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Old 2nd February 2010, 02:35 PM   #21
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Is that a kind of feedback ?

D.
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Old 2nd February 2010, 08:15 PM   #22
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Those white LED's look like white-hot sparks flying :O)
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Old 2nd February 2010, 10:19 PM   #23
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Quote:
Originally Posted by Nikon1975 View Post
Is that a kind of feedback ?

D.
No, just DC bias added to the LED.

Since an LED used like this works in constant-voltage mode, it doesn't effect the tube operation at all and also since the cathode is a low-Z point, the extra resistor also effects nothing in the signal.


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Those white LED's look like white-hot sparks flying :O)
Also not a sight unseen from George, I'm sure


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Old 31st March 2010, 11:52 PM   #24
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I am using led instead of the cathode resistors of the driver of the Simple SE. I needed to use one led only, as the voltage I was measuring on the resistor was around 1.7 V.

One positive side effect of using leds in this position is that you do not need to use coupling caps when using Ipod or something that had some DC in the output. With the Abdellah amp I noticed that the plate voltage of the drivers was going from 200 to 250 V when the Ipod was connected. I explained this thinking that in some way the ipod was offsetting the voltage of the grid.

With leds this does not happen. I am not 100 % sure that my explanation is correct. :-)

Thanks,

Davide
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Old 17th May 2010, 12:58 PM   #25
Skorpio is offline Skorpio  Denmark
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I have made some simulations in WorkBenck Pro to see the impact of LED versus resistor biasing on the SSE input stage.

The CCS is made of an ideal CCS in the libery, the +B is 450V, 10k i series and a 12AT7 tube, load 220K via 1uF:

Bias 220R//1500uF:
Gain:35.18dB, frequence range: 0,75Hz-3.16MHz, Va=250.9V, Vb=2.144V, Vo=19.73Vrms

Bias 220R:
Gain:34.48dB, frequence range: 0,75Hz-3.16MHz, Va=250.9V, Vb=2.144V, Vo=18.73Vrms

Bias 2x RED LED:
Gain:34.92dB, frequence range: 0,75Hz-3.16MHz, Va=251.8V, Vb=2.159V, Vo=19.70Vrms

Bias 2x RED LED, CCS+10k in anode replaced with 20k resistor:
Gain:30.82dB, frequence range: 0,75Hz-5.6MHz, Va=251.0V, Vb=2.155V, Vo=12.29Vrms

Judges by the above, it seems that decoupling the bias resistor does not change gain much? Perhaps due to CCS i anode?
Also, with resistor in anode instead of CCS you loose 3-4dB gain, but increase high cut-off from 3 to over 5MHz?

But the CCS will perhaps have better PSRR level, but could be interesting to try with resistor load...
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Old 17th May 2010, 04:54 PM   #26
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Hi,

Quote:
Originally Posted by Nikon1975 View Post
Did anyone try to use led instead of cathode resistors in the driver stage of tubelabse or simplese ?
I have not done in those specifically, but in a number of other applications.

There are a number of issues with LED's.

First, they are non-linear if operated at sensible currents, much more non-linear than a five cent resistor.

They are light sensitive (kid you not, they make nice photodiodes as well, if you are in a pinch).

And of course, the cathode of the tube is a signal input pin and one with possibly very high gain.

Of course there are solutions. If we apply CCS loading with a truely high impedance (such as the Tubelab SE) and we then buffer anode output we minimise the change of current through the LED and thus the non-linearity.

However, a five cent resistor is probably a better choice in the cathode, as it linearises the tubes and reduces the impact of tube sample variations.

The Light sensitivity can be fixed with black lacquer.

I suspect the final word will be your call. Try it out and then decide for yourself. I found that in subjective terms I personally preferred other solutions over solid state CCS's and Led's and similar design techniques. But tastes differ.

Ciao T
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Old 17th May 2010, 05:35 PM   #27
SY is offline SY  United States
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Quote:
They are light sensitive (kid you not, they make nice photodiodes as well, if you are in a pinch).
Forward biased? No, not really.

As a practical matter, measurement of the claimed LED nonlinearity effects shows nada, especially at constant current.

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it linearises the tubes
See Baxandall's paper in WW for the downside of using small amounts of feedback like that.
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