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Old 24th June 2008, 02:28 AM   #31
kenpeter is offline kenpeter  United States
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You could ground that biased and bypassed node
and it would still "work". But then you have to pull
the lower plate negative to shut off grid conduction.

I thought this way of bias might be convenient???
If bias works at all here, and I'm not entirely sure...
Way outside the box, even for me.
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Old 24th June 2008, 03:35 AM   #32
kenpeter is offline kenpeter  United States
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Same circuit, different drawing... New symbol.
Was giving me a headache trying to wrap my
mind around the abomination I had created.

I needed a different way to draw an inverted
triode. One that would convey the plate as the
control element, and the grid as the anode.
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Old 24th June 2008, 03:37 AM   #33
smoking-amp is offline smoking-amp  United States
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Ken, post #29 schematic:

Whoa! Is this thing still conducting normal electrons? OK, looks interesting, attenuate first with inverted triode (neg. plate input, hi Z input, pos. grid output), then amplify with grounded grid. This setup will definitely drive the serviceguy nuts someday. Makes sense to attenuate first with the inverted triode though with its grid output, but it will have trouble getting much current out for the subsequent grounded grid stage cathode. Assuming 1/Mu1 for the 1st stage, I think the 2nd gives Mu2-1 gain (the cathode variation eats 1 Mu from the output)

To address the current disparity, maybe should ground the grid of the 1st tube, but still use negative plate bias for input on it's plate. Then take it's non-inverted cathode signal as the output to the grid of a 2nd conventional grounded cathode stage. Then there's no loading on the weak 1st stage. The second stage provides normal Mu gain and inversion. 1st stage giving 1/Mu+1 and 2nd stage giving Mu gain.


"Oldeurope will have to define which modes of negative feedback are acceptable, if we are to understand the rules of his game."

Yes, we do need some clarification on just what the rules are here. But it's interesting to press the confines of the envelope for tube circuit design in any case. Really weird circuitry as SY noticed.


late edit to post #26:
I attributed the 1st quote there to Ken, but it should be to Michael Koster who had popped in. Sorry for the mixup.

Don
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Old 24th June 2008, 04:00 AM   #34
smoking-amp is offline smoking-amp  United States
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Huh, ya know, it looks like that last circuit would work just fine with conventional B+ on the V1 plate input too. With a CCS in the cathode, the plate Z goes way up, even in conventional (plate as plate) mode. So that Rb1 resistor could be a high value going to B+ instead of -Vb.

Don
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Old 24th June 2008, 04:23 AM   #35
smoking-amp is offline smoking-amp  United States
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The post #33 schematic gave me an idea for improving the screen driven pentode scheme. With constant current thru the tube, the screen current should be near constant, so the screen should have relatively high input Z. Total gain would be approx. 1/Mu_screen times Mu_plate. Since this will be a bit more than unity, the cathode resistor Rk can be used to degenrate gain a little further till unity gain is reached.

Don
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Old 24th June 2008, 04:44 AM   #36
smoking-amp is offline smoking-amp  United States
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Check this out!

Unity gain inverter and complementary current mirror all in one package. No more Mu+1 or Mu-1 versus Mu etc. Exact match now.

Don
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Old 24th June 2008, 04:47 AM   #37
Wavebourn is offline Wavebourn  United States
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Quote:
Originally posted by smoking-amp
[B]
"Take any tube and select load resistance and idle current to get exactly unity gain. No feedback, the problem solved. However, without feedback it will be highly dependable on the tube used, but it is what you need. Tube gurus will be happy with your design creating new fashions and urban legends about different sounds of different tubes." Anatoliy

Definitely the obvious solution, but Darius apparently doesn't consider cathode degeneration as no-feedback. (or he would just use the usual split load concertina splitter) Not sure whether Darius would consider a low plate resistor alone to be linear enough. He should add to his "rules" that nothing can be done easily.
What cathode degeneration?
I proposed a single solution with no feedback added.


Quote:
Originally posted by kenpeter
The triode has feedback between all three terminals. Even the plate
resistor presents a negative feedback. It is hard to call any triode
circuit (except perhaps the lower triode of a cascode) one without
negative feedback.

It is true, but such a feedback is unavoidable (like feedbacks through inter-electrode capacitances)


or... what if to hide a transformer inside of a glass bulb?

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Old 24th June 2008, 05:06 AM   #38
smoking-amp is offline smoking-amp  United States
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Oops, my last post #36 schematic is NON-inverting, darn. A clever piece of wire. Nice complementary current mirror anyway.



"What cathode degeneration?" Anatoliy

True indeed, but with just a low value plate load resistor to kill the gain down to unity, the tube will be running almost in pentode mode with high distortion. Some cathode degeneration could fix that.

Don
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Old 24th June 2008, 06:57 AM   #39
oldeurope is offline oldeurope  Germany
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Default @ smoking-amp

Quote:
Originally #26 posted by smoking-amp
Wonder what Darius will come up with now for his preamp.

Don
Hi Don,
why do you ignore my posts and hyper links?
Why do you tell porkies in this thread?

Read post #12.

You can see a picture of the preamp in the hyper link at post #1

Darius
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Old 24th June 2008, 07:19 AM   #40
oldeurope is offline oldeurope  Germany
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Default #29 #32

Hi kenpeter,
I tested the arrangement in post #32.
There are two mayor problems.
- First the gain of the inverted tube '< 1/ .
See post #12.
- Second, the inverted mode triode needs a special construction
to get low distortion and a reliable value of '.

Kind regards,
Darius
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