| runeight |
The regulator thread generated this post. Once again, I realize that most of the folks on this list understand what CCSs do for tube circuits (at least some of them). So, I thought that if you all would indulge me once again, we might talk about it some. :)
Grounded Cathode
To start we will use the basic grounded cathode amplifier. This diagram shows two identical gain stages except that the one on the right is loaded with a perfect constant current source. I'm going to look at the gain, distortion and frequency response of both of these and compare them. Once again, I will use PSpice for the simulator and the trusty old 6DJ8 for the tube.
First, we can look at the gain. The tubes are driven by a 0.1v 1KHz sine wave and Ca=Cb=10u. Here is the output from points A (green) and B (red):
Notice that the CCS loaded device has more gain, about 25 for A and 30 for B. This is helpful, but not that important.
To look at the distortion, we'll take the Fourier transform and look at the second harmonic at 2KHz. Here it is, blown up:
Notice that the 2KHz amplitude for the non-CCS amplifier is about 5mV, but for the CCS loaded amp it is 2.5mV. This reduction in distortion can be of great benefit. When compared with the increase in gain the relative distortions are: 0.2% for A and 0.08% for B. Small numbers to be sure, but in the world of audio perfection, well . . . . . :goodbad:
Why is this so? Without dragging out the plate curves, this change simply comes from the fact that in the CCS loaded amp the load line is completely horizontal (minimizing distorion), whereas in the non-CCS stage, the load line has a slope. I know that you all know this anyway.
Perhaps, more important, is the frequency response curves. Here we will sweep the input from 10Hz to 1MHz. Again, Ca=Cb=10u. Plots are in DB of gain. Here is the comparison:
Notice that the CCS loaded amp is very flat all the way to 10Hz. Why is this so? Because, as all the better circuit guys are screaming to me, if the current is constant through the tube, then the bypass capacitor is essentially out of the circuit and, therefore, does not limit the low frequency response. In fact, if the CCS device really were perfect, we could eliminate Ck altogether.
The only way to approach this response for the non-CCS stage is to put C=1000u. Still, you won't get there and you've got another big ugly electrolytic in the circuit.
However, the goodness of this result also depends on the response curves of the actual CCS device. Since it will be made of real components, it won't have a perfect response and, hence, any real circuit will deviate from this result. How much, you may ask? Well, that's the subject of the next post. :wrench: |
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| runeight |
Tube CCS
Will a tube CCS really be fast (or slow) enough? Here's a fairly typical CCS tube load for our previous grounded cathode amp.
This circuit draws the same idle current as above. I am setting Ck=10u and Cc=470n. The gain of this configuration is nearly identical to the perfect CCS above. Here is the three-way distortion comparson:
The new blue curve is the Tube CCS stage. Its distorion is identical (at 2KHz) to the perfect CCS stage.
What about the response? Here is the three way comparison:
Notice that the Tube CCS blue curve is very close to the perfect response curve. This graph shows just the perfect and Tube CCS response comparison:
The Tube CCS stage is only 0.2db down at 10Hz (with a 10u Ck). I would hope that this would be unoticeable to all but the most discriminating ears. :)
So, if you want to go tubes all the way, you can load your grounded cathode amp with its mating tube ccs and you should get very good results. Of course, the price for this is an additional 100V or so on the PS (from 200 to 300 volts).
I've looked at CCS loaded cathode follwers (at least for a few configurations) and there seems to be no significant effect from the CCS. But, real circuits may be different.
A further question is, does a CCS help a circuit that has no bypass cap? For example, does it help a common cathode amplifier? Next post. |
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| runeight |
| Say, I made an error on the CCS effect on CF stages. I'll post that too. |
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| Sch3mat1c |
Thanks for the review. Also try load capability drawing output from the top tube's cathode, and using a pentode instead.
Speaking of pentodes, I wonder what output would look like with a pentode as the amp! Hmmm...
Tim |
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| Bas Horneman |
Hi Runeight,
Love your postings!!!
