Direct Coupling with Zener Diodes

[jmaf]

ah man...I gonna start ignoring you now...I hope I can, but feel there will be plenty of erronious claims in the future as well...

I don't understand this comment at all....

[J M Fahey]

Looks like you have a bunch of mu-amp schematics on hand there.

on some of those mu-amps?

but mu-amp grunge

Where do you see a mu-amp in what I posted?

Do you know what a mu-amp is?

I humbly suggest you google it.

As a side note, and as this has been mentioned on a couple posts, in my particular case *this* is my "day job" (and nights, weekends and holidays), has been so for the last 42 years, in the meantime churning out over 10000 amplifiers (not a typo, over ten thousand).
It has paid for my houses, cars, trips, family , and any other expenses along the way.
I have *never ever* worked in anything else or for anybody.

[redelephant]

hmmm...were you being sarcastic? In that case I take it back.

[redelephant]

Wish I could have this as my main job.

btw, do you have any links to your amps? I'd be interested to see what spicy South American amps are like

[exclamationmark]

The CCS in the plate resistor's path did exactly what I thought it would (thanks for the explanation though!). I've come up with an alternate design 'borrowed' from my hi-fi friends that should keep the CCS out of the signal path (and zener/LED's among other things), and make calculations easier. Only thing is, is that it eats half my of my signal output, which I guess isn't too bad if I used higher gain tubes...

Basically the first schematic is what I was talking about earlier, with the CCS/LED/Zener used to provide the bias AT THE GRID with the cathode going straight to ground. If I were to use a zener, it would be located the opposite way around. The trimpot across the LED is used to control the bias voltage. So it's basically a fancy voltage control that costs about the same as a capacitor (provided the -Vs is already sourced from somewhere). Notice the capacitor still in the signal path blocking DC...

The second circuit is my *ahem* 'alteration' which consists of basically the exact same circuit but with a resistive voltage divider replacing the capacitor. Obviously the B- supply will need a larger string of LED/Diodes in order to counter-act the quiescent voltage. Maybe one day when my brain grows large enough I could even implement a DC servo for the bias voltages...

As to why the resistive voltage divider is needed... I have no freaking clue. I frequently use LTSpice to sim things and it tells me that this works (If I change the divider ratio in favour of signal swing, I'll need a larger bias supply). I initially thought that simply omitting the capacitor in the first circuit should work, with the B- fed through a 1M grid resistor. No dice.

I assume I've done some illegal thinking in the world of electronics, which told me by removing the capacitor, all would be fine. I'm guessing the reason has something to do with the voltage potential at the point between the anode and plate resistor. Or currents. Or something. Could anyone please elaborate on this?

[jmaf]

hmmm...were you being sarcastic? In that case I take it back.

redelephant, I think my braingrid is gone positive

I dug what you said, as I posted just before your post(simulpost almost, I was typing) it's implied that I agree with your analysis, I was not ignoring what you posted, I just don't have anything else to add(or, further remove ) from this discussion

[jmaf]

Where do you see a mu-amp in what I posted?

Do you know what a mu-amp is?

I humbly suggest you google it.

As a side note, and as this has been mentioned on a couple posts, in my particular case *this* is my "day job" (and nights, weekends and holidays), has been so for the last 42 years, in the meantime churning out over 10000 amplifiers (not a typo, over ten thousand).
It has paid for my houses, cars, trips, family , and any other expenses along the way.
I have *never ever* worked in anything else or for anybody.

Under what name or brand did you make those amps? The reason I ask is that in the repair shop I co-owned (2005-) I saw most Brazilian-made amplifiers. You said you lived in SP for some time, so if you made one amplifier every 1.5 days for the past 42 years, then surely I must have seen one of yours. I take notes on the interesting amps I see, maybe I took a note when I saw one of your amplifiers, something like "geez, this guy runs his grids positive" ...

[redelephant]

I must admit I don't quite understand your schematic. The pot across the LED isn't doing much btw. Ayhow, are you saying you are not going with the original threadstarting idea of levelshifting using a zener? Now it appears you are talking about biasing the tube using active devices...
Perhaps you can try to draw a more complete circuit of what your idea is?
Btw, the CCS is less in the circuitpath than the zener...

Oh another thing I forgot in the last post. Regarding LEDs (or other types of diodes) in the cathode for biasing. You say CCS are placed between the LED and cathode, to both linearize the LED, and isolate it from the tube so the tube isn't aware of it...Well, that is a bit wrong. First off, the CCS has too high an impedance to be placed there. Second, I am not sure you'll be able to design a CCS with complience of only a few mV that retains it's CCS-function. The CCS is commonly placed on the plate, instead of plate resistor. This is to present a high load impedance, and as a bonus sets the quiescent conditions. The tube will automatically find it's working voltage b/c the CCS can swing within it's voltage area (compliance). If a current too high or too low is set with the CCS, the tube will simply not swing symetrically enough, or be either too much on or off.

[jmaf]

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jmaf Merlinb redelephant


UH OH.....

[exclamationmark]

I must admit I don't quite understand your schematic. The pot across the LED isn't doing much btw. Ayhow, are you saying you are not going with the original threadstarting idea of levelshifting using a zener? Now it appears you are talking about biasing the tube using active devices...
Perhaps you can try to draw a more complete circuit of what your idea is?
Btw, the CCS is less in the circuitpath than the zener...

