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Old 9th May 2005, 09:28 PM   #21
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Default Dual Triode Version

figure attached of the dual triode version
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Old 9th May 2005, 10:02 PM   #22
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Default insight

Just doing a little thinking on the case of using a triode in place of the pentode current source in CCT1 (ie, jumper out the Mosfet in CCT4 above).

The impedance modulation caused by the 2nd triode, V2, has exactly the right form of a diode conduction curve to cancel out the even harmonics from V1. So using just the triode V2 in place of the pentode is N.G. One MUST have the Mosfet as in CCT4 (above) to preserve the SET sound accurately. The Mosfet's high gm effectively pins the cathode voltage of the main triode V1 so it operates in a SET like environment. The Mosfet still accurately conveys the complementary current to V2, and V2 conveys it accurately to the xfmr. with power gain.

So JUST running a dual triode LTP pair with a single input will NOT give you SET sound. Use CCT4 ( figure 4 above).

Don
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Old 10th May 2005, 02:43 AM   #23
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Default Re: Kevlar Balloon!

Quote:
Originally posted by smoking-amp
Hi Tim,
I'm quite aware of the non-linear 3/2 power law for pentode current versus drive signal. But if you will notice, the CCT1 and CCT3 designs uses the bottom pentode (or Mosfet) in the grounded grid configuration.


And this magically eliminates its distortion...?

Quote:
The same current comes out as goes in the cathode except for screen current siphoned off in the case of the pentode.

As long as the plate voltage does not drop to near the screen voltage, the screen current is a near constant percent of the plate current due to interception, so has low distortion.
It has nothing to do with plate to screen current ratio and everything to do with the Gm vs. Ip characteristic of the tube (which is different for different types, styles, loads and so forth). Once that is fixed, we can argue such subtleties as screen current.

All these circuits are *approximately* what they claim to be, but aren't as they are claimed. The claimed increase in power output only works if you don't count transistors or pentodes as amplifiers, which sounds distinctly pro-triode racist to me...

Tim
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Old 10th May 2005, 05:36 AM   #24
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Default clarification called for

Hi Tim,

The signal we are interested in reproducing accurately at the opposite side to the triode is the complementary CURRENT. Not voltage. The grounded grid or grounded gate circuits are indeed linear for current input to current output (ignoring screen current of course for the moment). In fact they are EXACTLY unity current gain, otherwise Kirchoffs Laws would be violated, since no current flows out of the grid or gate (at least not before the component fails anyway).

The gm factor only comes in when we figure the variation of the CCS voltage required to get the demanded current. This variation of the tail voltage can cause a distortion problem for the main triode by varying its cathode voltage. Often referred to as impedance modulation (Z=1/gm of the pentode or Mosfet at the tail point) By using a LARGE gm we make this impedance near zero, so it doesn't matter how badly it behaves around zero. Hence the preference for using a Mosfet there. (so if using a pentode, we would want one with high gm) The Mosfet effectively pins the tail voltage. We could also use a bipolar transister, which has even higher gm, but it has base current that subtracts from the emitter current, affecting our unity current gain or linearity somewhat in grounded base op. (only a tiny bit actually since it is mainly a constant tiny fraction).

So, I guess the answer is it DOES magically eliminate distortion, because it does. Its called optimum circuit topology.

On the power gain figure, I did not mean to imply some magic gain of power from the triode. The pentode or Mosfet is indeed supplying the extra power. The point is that if you have some triode that can produce X amount of watts in a SET circuit, this circuit can give you 2X amount of watts of the same sound quality (and DC compensation for the xfmr). Some people would be very happy to achieve that without having to parallel more triodes. (4X with the H bridge circuit)

One spot where the circuit does magically save on power waisted, is efficiency, versus just using a CCS load on a triode to get DC current compensation for the xfmr. (50% max eff. versus 25% max eff.) But it is the same efficiency obtained using an inductor for parafeed.

Don
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Old 10th May 2005, 06:49 AM   #25
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Default Linearity in the CCT2/CCT3 circuits (figs. 2 & 3)

Lets look at the top devices in the fig. 2 and 3 circuits now. The R2 resistor has to be sized a little larger than the R1 resistor to provide drive signal to the Mosfet. If the Mosfet had a linear gm this would not affect linearity. But since the Mosfet does have a non-linear gm (same for a pentode there too) it will potentially produce distortion. The accuracy of the complementary current generation depends on a constant voltage across R1 + R2 (ignoring the extra delta R for the linear drive signal).

The non-linear portion of the drive required by the Mosfet (or pentode) makes this voltage across R1+R2 vary a small amount, distorting our complementary current generation. So it is important to minimize it. The way to do this is to use a large gm device so its drive voltage requirements will be small compared to the IR drops on R1+R2. We can also increase R1+R2 in value, but this eventually affects our amplifier output impedance and damping factor. So there is some practical limit to the value of the R1 and R2 current sampling resistors.

A mosfet (or even a bipolar transistor could be used, has even higher gm) has very large gm. The Mosfet may require around a volt or so of drive signal. Of this, some few % of that will be the non-linear component, say 10 mV. If the R1 and R2 resistors are dropping say 10 V, then our accuracy will be off by 10 mV/10 V or 0.1%. Which is at about the threshold of audibility. So this is in the ballpark of useability.