From an amateur's point of view I am somewhat suprised however by the performance of the triode CCS...
Alan Kimmel says that a triode cannot be considered a true CCS since "because it hasn't enough gain to respond adequately to the small voltage changes which occur across Rk2"
http://home.zonnet.nl/horneman/ml/akimmel.htm
Regards,
Bas |
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| Bas Horneman |
Another thing I would be interested in is how could the shunt capacitance be lowered without a cascoded CCS? Is that something you can easily measure/simulate.
From Gary Pimm's pages...
http://home.pacifier.com/~gpimm/Act...nt_control.html| quote: | Another poor performance aspect of single mosfet CCS’s is high shunt capacitance. Mosfets look like great parts until you take into account their high capacitance issues. I suspect that the shunt capacitance is the source of most of the sound coloration’s that make CCS’s sound different.
The use of a cascode circuit will greatly reduce the shunt capacitance and improve the DC performance of a CCS. |
As a diy'er I was thinking...how about just using a 1k resistor or so? Would that "hide" the shunt capacitance of the mosfet CCS??? In the same way that you can hide capacitance from a tube rectifier with a choke or resistor???
Cheers,
Bas |
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| EC8010 |
Hello Runeight, audio runs on dBs. It produces slopes of 6dB/octave etc, and humans respond to sound logarithmically. We just can't get enough dBs...
Although constant input voltage is fine when looking for changes to gain, if you want to look at how a CCS changes distortion, it's best to hold the output voltage constant. |
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| SY |
| And, of course, you have to look at loading from the next stage CCS circuits are particularly vulnerable to loading effects. |
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| Sch3mat1c |
...Hence why you'd want to look at the output from the top tube's cathode. :nod:
Tim |
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| fdegrove |
Hi,
| quote: | | Hence why you'd want to look at the output from the top tube's cathode. |
And...surprise, surprise...I'd like to see a penthode as the top tube, not just to make pentahodos Tim happy.
Cheers,;) |
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| Sch3mat1c |
Dear God, for once we agree on something!!! :bigeyes: :bigeyes: :dodgy: :rolleyes: :p
Tim |
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| runeight |
| quote: | | Hello Runeight, audio runs on dBs. It produces slopes of 6dB/octave etc, and humans respond to sound logarithmically. We just can't get enough dBs... |
Thanks EC. I guess this was in response to my line about discriminating listeners? I think my hyperbole was a little more effective than I meant it to be. :) I do know how the ear hears loudness, although my wife might argue with that.
| quote: | | Although constant input voltage is fine when looking for changes to gain, if you want to look at how a CCS changes distortion, it's best to hold the output voltage constant. |
Could you explain this another way. I'm not sure I understand it. Do you mean that to get an accurate picture of distorion vs. frequency, we should keep the output signal the same amplitude? Please help.
| quote: | | Thanks for the review. Also try load capability drawing output from the top tube's cathode, and using a pentode instead. |
Ah yes. I was trying take the very simplest case for comparison, but if we're looking at Zo then, of course. I've added point B to the picture above at the cathode of the CCS device. Just for completeness, here is the simulated Zo for:
No CCS = 3200R
Tube CCS A = 3700R
Tube CCS B = 135R
Taking advantage of the follower is the natural thing to do if you're driving a load or even if you're not.
OK, so I'll try some pentodes, but at least according to this simple model, the triode will do quite well. I'll have to read Mr. Kimmel's paper in detail.
| quote: | | Dear God, for once we agree on something!!! |
If I have performed no other service to this forum, this happy agreement may be enough. :clown: |
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| EC8010 |
What I meant was that graphs expressing distortion in terms of dB down on the findamental are clearer than absolute distortion voltages.
As for my other point, when you bypass a cathode resistor with a capacitor, you change the gain as well as the distortion. Because distortion is proportional to output level, if you want to compare the effect on distortion, you need to adjust the input level so that both circuits produce the same output level.
