KT88 PP UL Design Help Needed

[jwsplatjw]

Scott,
You nailed it. I'll be studying Steve's site for a while now. Thanks for your help!

JW

[nigel pearson]

This is useful also . Circuit principles apply to hi fi also . How we see things is often refereed to . Excellent description of SRPP .

How to design valve guitar amplifiers

[boywonder]

and Patrick Turner's education/DIY pages...

loadmatch4-pp-beamtetrodes

[jwsplatjw]

I worked through Patrick's guide but I don't have confidence in my results. The curves I have show bias voltages from 0 to -70, the data sheet for the KT88 suggests a bias point of -75 volts. Does anyone have a set of curves for the KT88 with a wider range of bias voltages? Have I made a mistake?

JW

[nigel pearson]

One way to do is to set bias at - 100 V ( g1 ) and gradually work down . Have a 10 R resistor at the cathode so as to read current (V/R = I ) . Don't go above 70 mA I would say . We did it that way at college . We had to plot the curves and say which was best and why . You can use a spectrum analyzer to see the results ( cheap one , freeware perhaps ) . A KT 88 should give about 1% distrotion I suspect . Set HT to about 400 V . Smoothly reducing harmonics will say you got it right . Real KT 88's are not all the same . I suspect JJ are different and need tweaking . Also one can then use any similar tube as you know how . The people who did it first did it this way in the 1920's . Always fit a grid stopper ( 1 K is safe usually , no magic it is a RC filter ) .

Good luck with it .

[jwsplatjw]

Nigel,
I am still at the pencil and paper stage I am trying to learn how to plot load lines on plate curves. The bias voltage given on the KT88 data sheet is -75 V but the plots of plate current vs bias voltage I find only go up to -70 V. I have read that I only want to put about 50 mA on the cathode to get full power out of the tubes. Am I confusing two different values? Is the bias grid voltage shown on the plot the DC offset to allow a full swing of the signal input voltage? If this is the case I only need a negative bias of one half of my peak to peak drive voltage, true? The cathode bias I can set independant of the grid by say using an LM317 twisted into a constant current source of 50 mA. Am I close to understanding what is going on here?

JW

[boywonder]

Originally Posted by jwsplatjw

Is the bias grid voltage shown on the plot the DC offset to allow a full swing of the signal input voltage? If this is the case I only need a negative bias of one half of my peak to peak drive voltage, true?

For class AB1 operation, yes.

[nigel pearson]

Originally Posted by jwsplatjw

Nigel,
I am still at the pencil and paper stage I am trying to learn how to plot load lines on plate curves. The bias voltage given on the KT88 data sheet is -75 V but the plots of plate current vs bias voltage I find only go up to -70 V. I have read that I only want to put about 50 mA on the cathode to get full power out of the tubes. Am I confusing two different values? Is the bias grid voltage shown on the plot the DC offset to allow a full swing of the signal input voltage? If this is the case I only need a negative bias of one half of my peak to peak drive voltage, true? The cathode bias I can set independant of the grid by say using an LM317 twisted into a constant current source of 50 mA. Am I close to understanding what is going on here?

JW

I can never get my head around them . I was taught by Sid Smith the designer of Marantz amps . He gave up . He agreed my way works . Very nice man was Sid . Sid used LM317 for HT smoothing . It was arranged to work at 400 V and was a famous modification . I always would use cathode bias as it adjusts the KT88 as it ages . Sid always triode connected his amps at home . Sowter in UK make transformers about as good as SIds .

[jwsplatjw]

Nigel,
I would like to see how to make LM317 regulate HT voltages, do you have a schematic?

JW

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