Oh another thing I forgot in the last post. Regarding LEDs (or other types of diodes) in the cathode for biasing. You say CCS are placed between the LED and cathode, to both linearize the LED, and isolate it from the tube so the tube isn't aware of it...Well, that is a bit wrong. First off, the CCS has too high an impedance to be placed there. Second, I am not sure you'll be able to design a CCS with complience of only a few mV that retains it's CCS-function. The CCS is commonly placed on the plate, instead of plate resistor. This is to present a high load impedance, and as a bonus sets the quiescent conditions. The tube will automatically find it's working voltage b/c the CCS can swing within it's voltage area (compliance). If a current too high or too low is set with the CCS, the tube will simply not swing symetrically enough, or be either too much on or off.

Hmm I suppose you could say I'm looking at active devices. Though I don't really know where the distinction lays. The idea is just to somehow induce a level shift using lord knows what devices.

The B- nodes on the schematics are supposed to be connected together and in the case of the first circuit (the one with a coupling capacitor) it sets the grid at whatever the voltage drop across the diodes is. So if the voltage drop is -3 volts across the diodes, it sets the bias as such. So it's just a biasing method that I somehow figured could be clobbered into a level shifting circuit. Oh and about the pot across the diodes/LED's, I don't think it is connected properly in the schematic, but I'm fairly sure it works. Essentially at 0 ohms it would bypass the the LED/diode string by means of short circuit and at some resistance would enforce the maximum voltage drop... What happens in the middle is beyond me. Will test this out soonish.

Also with the cathode LED bias example, the current source is not actually in series with the cathode/LED but 'injected' into the junction at a positive potential. I don't know if I made that clear or not in my other post. If I did, just say it again

There's not really anything wrong with the other schematic, it's just that the extra current being drawn through the plate resistor (and voltage drop) is not so good for lowish voltage designs. I'm only using a ~100v SMPS that was originally in some sort of audio equipment. So I would probably need to reduce the current source(which would somewhat defeat the purpose of, well, itself).

[redelephant]

The B- nodes on the schematics are supposed to be connected together and in the case of the first circuit (the one with a coupling capacitor) it sets the grid at whatever the voltage drop across the diodes is. So if the voltage drop is -3 volts across the diodes, it sets the bias as such. So it's just a biasing method that I somehow figured could be clobbered into a level shifting circuit. Oh and about the pot across the diodes/LED's, I don't think it is connected properly in the schematic, but I'm fairly sure it works. Essentially at 0 ohms it would bypass the the LED/diode string by means of short circuit and at some resistance would enforce the maximum voltage drop... What happens in the middle is beyond me. Will test this out soonish.

Ok, got it. The pot must be connected as shown, but the wiper goes to the bias node, providing a adjustable voltage for the grid. The wiper should have a cap to gnd so the voltage reference stays low impedent, unless it is relatively low ohmic. With the voltage tapped from the wiper, you see what happens in the middle...

Edit: forgot some... The Ac coupled circuit will work fine. But the DC coupled one will not. The DC current going from the preceding stage thru the resistors will swamp the LED bias voltage. If you get the voltage right it's b/c the resistor values are such that you end up losing too much signal. What's the point? If those zeners you got function ok at low currents, well half a mA or so aint gonna mess up the stage that much. Since you are going hybrid (mixxing SS and tube) how about using a CCS for the plate load? That way you can have large plate resistance, and still have the voltage drop you want over load.

Also with the cathode LED bias example, the current source is not actually in series with the cathode/LED but 'injected' into the junction at a positive potential. I don't know if I made that clear or not in my other post. If I did, just say it again

That's a brilliant idea. I've only tried this with a LED/diode from cathode to gnd and the tube's cathode current is the only current going thru the LED. 2-4mA isn't a lot for a steady LED voltage, maybe that's why I've never liked that scheme...Gotta try that CCS trick! When the current thru the LED can be much higher than cathode current, the LED should indeed provide a constant voltage and signal swing will not affect it.
Thanks man, learned something new today
Check circuit here, is that what you mean?

[redelephant]

I've had plenty brainfarts myself from time to time and will keep having them...it just happens.

[jmaf]

I've had plenty brainfarts myself from time to time and will keep having them...it just happens.

My favorite tech friends(fluke and techtronix) have just helped me test a lot of the stuff that was argued at this thread. It is good to go home from the shop not feeling like a complete fool. I hope my mindfart, as you called it, is found to be useful to those not interested in forum politics.

[Chuck H]

this has got to be the worst thread I've ever read.... I'm either laughing my ^ss off with some of these comments, or just being taken to new lows in the world of electrical theroy... I sure hope some of you don't have any plans on quitting your "day jobs"......



-g

Why would you bother to write this? This isn't going to make the thread any better and your just isolating yourself by insulting other long time members. How is it you thought this would make anything better, help anyone, further your own goals, serve your god or do anything useful at all?

Not a rhetorical question.

[imaradiostar]

I've tried to understand why everyone is arguing but I don't.

KG posted datasheets of triodes operating with the grid positive so it's scientific fact that they continue to amplify whatever is showing up on the grid given a low enough driving impedance.

As for the original question of direct coupling with zener diodes, it works but you end up having to put effort into keeping the zeners conducting during transients and heavy distortion. I like the multiple power supply idea- there are quite a few old tube scopes that relied on multiple supplies and direct coupling.

What IS the point of the thread really? If it's just spiraling out of control let's either rope it in or lock it up. This isn't the type of thing I expect on this forum.

jamie