I'm going to have to say that I don't think any of the PSPICE triode models are sufficiently good to accurately predict the distortion of a real valve. I have seen people trying to show distortion cancellation/reduction schemes down to -80dB based on PSPICE. All it "proves" is that the model cancels. No more. |
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| runeight |
| quote: | | I'm going to have to say that I don't think any of the PSPICE triode models are sufficiently good to accurately predict the distortion of a real valve. I have seen people trying to show distortion cancellation/reduction schemes down to -80dB based on PSPICE. All it "proves" is that the model cancels. No more. |
I mostly agree with this. I don't think that the spice models can predict absolute levels of distortion with detailed accuracy. But, I do think that they can give you order-of-magnitude results and I think that they give very good relative results. That is, they can tell you if one topology is better than another. For example, I have simulated transcendent's grounded grid preamp and the distortion figure that I get is very close to the published value (on the assumption, of course, that the published value is accurate).
| quote: | | What I meant was that graphs expressing distortion in terms of dB down on the findamental are clearer than absolute distortion voltages. |
Yes, I just don't have that capability in the PSpice that I have. And, also, we are accustomed to seeing distortion figures in % values too.
| quote: | | if you want to compare the effect on distortion, you need to adjust the input level so that both circuits produce the same output level. |
Yes, I should have done this. Although, in this case, I think the non-CCS device will only get worse in comparison. |
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| EC8010 |
| Agreed, PSPICE is certainly good enough for comparisons between topologies and order of magnitude predictions. What a shame that your version insists on giving distortion as an absolute voltage. We'll just have to live with it... |
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| fdegrove |
Hi,
| quote: | | What a shame that your version insists on giving distortion as an absolute voltage. We'll just have to live with it... |
While I quite understand the objections uttered, it doesn't bother me all that much.
All it takes is a little intellectual effort to extrapolate these figures and see how they correlate.
All in all I think runeight is doing us a great service investigating/demystifiying the CCS and its' impact on the circuit.
Cheers,;) |
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| runeight |
I'll keep plugging along Frank. :)
To adjust for EC's correct assertion about distorion, here is the comparison for the non-CCS gain stage against the tube CCS gain stage where the input to the non-CCS stage is set to give the same output level (.118V in). Non-ccs distortion is green, tube ccs is red. Ck=10u.
As everyone expected the second harmonic increased to 7.5mV which is about .25%. Worse than before.
We agreed before that these absolute numbers cannot be counted on to be precise because of other circuit affects. But, I believe that the trend is correct, even if it is not necessarily important.
In case you're wondering, this is a graph of the current draw from the PS for the tube CCS stage. I have placed a 100K load through a 1u cap to provide something for the stage to drive. Output is taken from the cathode of the ccs device (point B), so that this is a mu follower (Ck=10u):
So that when the device is loaded, it draws about 20uA peak from the PS. I don't know if the PS designers would consider this to be constant current enough.
This little bit of work led me to look at a constant current draw pair. Here is the diagram:
The idea here is that since both tubes are at the same operating point, but their currents are 180d out of phase, their demands on the PS will cancel. And, therefore, they can share a common Rk which won't need much, if any, bypass. Since the output is a follower you still get low Zo.
First, the distortion comparison to the tube ccs device:
This is as expected, because, although the pair of tubes is constant current, each tube still sees a load resistor and doesn't get the benefit of a CCS load. So the distortion is higher than for the pure CCS device.
Now we may ask, does this device's PS current requirements differ from the mu follower for the same load? Here it is at 100K load. I've taken the fourier transform and am showing the current amplitude at 1KHz. Ck=10u.
So, from the PS point of view the current draw should be nearly identical. However, the mu follower needs 100 extra volts on the PS while the constant current pair needs twice the idle current. Also, the mu follower doesn't depend on the tubes being in balance (which they usually aren't).
What about the frequency response? If we set Ck=10u for both stages and if we normalize the output, we get:
If we set Ck=0 we get:
For Ck=0 the curves are nearly identical. Even for Ck=10u they are very close. (I realize that I haven't normalized the outputs precisely, but they're close enough).
