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E80CC/KT88 SE Triode/UL question
E80CC/KT88 SE Triode/UL question
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Old 18th May 2018, 05:06 PM   #11
jazbo8 is offline jazbo8
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E80CC/KT88 SE Triode/UL question
Your operating conditions are better than the 425V proposed above, but you should still draw the loadline and make sure everything looks ok.
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Old 18th May 2018, 06:49 PM   #12
Viking83 is offline Viking83  Norway
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Well, I thought so too, but now I am not so sure anymore. Iíve drawn loadlines, made calculations etc, everything worked out quite nice actually. About to start looking at a power supply this evening.

So just to make sure, can I still use these data even though my working point is at a much lower Vq?

And another thing; below the Va line there is a line for Vg2 and Va=300 V corresponds to 375 V Vg2. Is this the voltage ratio between Va and Vg2 at this given operating point?
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Old 18th May 2018, 07:38 PM   #13
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E80CC/KT88 SE Triode/UL question
Yes, the same characteristic can be used. In your case, Va = Vg2 = 300V at idle.
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Old 18th May 2018, 07:49 PM   #14
Viking83 is offline Viking83  Norway
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Sounds perfect! Thanks for replying everyone.
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Old 18th May 2018, 08:22 PM   #15
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by jazbo8 View Post
Yes, the same characteristic can be used. In your case, Va = Vg2 = 300V at idle.
I have to disagree
With other Vg2 (40% of the drop from 300V not 425V) the curves end up much lower (current).For every supply voltage (and tap %) there is another set of curves.
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Old 18th May 2018, 08:33 PM   #16
jazbo8 is offline jazbo8
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E80CC/KT88 SE Triode/UL question
Yes and no... The Va-Ia scales have to be changed but the characteristic can still be used. Doing it manually is difficult but much easier with a simulator or other software.

But I hope you agree that 425V isn’t the ideal voltage for SEUL operation.
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Old 18th May 2018, 08:38 PM   #17
6A3sUMMER is offline 6A3sUMMER  United States
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That is one reason why UL is so hard to predict. Need more sets of UL curves at different screen voltages.
Or try and use another mode (Pentode or Triode wired).

Even some old manufacturers of tube curves, i.e. 6L6 and 807 have multiple sets of pentode curves that are for screen voltages of 150, 200, 250, 300V. It makes it easy, just make your choice of screen voltage, and quiescent conditions, and draw a load line.

You can get some ball park numbers of what kind of power you will get:

Look up and down the load line, and see the peak watts from quiescent bias to 0V grid (Delta Vp/Load Z), and peak watts from quiescent bias to 2 times quiescent bias (Delta Vp/Load Z). Then you can easily calculate the approximate results for single ended, or for push pull.

Suppose from quiescent bias to 0V the plate volt change into the plate load is 12 (peak) Watts, and from quiescent bias to 2x quiescent bias the plate voltage change is 4 (peak) Watts.
For single ended, you get (12 + 4)/2 = 8 peak watts , and ((12 + 4)/2)/2 = 4 Watts rms (but of course at very high distortion for this made up example). This will have dominant 2nd harmonic distortion.

Then for push pull, if you use a plate to plate impedance = 2x the (SE) load line, then the peak Watts is 12 + 4 = 16 Watts. That is 8 Watts rms.
The distortion of this made up example will be far less than for the SE mode.
But it will be dominant 3rd harmonic distortion.

These are over simplifications, but make for quick estimation of results.

This also works for SE and Push Pull triode wired mode, if you can find a single set of triode wired curves for your favorite Pentode or favorite Beam Power tube. And for triode wired curves, you only need one set (you do not need a separate set for 150V, 200V, 250V, 300V on the screen, because the plate and screen voltage are always the same (except for a very low voltage drop through a (i.e. 100 Ohm) screen stopper resistor).
Pick your quiescent state, draw a load line, and Go!

Last edited by 6A3sUMMER; 18th May 2018 at 09:08 PM.
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Old 18th May 2018, 09:11 PM   #18
jazbo8 is offline jazbo8
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E80CC/KT88 SE Triode/UL question
Well said, I should have been clearer in my earlier posts, sorry for any confusion caused. Even though we know the “shape” of the UL curves, the scales will change depending on the actual operation conditions as Ketje rightfully pointed out.
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Old 18th May 2018, 10:09 PM   #19
Viking83 is offline Viking83  Norway
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Hmm, ok, guess I stepped in a rookie-trap there... Is it possible to make an estimate on Iq based on the ratio 425Va/300Va? If things change fairly linearly will Iq change in direct proportion to the voltage difference? I’m suspecting it won’t... The datasheets have curves for different Vg2 in pentode mode, and it doesn’t look like there’s to much predictability.

Since I will probably order a lot of stuff from overseas, Edcor transformers for example, I want to be sure I have something that might work ok going on.

Maybe triode mode will be the easy way out, but I liked the idea of the extra power with ok distortion numbers...
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Old 18th May 2018, 10:10 PM   #20
PRR is offline PRR  United States
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Quote:
Originally Posted by 6A3sUMMER View Post
That is one reason why UL is so hard to predict. ....
Why "predict"?

Design a straight Pentode stage only with Vg2=Vp. Use that bias and load, but buy the OT with a UL tap. Try both ways. (And also Triode; solder is cheap.)

Yes, the Pentode will NEED a heap of NFB. But UL usually needs more than just UL NFB, so no change there.

One thing is that a Pentode can usually run Vg2 lower than Vp, with savings of Ig2 and G1 drive. The tap-UL can't. There is a form with a tertiary winding for G2 but nobody does that.
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