any set of curves, because only DC is involved. Apart from mistakes or experimental error, the curves should give you exactly the same current for a given set of g1, g2 and anode voltages - whichever set of curves you use. At DC there is no difference between triode, UL or pentode. Choose a point which is nearest to your chosen bias, then interpolate/extrapolate as necessary. 420V on the screen is 420V however it gets there. The only minor difference is that the anode and screen currents move in opposite directions by a small amount as the anode voltage increases past the screen voltage.

Remember that whatever g1 voltage you choose will need to be adjusted anyway to match particular valve specimens.
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Old 19th March 2012, 07:07 PM   #7
avp1 is offline avp1  United States
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Datasheet here http://www.r-type.org/pdfs/kt88.pdf has tetrode mode operational information. It should give you a clue.
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Old 19th March 2012, 07:41 PM   #8
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What I concern about is the kt88 burn out due to wrong bias range when firing up the amp the first time. For fixed bias operation, we usually turn the bias up to the full negative voltage for safety and readjust the bias after the tubes warm up. Of course there will be fine tuning as the tube run in. Can you recommend the range of negative bias supply? For eg. -50 to -70V?
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