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E80CC/KT88 SE Triode/UL question
E80CC/KT88 SE Triode/UL question
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Old 16th May 2018, 06:35 PM   #1
Viking83 is offline Viking83  Norway
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Default E80CC/KT88 SE Triode/UL question

Hi

Working on a schematic for a E80CC/KT88 SE UL amp,and I thougt this might be my thread for newbie questions. I know there are quite a few of threads like this but it takes time to search for just the right answer, so I hope youíd bear with me.

I am using the KT88 in SE UL mode, it should give sufficient power and has rather nice distortion numbers.

When in UL mode with g2 at the 40 % tap, how do I find the current passing in it? Read somewhere that the relation between Ik and Ig2 would be fairly constant across different operating points, and this gave a fair approximation? JJ states 140/7 mA, which is 5%

Also, calculating output power with a 3,5k OPT gave 10,5W, using formula P=V2/R. Not considering OPT losses, but still seems a little low for UL?
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Old 17th May 2018, 02:42 AM   #2
PRR is offline PRR  United States
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Quote:
Originally Posted by Viking83 View Post
....how do I find the current passing in it?
...Also, calculating output power with a 3,5k OPT gave 10,5W, using formula P=V2/R. ...
Why do you care what Ig2 is? It is surely "small" compared to Ip.

Show your math on that power computation. What "V"? or "V2"?

The DC dissipation is related to, but not the same, as the sine audio power output.

The audio output for class A is at-most 50% of the idle dissipation. For a 40W tube, 20W. For a real tube, 40% is doing very good, so 16W out. For real tubes with real transformers, working 6550 at 39W dissipation I got 13W clean and 17W with guitar-tolerable THD. For a less aggressive condition 10W might be what it does.
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Old 17th May 2018, 07:59 AM   #3
jazbo8 is offline jazbo8
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E80CC/KT88 SE Triode/UL question
Arenít there some datasheets or published UL characteristic for the KT88 that you can use for the calculation? Should be straightforward...

[EDIT]Found it.
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Old 17th May 2018, 09:51 AM   #4
Viking83 is offline Viking83  Norway
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Hi

Iíve got the tubedata, do not remember whose it is but I suppose thatís not important.

The reason for asking was it ads to the total current, needed to know to calculate Rk... Anyhow, I just went with what the diagram said. Every circuit Iíve seen just connect r2 to the UL tap, maybe a with a grid stopper but nothing else.

Tried different Zopt, appearantly approx 2k is a sweetspot between power and 2nd h. distortion. With 2k I should be able to get 14-15W in UL mode. 3,5k lowered it to 10,5 W not considering OPT losses, those are triode territory if Iím mistaking...

Have to run for now.
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Old 17th May 2018, 10:28 AM   #5
jazbo8 is offline jazbo8
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E80CC/KT88 SE Triode/UL question
Right, I see you mean, the datasheet's UL figures are for push-pull, not really suitable for SE.
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Old 18th May 2018, 01:07 PM   #6
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by jazbo8 View Post
Right, I see you mean, the datasheet's UL figures are for push-pull, not really suitable for SE.
Sure they are but only for a supply voltage of 425V
Mona
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Old 18th May 2018, 01:13 PM   #7
jazbo8 is offline jazbo8
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E80CC/KT88 SE Triode/UL question
425V for SEUL?!
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Old 18th May 2018, 03:01 PM   #8
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by jazbo8 View Post
425V for SEUL?!
Yes, all alone
Mona
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Old 18th May 2018, 03:20 PM   #9
jazbo8 is offline jazbo8
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E80CC/KT88 SE Triode/UL question
Sure, you can run it as such, but the distortion is quite high, so much for being "ultra-linear".
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Old 18th May 2018, 04:59 PM   #10
Viking83 is offline Viking83  Norway
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Well, I didn’t know that�� I’ve got it at 300V/Vg1 -35, with a 2k OPT. Never paid too much attention to the dotted line at 425V, doesn’t look so UL over there.....
Do I need to start over or might this work? Suppose g2 voltage would change the picture quite a bit?
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