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Old 15th March 2014, 12:26 AM   #1
00940 is online now 00940  Belgium
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Default Yet another question about cathode bias (led+pnp)

Any reasons not to use the following to bias the tube ? Besides complexity and cost of course, which is rather marginal compared to the rest of the amp.
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Old 15th March 2014, 05:14 AM   #2
DUG is offline DUG  Canada
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Three diode drops and an LED with a current source in parallel with two of the diode drops.

I would like to see the V-I curve for that combination.

Would it be linear?

Does it compensate for tube non-linearity?

Interesting.
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Old 15th March 2014, 08:54 AM   #3
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Hi

I bias my 300B driver stage like this. It sounds excellent

Paul N
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Old 15th March 2014, 09:24 AM   #4
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@needsp: yes, I've seen similar schemes. The idea was to get it but in a two-terminals fashion.
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Old 15th March 2014, 01:33 PM   #5
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Quote:
Originally Posted by DUG View Post
Three diode drops and an LED with a current source in parallel with two of the diode drops.

I would like to see the V-I curve for that combination.

Would it be linear?

Does it compensate for tube non-linearity?
It's only sims but it seems like less is more.

edit: using a ccs in the simple version brings a very marginal gain.
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Last edited by 00940; 15th March 2014 at 01:36 PM.
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Old 15th March 2014, 02:15 PM   #6
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Quote:
Originally Posted by 00940 View Post
It's only sims but it seems like less is more.

edit: using a ccs in the simple version brings a very marginal gain.
Better than I expected.

Thank you.
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Old 16th March 2014, 01:34 PM   #7
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There's a Walt Jung article about various different CCS. Have you read it?
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Old 16th March 2014, 02:15 PM   #8
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Yes, I did. But aren't we looking for constant voltage here ? I don't exactly understand what I should look at in his article.
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Old 16th March 2014, 03:13 PM   #9
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Yes, I did. But aren't we looking for constant voltage here ? I don't exactly understand what I should look at in his article.
Perhaps you don't realize that a constant current through a constant resistance produces a constant voltage?

Otherwise, why not use a shunt type voltage regulator like a TL431? (it appears that you are trying to make something like that now)

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Old 16th March 2014, 04:51 PM   #10
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Perhaps you don't realize that a constant current through a constant resistance produces a constant voltage?
I do realize that... problem is that we don't have constant current. We need constant voltage with variations of +/-10ma, centered at 20ma.

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Otherwise, why not use a shunt type voltage regulator like a TL431? (it appears that you are trying to make something like that now)
A tl431 would indeed work perfectly. It's also a lot more complex (internally that is). But some have raised concerns about noise when using a tl431 for bias.
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