12at7 clipping

[catalin gramada]

https://www.circuitbread.com/ee-faq/...different-leds

[Helmholtz]

A constant current device cannot be used for cathode biasing as required with this paraphase PI.

But it might make a fine "long tail" in a LTPI (with a separate biasing resistor in series).

[vintagekiki]

https://www.circuitbread.com/ee-faq/the-forward-voltages-of-different-leds]

A large number of LEDs are required for matching several identical diodes in terms of calculated current and voltage.

[catalin gramada]

A large number of LEDs are required for matching several identical diodes in terms of calculated current and voltage.

I just sorted from my box. Unfortunately all I have get 1.9- 2.1 V at 10-12 mA no matter the colour (red,yellow, green), so I stucked with -2V dc bias. But as it is it still not solved the problem. I rebiased at -2.6V with a common 220 ohm resistor and it start to squash at 1.8Vpk around, little bit early than inverter clip at 72Vpk (the voltage gain of PI stage is 36 , a 330uF bypass cap makes no difference)

[vintagekiki]

...I stucked with -2V dc bias...

How much cathode current do you expect through the PI tube(s)?

For 2V
https://eu.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Current-Regulator-Diodes/_/N-ax1ml

[Dave H]

Current regulating diodes provide constant parameters for the selected operating point unlike DUS or LED diodes.

The current through the LED will be constant because the right hand triode is a unity gain inverter. That's why a bypass capacitor is not required when a resistor is used in place of the LED.

[J M Fahey]

I wanna clip the input at 2Vpk

Why?

The GK diode clearly starts passing current at 1.4V Pk

[catalin gramada]

Why?

The GK diode clearly starts passing current at 1.4V Pk

Yes it is and I wonder why ?

[vintagekiki]

If DUS or LED diodes were better than current regulator diodes they would be seen in amp constructions.
DUS or LED diodes do not have constant U/I parameters like current regulator diode or cathode resistor, so each their replacement without previous matching will give different operating point.

Schematic fir PI ( post #4 ) has the same configuration as Vox V100 ( post #13 ) and Vox AC100, in the cathode has 1k5 without bypass capacitor.
https://www.voxac100.org.uk/schematics.htm
https://www.voxsupreme.org.uk/images/v100/closeups/schema.jpg

catalin gramada did you try anything about post #13, to see if there is a difference in PI behavior. If you don't try and compare results you won't know what the problem is.
1)
2VAC on g1 PI is sufficient drive for the full power on output stage. The output tubes is biased with -70VDC, how much is quiescent current (measured in cathode).

[Dave H]

If DUS or LED diodes were better than current regulator diodes they would be seen in amp constructions.
DUS or LED diodes do not have constant U/I parameters like current regulator diode or cathode resistor, so each their replacement without previous matching will give different operating point.

Is DUS Diode Universal Silicon?

Yes, you would have to select the LED. Traditionally it was a resistor because that was all that was available at the time. Looking at the circuit I think a resistor (bypassed or not), selected LED, diodes or constant current sink should all work. I don't know which is best. I'd just use a resistor.

[vintagekiki]

Is DUS Diode Universal Silicon?

Yes, DUS is a Universal Silicon Diode.
Unlike current regulator diodes where current is a few mA, at DUS and LED diodes this same current is a few tens of mA, so they are impractical for wide consumption.

The cathode resistor with its value exactly defines the voltage, it uses over 95% amps, and since I am "old school" (y74) I use resistor everywhere (which is sure, its sure).
Personally, I think that current regulator diodes are more intended for measuring technology and for professional use.

[Helmholtz]

With a current regulator at the cathodes the bias voltage will not be well defined. Just look at the characteristics of the device.

Dave H is right that a cathode resistor doesn't necessarily require a bypass cap for low AC impedance - as long as the PI is perfectly balanced, i.e at low output.

A shared unbypassed cathode resistor will introduce some cathode coupling between the triodes, which would be prevented with a bypass cap or when using the LED.
The result is a mix of paraphase and LTP operation.
Merlin calls this a Floating Paraphase Inverter.

[vintagekiki]

Depending on the characteristics of device, selected appropriate diode.

useful link

https://eu.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Current-Regulator-Diodes/_/N-ax1ml

[Helmholtz]

If you look at the voltage-current characteristic of a CRD (http://www.semitec.co.jp/uploads/eng...P22-23-CRD.pdf) you can see the above a threshold voltage of maybe 1.5V the device gets into constant current mode, where the voltage across it may vary over a large range (say 2V to 100V) without changing the current.

That means that there will be no defined or constant bias voltage.

A device that regulates voltage and current at the same time is not possible.

[vintagekiki]

About CRD

http://www.semitec.co.jp/uploads/english/sites/2/2017/03/P22-23-CRD.pdf

Current regulating diodes (CRD hereunder) are diodes that maintain a constant current flow despite voltage fluctuations. CRDs supply constant current over a wide range of voltage from less than 1V to 100V. Constant current is supplied regardless of fluctuations in voltage applied, load resistance changes and ripple voltage. Creating a constant current circuit generally involves multiple components, but with SEMITEC CRDs only one part is required to accomplish the same function.

The selection of CRD diode is according to required voltage or required current.
One parameter conditions another.
Example
If required voltage is 2V, choice is an S-152T diode with a typical 1.5 mA current.
And that's it, no mysticism, no philosophy.