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Author Topic: LND150 tube emulator attempt.  (Read 24941 times)

Camembert

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Re: LND150 tube emulator attempt.
« Reply #15 on: December 22, 2017, 06:37:13 AM »

Hello,

Can someone explain why, in the final version of this circuit, there are two diodes in parallel between gate and source, in stead of a single one?
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Katoda

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Re: LND150 tube emulator attempt.
« Reply #16 on: December 22, 2017, 07:20:10 AM »

My guess would be that it's just easier to solder, but two diodes would conduct twice the current, so I'm not really sure.
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Camembert

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Re: LND150 tube emulator attempt.
« Reply #17 on: December 22, 2017, 09:28:59 AM »

You are right about twice the current of course, but there is not a wholotta current flowing there.
I did find out that when you put two diodes in parallel, the voltage drop across the pair is slightly smaller than across a single diode. Maybe that is good for something.
But actually, I think my problem is that I don't really understand the "grid" limiter thing of wich the diodes are part...
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blackcorvo

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Re: LND150 tube emulator attempt.
« Reply #18 on: December 22, 2017, 01:24:36 PM »

You are right about twice the current of course, but there is not a wholotta current flowing there.
I did find out that when you put two diodes in parallel, the voltage drop across the pair is slightly smaller than across a single diode. Maybe that is good for something.
But actually, I think my problem is that I don't really understand the "grid" limiter thing of wich the diodes are part...

After reading on the link posted at the end of pg1 of this thread, what I understood is that the source diodes are there to help shape the clipping curve for the stage, as well as act like a protection circuit against overvoltage between source and gate. I might be mistaken, however.
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Katoda

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Re: LND150 tube emulator attempt.
« Reply #19 on: December 23, 2017, 06:52:43 AM »

You are right about twice the current of course, but there is not a wholotta current flowing there.
I did find out that when you put two diodes in parallel, the voltage drop across the pair is slightly smaller than across a single diode. Maybe that is good for something.
But actually, I think my problem is that I don't really understand the "grid" limiter thing of wich the diodes are part...

The diode conducts when there is a voltage greater than cca 0.7V across the conducting direction.  So when the signal swings in the positive direction, voltage on the source rises. If the source voltage is then greater than 0.7V  to base, the diode clips the signal. The 1k resistor limits the current through the diode, making the knee of the clipping softer.

It simulates the asymmetry of valve clipping.

I hope I'm not completely wrong, somebody please correct me if I am.
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phatt

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Re: LND150 tube emulator attempt.
« Reply #20 on: December 23, 2017, 09:15:28 AM »

Look at the transfer curves shown here;
http://www.radiomuseum.org/forum/the_trioderizer_a_solid_state_triode.html
posted by Steve Conner on previous page.

The diodes are not causing distortion like a dirt pedal which just clips the audio, they are shifting how fast the Fet reacts (for want of better teck terms) .
As mentioned in the above link Fets gain curve goes straight up as do Pentodes whereas triodes the transfer is much slower to react and never quite gets there.
Pentodes were invented to overcome the limitations of triodes.

As to the Q; why 2 diodes in parallel,,, I dunno. Maybe KMG will see this and respond. He may have mentioned it in a another posting but too late now I'm off for sleep.
Phil.
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Camembert

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Re: LND150 tube emulator attempt.
« Reply #21 on: December 24, 2017, 05:24:15 AM »

Well, I have been reading a bit about grid current and tubes and diodes, and the most shocking thing I learned is that this 'threshold' where the diode starts conducting doesn't exist! actually, it's a smooth exponential curve with a knee. Beyond the knee the diode starts to conduct 'a lot'. Before, it conducts little, but it does. Now,the grid current is in the order of microamps, so this all takes place way before the knee. And different diodes can behave differently in this region.
When I played with this in SPICE it appeared that two bav70 diodes in parallel behave very similar to a single 1N4148, wich was used in the first version of the circuit. So maybe the answer is quite simple: our KMG ran out of 1N4148 diodes and started using the parallel bav70's  as equivalent circuit..
Anyway, although I have no definite answer to my question, I learned a lot by investigating. Thank you guys for your kind replies.
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