The Zo of the constant current pair is about 80R. The tube mu follower has about 135R.
So, the only distinguishing feature of a real CCS loaded device compared to what could be done with a constant current draw pair is the level of distortion. Assuming that a real triode performs the way the models do, then this difference might be enough to choose the CCS loaded amplifier. ANd, of course, the need for some balance in the tubes for the constant current pair.
I guess the PS trade-off depends on what you like to do, supply current or supply voltage.
Some caveats again. This is simulation and these are perfect Rs and Cs. Also, I've only explored a few possibilities. There are many others, but I think these get the ideas across. At least, this helped me to see what's going on.
Pentodes next. But these are a real pain in the :censored: compared to triodes (model-wise). |
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| runeight |
Pentodes
Ah yes. The claim is that pentodes are much better CCSs than triodes. Perhaps they are, but are we straining out gnats while swallowing camels?
I loved pentodes when I was a geek kid. They were the best tubes to break open to see the internal construction. I used to buy tons of them for $0.10 apiece at surplus stores. In case you want to try this, put the tube into a paper (plastic) bag and whack the glass with a hammer. Just hard enough to shatter the envelope. The bag will collect the junk from the implosion and you can retrieve the tube to pick apart the internals to see how this stuff is really made (warning: be very careful with the glass and tubes have heavy metals in them). I just wanted you guys to know that the stuff you pay big money for and view as new age crystals that bring cosmic goodness to your gear, I used to smash regularly. :devily:
Bas, I think this begins to answer your post. I have read Alan Kimmel's article.
This post will compare triode and pentode CCSs. Mr. Kimmel's article recomends high S tubes. I didn't have a really high S pentode, so I created a 6JC6 (S~=16000 umohs) model. It's not perfect, but it is close enough for this analysis. Here is the circuit:
The left side is the triode CCS from before. The right side is a topologically equivalent pentode CCS using the 6JC6. The biasing is set so that the idle current is the same (within a few uA) for both circuits.
The cap that locks the screen to the cathode is 10u. Mr. Kimmel recommends that Xc for this cap be no more than 1/10 the value of Rsc (100K). I believe that 10u does the trick. Note that this is a high Rsc, but is what seems to be necessary to make this configuration work. Besides, Isc hardly changes at all.
First, let's look at the CCS characteristics of both configurations. This graph shows the current variation in the bottom triodes at 1KHz input (green=triode, red=pentode):
Indeed, the pentode current variation is smaller. To see this better, here is the fourier transform at 1KHz:
So the triode CCS keeps the current constant to within 4.5uA and the pentode CCS keeps it constant to within 0.5uA. The pentode is a remarkably good CCS, but does this difference really matter? I'd like to hear from you guys about this.
How does this effect the distortion? Well, both bottom triodes are now operating close to maximum possible gain, with very flat load lines. And so, there is very little change to the distortion figures (looking at second harmonic):
And finally, does the more perfect pentode lead to better frequency response? Well, here's what the simulation says:
Gain is practially identical with the pentode approaching mu a little better, but still they are within 1db. The triode rolls off a little more quickly above 200KHz, but I can't imagine that this would be noticeable since it's way less than 1db at 300KHz.
Mr. Kimmel shows another, more sophisticated CCS for pentodes. I can't imagine that could help much. But, he does indicate that he built all of his designs and compared them.
So, my take on this, recognizing the limits on the simulations, is that the pentode CCS may not be worth the extra trouble or parts. Go ahead, fire away . . . . :)
Next, I think I'll look at a 6BM8 for the pentode as these may be more available today. 6BM8 has about half the transconductance of the 6JC6. |
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| Sch3mat1c |
Thanks for the review.
For tube models, get in contact with Robert Casey from rec.audio.tubes, uhm.. e-mail is wa2ise @ ix.netcom.com.
