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Definition of Class A?
Definition of Class A?
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Old 8th December 2017, 02:38 PM   #111
jazbo8 is offline jazbo8
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Definition of Class A?
Broskie has a bunch of blog posts about non-gm doubling, e.g., Constant-gm Output stages part two
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Old 8th December 2017, 03:54 PM   #112
smoking-amp is offline smoking-amp  United States
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A little more playing with the 4 tube (2 grounded cathode in class AB, 2 CCS tail in class A) class AB stage shows that the bump and the valley can be tuned to cancel by adjusting their widths equal (by biasing V's) and by adjusting magnitude (by CCS current setting). This no longer leaves two valleys outside the central flat section, very good. But.... the gm curves outside the central flat section are increasing rapidly and winging out into saturation. A flat bottomed V with wings.

So the 4 tube "solution" just gets one to the flat bottom and winged out V shape gm sum (which I thought class AB grounded cathodes would already do).

Clearly there IS a BIG difference between class AB and class A. This promoted idea that one can operate in the flat gm section of class AB is utter BS!!!! (high Odd harmonics) The restrictive class A definition should be RIGIDLY adhered to.

Conclusions:

1) Class AB, grounded cathodes, absolutely sucks. Big V in the gm curve. No flat bottom as promoted. High Odd expansive harmonics.

2) Class A, CCS tail, absolutely sucks, Big bump in the gm curve. High Odd compressive harmonics.

3) Combined 2+2 tube class AB/A (grounded cathode + CCS tail pairs) gets you a flat bottomed V gm sum with winged out edges. Tolerable for small signals, but not good overall. (this is actually the real embodiment of what the class AB, restricted to class A portion, were claiming. Sorry, takes 4 tubes to get that!)

4) Broskie's non gm doubling designs are using diode/resistor limiters. This will give a sort of piecewise approximation to constant gm, but the kinks in the gm curve are going to produce high harmonics. Really prefer some kind of natural fit.

5) Twin/Crazy Drive can produce constant gm over most of its operating region (see actual Ip versus Vg graph below, slope is gm), but reverts back to essentially grid 1 operation near its cutoff end, where P-P would crossover. One would expect the gm curve to look -something- like pic 2 below. Which -could- be smoothly overlap-able. I need to take some detailed measurements (to derive gm) in this cutoff region (on the curve tracer). So that's next, some measurements. Start praying.
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File Type: jpg rsz_26lx6_twin_54v_54vs_50_50.jpg (79.1 KB, 99 views)
File Type: jpg Hoped_for_gm_curves_please_please.jpg (10.1 KB, 98 views)
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Last edited by smoking-amp; 8th December 2017 at 04:06 PM.
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Old 8th December 2017, 04:22 PM   #113
20to20 is offline 20to20  United States
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Quote:
Originally Posted by smoking-amp View Post
Start praying.
Ya, I pray this stream of conciousness thread ends soon.
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Old 8th December 2017, 07:48 PM   #114
Wavebourn is offline Wavebourn  United States
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Definition of Class A?
So, it was symbolic, to use SE amp for PA during BAF in San Francisco this year. No gm doubling! ;-)
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File Type: jpg IMG_3963.jpg (465.2 KB, 85 views)
File Type: jpg IMG_3962.jpg (791.8 KB, 73 views)
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Old 8th December 2017, 08:12 PM   #115
SpreadSpectrum is offline SpreadSpectrum  United States
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Quote:
Originally Posted by smoking-amp View Post
RE: SpreadSpectrum
That's interesting.
Any chance one could combine a humped up with a valley down set to get near constant gm? (4 tubes then)
I bet you could.

