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Old 9th April 2012, 11:51 AM   #1
M Gregg is offline M Gregg  United Kingdom
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Default LED's in the cathode..

Just a quick question,

Does any one know the effect of using LED's in the cathode of all pre stages of a tube amp? Is there any noticable effect good/bad or indifferent?
I have this in one stage now and was wondering if its worth trying in the other stages before I actually do it..

Is there any colouring if used in all stages? I know it might cast some light on the subject..LOL

I guess I should add its been quite promising so far!

Also what is the effect if used with CCS or gyrator<<<for and against or preference and why?

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M. Gregg
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Old 9th April 2012, 12:08 PM   #2
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Is there any colouring if used in all stages? I know it might cast some light on the subject..LOL
I think you'd should expect some pink noise if you use red LEDs !

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Old 9th April 2012, 12:25 PM   #3
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Quote:
Originally Posted by M Gregg View Post
Just a quick question,

Does any one know the effect of using LED's in the cathode of all pre stages of a tube amp? Is there any noticable effect good/bad or indifferent?
I have this in one stage now and was wondering if its worth trying in the other stages before I actually do it..
I believe it degrades audio quality adding semiconductor alinearities and destroying tube sound. Also, I strongly dislikes mixing tubes and semiconductors in a set. Bit it is only personal taste.
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Old 9th April 2012, 02:15 PM   #4
TheGimp is offline TheGimp  United States
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The RLD uses them in the output stage and has been reported to be very good by several people.

I've only used LED cathode bias in the first stage, but of several amps and find it to be quite acceptable. It's non-linearity does not come into play in a fixed bias mode such as a cathode bias position since the voltage variation is minimal.

What other stages were you thinking about?
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Old 9th April 2012, 03:21 PM   #5
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Several weeks (or months?) ago I've yet posted my opinion that I don't see any benefits of such an arrangment, because it pairs the disadvantages of both fixed and cathode bias.
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Old 9th April 2012, 03:24 PM   #6
TheGimp is offline TheGimp  United States
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Link to the thread please.
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Old 9th April 2012, 03:42 PM   #7
dgta is offline dgta  United States
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The LED acts as a constant voltage source. Or close to it, depending on the particular LED's dynamic impedance.

Whether it's appropriate and of benefit for any given stage depends on the design. It's just another component available to the designer and must be considered in context.

It is typically not used in a second stage because the second stage is often a concertina or LTP splitter. But if your second stage (or any other stage) is a simple common-cathode voltage amplifier, then I see no reason you can't use it.

As noted, it is also fine to use for cathode bias in a final stage.
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Old 9th April 2012, 04:07 PM   #8
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I've only used LED cathode bias in the first stage, but of several amps and find it to be quite acceptable. It's non-linearity does not come into play in a fixed bias mode such as a cathode bias position since the voltage variation is minimal.
OK, voltage variations donīt care, but really import are current variations, plate of a triode you found escentailly current variations translated to voltage thanks to the plate to +B resistor.
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Old 9th April 2012, 04:11 PM   #9
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This is unhelpful, but I like batteries better for that kind of gimmick.
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Old 9th April 2012, 04:14 PM   #10
dgta is offline dgta  United States
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Osvaldo, the LEDs are used in the cathode, for bias. Not in the anode.

Ronsonic, LEDs are much like batteries, except more (and different) voltages are available. And batteries and LEDs are not gimmicks, just circuit components like any other.

Last edited by dgta; 9th April 2012 at 04:17 PM.
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