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Old 27th January 2010, 01:15 AM   #1
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Default LED instead of Cathode resistors

Did anyone try to use led instead of cathode resistors in the driver stage of tubelabse or simplese ?

Best Regards,

Davide
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Old 27th January 2010, 02:01 AM   #2
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Given the currents involved, you'd need a lot of LEDs. Can be done, though, if you like tedious soldering...just ask SY.
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Old 27th January 2010, 02:35 AM   #3
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Lotsa LED's:
The Red Light District - another PP EL84 amp
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Old 27th January 2010, 03:08 AM   #4
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Ahem - he did say driver stage. The RLD used LED's in the output stage, no?

I've thought about this myself, but have not tried it.
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Old 27th January 2010, 03:14 AM   #5
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Whoops...there are words in there. Yup, that would be a lot "simpler", so to speak.
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Old 27th January 2010, 12:36 PM   #6
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That was the speaker driver stage -- mea culpa.

Morgan Jones 3rd Ed shows how to use an LED with a phono input stage, and the SY's Impasse Pre-Amp uses a pair of LED's on a 6SN7 -- with a depletion MOSFET current source as well.
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Old 27th January 2010, 12:54 PM   #7
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I have used leds in the M Abdellah design and saw it in the Morgan Jones book. I am sure it will work, just curious to know if someone tried such a simple mod.

George, did you ?

D.
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Old 27th January 2010, 01:34 PM   #8
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It is indeed possible to use an LED in place of the cathode resistor and capacitor in the Simple SE and the Tubelab SE. As with any amp modification some experimenting is necessary. There is a fairly wide variation in the 12AT7's used in the Simple SE and a really big variation in the 5842 used in the Tubelab SE, which is the reason for the pot. LED's are usually consistent from device to device within the same batch, but exhibit some variation in voltage drop from batch to batch.

The easiest way to do this is to build the amp normally, and then measure the plate voltage on the driver tube. Remove the resistor and cap (or resistor cap and pot) and substitute in several different LED's until a similar plate voltage is reached. If you have several driver tubes, you could try rolling several of these through to find the best combination of tube and LED. I have done this on both amps. The Tubelab SE took a lot of tube and LED swapping to find a good combination.

It is possible to use LED's in the output stage on some amps, but this is really only practical with tubes that work with a low bias voltage at relatively low current. The EL84 is one of those tubes, as evidinced by SY's RLD. Of course I had to try this too, so I made the WLD! It is a prototype Simple P-P board. In place of the cathode resistors I used a little ceramic substrate with 3 SMD white LED's in series.
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Last edited by tubelab.com; 27th January 2010 at 01:37 PM.
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Old 27th January 2010, 02:34 PM   #9
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Did you measure the dynamic impedance of the white LEDs?

edit: from the I/V graph in the datasheet for a VAOS SP4W4, which looks a lot like yours, the dynamic impadance at 100mA looks like about 2-3 ohms at Vf of 3.2V. Pretty darn good!
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Old 27th January 2010, 02:44 PM   #10
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Quote:
Originally Posted by SY View Post
Did you measure the dynamic impedance of the white LEDs?

edit: from the I/V graph in the datasheet for a VAOS SP4W4, which looks a lot like yours, the dynamic impadance at 100mA looks like about 2-3 ohms at Vf of 3.2V. Pretty darn good!
Hmm, would you consider replacing the 2 red led's in the impasse with one white one?

I am also wondering whether it would be desirable to reconfigure the RLD display in the manner of "Ol' Betsy" with red, white and blue LED's. Think of the combinations
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