10V drop on the resistors with a 300V plate swing would be about 10/300 times the 8 Ohm output or .26 Ohm equivalent in the secondary. This would give a 30 to 1 damping factor if the triode's output impedance were zero. (Actually, R1 and R2 are in parallel as far as AC output goes, so would be more like 1/4 of .26 Ohm equiv. or 120 to 1 damping) So we can twiddle the R values some and use a higher gm part if necessary. Not quite as clean as the CCT1 (fig. 1) design, but probably acceptable with a little tweaking.

One could, by the way, use an extra Mosfet to make a Mosfet current mirror configuration to fix this non-linearity problem altogether, but that gets into selecting matched parts. Might be more feasible using bipolar transistors. (hard to match Mosfets up well) One could also put a floating Op Amp. in the Mosfet drive loop to push the effective drive level requirement down to maybe nanovolts, a little overkill I think.

Don
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Old 10th May 2005, 08:04 AM   #26
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I applaud you in your intrepid effort to maximize the potential of the power triode. I have to tell you though that following the thread is making my haid hurt. . I am willing to give it a shot though when I finally get a breadboard set up. Bravo!
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Old 10th May 2005, 05:16 PM   #27
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Hi G,
Sorry about the headaches!

Whenever a new idea crops up, it inevitably goes thru some flux and change as weaknesses are discovered or re-thinking occurs. I also would like to express my appreciation for all critical remarks, one often finds weaknesses in design or better ways to do things, when having to critically re-examine how things are working. Let the bullets fly!! (putting on my Kevlar suit hurriedly)

Having noted earlier that the figure 2 and 3 (CCT2 and CCT3) totem pole circuits were a little weak on precision, I have come up with a more precision design, see attached figure 5. Q1 and Q2 matched pair are used as a mini-op amp to control Q3 more accurately. The constant voltage across R1 and R2 will be maintained quite accurately now, so will be enforcing the complementary current generation. (I1*R1 + I2*R2 = constant, R1 = R2, so I1 = constant/R1 - I2 ) Q4 is used just to keep high voltage off of Q1's collector.

This circuit can also be easily changed to perform precision SRPP or Mu follower by simply moving the transformer connection to the top of R1 or bottom of R2 respectively.


Don
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Old 10th May 2005, 10:31 PM   #28
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Default analyzin away

Been up to a little more analyzin of the earlier CCT2/3 (totem pole type) circuits (figs. 2 & 3) using a pentode on the top part.

We looked at the general magnitudes of distortion earlier. But we also know the general nature of these gm non-linearities as well.

The nature of the distortion from the non-linear gm of the pentode is one of a thermionic diode conduction curve (3/2 power law), and is of the correct polarity to REMOVE even harmonics from the main triode's even harmonics when in the top totem pole position in CCT2/3.

(Notice that putting a pentode or triode of COMPARABLE gm to the main triode in the BOTTOM right section of CCT1/3 circuits also tends to cancel even harmonics from the triode's even harmonics, actually the pentode may likely overwhelm the triode's even harmonics)

If the gm of the top totem pentode is comparable to the main triode you will also get even harmonic cancellation and maybe even generation of inverted harmonics from the pentode overwhelming the triode's harmonics. The triode's harmonic generation is rather less than on a 1 to 1 gm comparison since it operates with plate feedback and so depends on the load impedance as to how much 3/2 power distortion is present. So you really need a very high gm pentode to preserve the SET sound.

Using a Mosfet on top in the CCT2/3 (totem pole) circuits will likely preserve 99% of the even harmonics due to the much higher gm of the Mosfet. (the Mosfet also has a gm non-linearity somewhat like the thermionic diode curve, square law instead of 3/2 power law) So using the last CCT6 version (figure just above) is probably unnecessary overkill just to get that last 1% restored.

Don
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Old 11th May 2005, 04:20 PM   #29
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Default correction

Oops, got the decimal point wrong on the final paragraph above. For a typical SET triode stage we would expect around a couple % of distortion, and the Mosfet on top will be contributing around -.1 or -.2% cancelling even harmonics, so will preserve about 90% of the SET even harmonics.

However, the CCT3 design (figure 3, H bridge, totem pole design) has the top Mosfet stages on BOTH sides of the xfmr and they will cancel each others even harmonics out. This leaves 100% of the SET even harmonics preserved. (although, the bottom Mosfet may still remove about 5%) Maybe would be worthwhile to use a bipolar transistor on the bottom section to knock that 5% loss down to maybe .5% loss of even harmonics.

For the H bridge design, using the more precision fig. 5 (CCT5) design on each side doesn't gain anything as far as the even harmonics go (nothing to fix), but would knock out the extra <.1% odd harmonic distortion from the Mosfets (should actually be well below the .1% total distortion figure, since its a 3rd order product or higher, and the device is a near square law curve with little odd order contribution). Maybe some golden ears can hear these <.01% effects?

(one might take a more detailed look at the odd order distortions here, the triode changes over from expansive 3/2 power effect to a compressive odd order effect due to saturation as signal level increases, whereas the top Mosfets continue as an expansive odd order distortion effect, this means the odd order distortions add at low levels and change over to cancelling at higher signal levels. The bottom section Mosfet only contributes even order, primarily 2nd harmonic, about -.05% with respect to the triode's, distortion.)

Don
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Old 13th May 2005, 06:53 AM   #30
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I HAVE TO i ADMIRE YOUR DEDICATION TO THIS. GOOD JOB!
How far are you from building a prototype

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