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LED cathode bias - how to pick LED?
LED cathode bias - how to pick LED?
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Old 26th December 2008, 04:44 PM   #11
dave slagle is offline dave slagle  United States
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Just curious.... Why do they have to be LED's? has anybody played with other diode technologies? (shottky, hexfred...etc?)

dave
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Old 26th December 2008, 04:45 PM   #12
Michael Koster is offline Michael Koster  United States
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What about using the amber LED with a couple of schottky diodes?
That would get the voltage with the lowest Rdyn short of a battery.

As the OP said it's a spud amp, so I'm assuming it's an output stage
and low dynamic cathode resistance is king.

If it's a driver stage the Rdyn may be less important.

If it's a high-u spud amp using a 6C45pi for example, then you really
want low low cathode resistance as every ohm in the cathode gives
you ~50 ohms at the anode.

What tube is it?

Cheers,

Michael
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Old 26th December 2008, 05:17 PM   #13
SY is offline SY  United States
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LED cathode bias - how to pick LED?
Quote:
Originally posted by dave slagle
Just curious.... Why do they have to be LED's? has anybody played with other diode technologies? (shottky, hexfred...etc?)

dave
I've measured conventional silicon diodes and high speed versions (1N4007/UF4007) and saw numbers in the 4-5 ohm range. Putting enough of these in series to get the desired drop brings the impedance higher than the corresponding LEDs. I haven't tried Schottkys yet, but their whole raison d'etre is the low drop, so even more are needed in series.

If the Scottkys measure low, then Michael's suggestion or combining them with the amber LED is a good one. Otherwise, you can get a 4 ohm impedance with a couple of IR diodes in series.
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Old 26th December 2008, 06:12 PM   #14
sharpi31 is offline sharpi31  United Kingdom
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LED cathode bias - how to pick LED?
I will try the reds when they arrive and spend a while longer looking for a alternative with lower dynamic resistance :-)
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Old 26th December 2008, 06:48 PM   #15
sharpi31 is offline sharpi31  United Kingdom
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LED cathode bias - how to pick LED?
SY,

Would this be a good choice for an IR LED (2 in series required)?

OPE5594A

The forward voltage increases from 1.3V to 1.4V over 7.5mA to 90mA forward current.

My amp is the Hawthorne Spud kit so this is to bias the 6CL6 tube. Apparently the bias should be 2.5-3V so two of these IR LEDs puts me right in the middle of the recommended range.

Thanks for all the help so far.
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Old 26th December 2008, 11:59 PM   #16
rman is offline rman  Canada
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Hi.

How about a lm336 precision voltage reference
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Old 27th December 2008, 01:13 AM   #17
SY is offline SY  United States
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LED cathode bias - how to pick LED?
sharp, it looks superb at currents over 10mA. Resistance is deltaV/deltaI or about 1.1-1.2 ohms, if my squinting at the chart is correct. Two in series is 2.2-2.4 ohms which is as good as you'd ever want.

The other nice thing about this bias scheme is that, unlike capacitor bypasses, they don't go inductive at high frequencies.
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Old 28th December 2008, 07:56 PM   #18
Michael Koster is offline Michael Koster  United States
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Quote:
Originally posted by sharpi31
SY,

Would this be a good choice for an IR LED (2 in series required)?

OPE5594A

The forward voltage increases from 1.3V to 1.4V over 7.5mA to 90mA forward current.

My amp is the Hawthorne Spud kit so this is to bias the 6CL6 tube. Apparently the bias should be 2.5-3V so two of these IR LEDs puts me right in the middle of the recommended range.

Thanks for all the help so far.
These look great! Who stocks them?

Thanks,

Michael
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Old 28th December 2008, 08:28 PM   #19
dave slagle is offline dave slagle  United States
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allied has them. (ope5594's)

In looking at all of the various data sheets out there I do not see any hard and fast rules WRT color and behavior.

here the Red and Yellow have the same characteristics.
LED cathode bias - how to pick LED?


Red and yellow with different characteristics
LED cathode bias - how to pick LED?


dave
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Old 28th December 2008, 08:35 PM   #20
Michael Koster is offline Michael Koster  United States
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They're in the Allied catalog but non-stock.

RS online stocks them; guess it's time to sign up with RS...

Michael
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