Now, I did once make a serious attempt to measure distortions caused by these variations in gm with my 30% LFB KT88 amp. I made a lot of distortion measurements at 50mA, 60mA, and 70mA bias levels. I was unable to detect differences in distortions (I measured many signal levels) at those bias levels. Both harmonic content and quantity of distortion seemed to stay about the same. It could be that my driver (CCS-loaded 6BL7) was the dominant source of distortion in the amp or it could be that the distortion caused by these variations of gm are just not that big. One thing that did change a lot with changing bias current was idle Zout of the amplifier, as expected from the change in idle gm.
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Old 8th December 2017, 10:01 PM   #116
smoking-amp is offline smoking-amp  United States
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Here is a magnified section of Crazy/Twin drive (26LX6) Ip versus Vg2 near cut-off. Vertical is 10 mA/div. and steps are 2V on grid 2.
0 V on grid 2 is at the center line. So right side bar is +13V on g2 and left side dot is -11V on g2.
(takes some neg. Vg2 and Vg1 to counter the positive plate going to +370V, this is all taken on a 650 Ohm load line)

Using a bigger scale (8V steps and 50 mA/div) the Ip versus Vg2 is linear (using bigger scale) from about Vg2 = +4V up to +68V (520 mA)

From the pic it is obvious the cutoff region has a roughly parabolic shape for Ip between -11V and +4V on Vg2. Which would correspond to a linear slope of gm.

While the (earlier) bigger scale Ip plot has a constant slope from +4V (50 mA) up to +68V (520 mA) which would correspond to constant gm over most of the operating range. I'll take some detailed # measurements off the CRT to tabulate and calc the slopes for gm.

Looking good so far!
The tube hobby might be saved. Grip your chair for the final gm results....

.
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File Type: jpg rsz_26lx6_g2_g1_10ma_2vsteps_0center.jpg (122.5 KB, 68 views)
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Last edited by smoking-amp; 8th December 2017 at 10:12 PM.
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Old 9th December 2017, 12:40 AM   #117
smoking-amp is offline smoking-amp  United States
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Here we go. Gm and Ip curves for the 26LX6. Rg2g1 = 10.9K Ohms, Rg1K = 622 Ohms

Table data:
Vg2.....Ip mA
-11........0
-9..........1.45
-7..........4.15
-5..........8.9
-3.........15.8
-1.........24.3
+1.........34.3
+3.........45.7
+5.........58.4
+7.........72.2
+9.........86.6
+11......101.2
+13......115.85
+15......130.55
+17......145.25

+68......520

Not a perfectly symmetrical gm S curve, but reasonable enough for overlapping. And the 1/2 max gm point is at low current, like only 20 mA idle.

I'm going to play with the Rg2g1 and Rg1K now to see if that has any big affect on the Ip curve shape. One could also play around with a small offset voltage between bottom Rg1K and the cathode. Some tubes have obviously needed a few volt offset there. Need to get a dynamic gm tester going, then I can play with the R dials.
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File Type: jpg gm_curve_26lx6_g2g1.jpg (453.4 KB, 19 views)
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Last edited by smoking-amp; 9th December 2017 at 01:07 AM.
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Old 9th December 2017, 02:02 AM   #118
smoking-amp is offline smoking-amp  United States
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OOPs, just found a minor error in the Ip table. First entry for -11V should be 0.3 mA, not 0 mA.

That has a small effect on the gm graph, so here it is corrected:
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File Type: jpg gm_curve_26LX6_g2g1.jpg (452.7 KB, 18 views)
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Old 9th December 2017, 05:14 AM   #119
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I played around with Rg2g1 and Rg1K (pots) while looking at the cut-off range Ip curves (low current), and they have large effects in this low current range. Since gm is the derivative of Ip, the effects will be significantly larger there. So there is no doubt that the shape of the gm curve can be altered. However, only a certain range of the R parameters gives the constant gm at higher currents. So this will have to be explored.

This will need a dynamic gm display to sort out what can be done for P-P crossover steady gm control. Seeing as the 1/2 max gm point occurred at only around 20 mA for the 1st test indicates that good class AB efficiency can be obtained.
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Old 9th December 2017, 12:10 PM   #120
Bandersnatch is offline Bandersnatch  United States
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Nice work there SA...

So, with your constant gm zone from 20 mA onwards, it looks like there is no need to do a full overlap Class A.

Now with a full overlap design, the cut off areas would lead to a slightly decreasing gm at the extremes...flat in the middle, but turning down as we get to the 20 mA line...
cheers,
Douglas
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