I'd also like to see what the high plate resistance alone does (i.e. pentode with grounded-to-cathode grid, no drive) as far as CCS-ness.
Tim |
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| runeight |
Tim, thanks for the lead on the models. It's amazing how the addition of one electrode really complicates, not only the model, but matching the curves.
I will try connecting grid to cathode and post it the results. |
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| runeight |
Tim, I assume you mean this?
If so, here is the current regulation (green=driven, red=strapped):
The GK strapped configuration shows about 26uA. Much worse than the 0.5uA when the grid is driven. |
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| SY |
| quote: | | Perhaps they are, but are we straining out gnats while swallowing camels? |
We're doing that when we use tubes in the first place. :devily: |
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| Sch3mat1c |
| quote: | Originally posted by runeight
Tim, I assume you mean this? |
Yes. Except, you can get rid of the 10k between stages and adjust the screen supply resistor for operating point.
| quote: | | The GK strapped configuration shows about 26uA. Much worse than the 0.5uA when the grid is driven. |
Not suprising, it's running purely on plate resistance (high though it may be), rather than amplifying the error signal. Likewise, it'll also trash out pretty good when driving anything.
Oh - how about putting a CCS in place of the "current sense" resistor? I bet that would perfect it.
Thanks for satisfying my curiosity, :)
Tim |
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| runeight |
| quote: | | Oh - how about putting a CCS in place of the "current sense" resistor? I bet that would perfect it. |
Good idea. Kind of an infinitely recursive design. Each CCS with another CCS inside of it. I like it.
One last thing on the pentode option. I thought I would try a 6BM8 because it is a triode/pentode package and because I have the model. The pentode has half the S of the 6JC6. I wondered if it would work. Also, you get your triode and pentode in the same bottle. Only two sockets for a stereo line stage. :)
Here's the diagram.
To get both devices into good operating region (as I judge it) I raised the B+ to 400V. Idle current is about 4.6mA.
Here is the current draw with a .05v 1KHz input signal. Again the fourier transform:
Still below 1ua and so, from the perspective of simulation vs. reality, no different from the 6JC6.
The gain is almost twice the previous configuration. Here is the 2nd harmonic for about the same output voltage:
About 0.25%.
And, finally, the response curve:
My conclusion is that if you really want to use a pentode as the CCS device, except for the PS change, this is a pretty good option.
Frank, I wonder if a 6GV8 would be a better choice?
However, all of this work still tells me that a triode pair will be just as good. You can get them two to a package too, while keeping a reasonable supply voltage. |
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| fdegrove |
Hi,
| quote: | | Frank, I wonder if a 6GV8 would be a better choice? |
Not necessarilly, that's a fine reg. for low current/ low voltage demands though....around 200 to 250VDC @ 5 mA at most.
One of my favourites is the ECL82/6BM8 for lowish voltages.
Cheers,;) |
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| fdegrove |
Hi,
And what if I tell you that some penthodes can be used to replace a choke? ( Tim loves me already)
For, all things considered, this is what we're doing here except we try to keep current constant.
Moreover, I know of at least one tetrode that can mimmick a nice choke... and it ain't all that small a H value either.
There's a lot of clever things you can do with tubes...there really is no end to it.
Cheers,;) |
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| PRR |
> Kimmel says that a triode cannot be considered a true CCS since "because it hasn't enough gain...
It Depends.
The usual trick is about a 470Ω cathode resistor, with the grid tied to the bottom of that. The CCS effective impedance is then essentially Mu*(1/Gm) which is Rp which is about the same as a simple plate resistor.
Runeight is using a 20K cathode resistor. That forces 3 extra parts to keep DC bias. But then the CCS impedance is Mu*(20K+(1/Gm)). For the 6DJ8, that comes out to about 600KΩ, a very high value indeed. You get 90% of the benefit of a "perfect" CCS.
Another point: 1/Gm is 200 to 2K for most tubes, and 200Ω happens only with "best" (most fragile) tubes at high current. But with the 20K resistor, they all work about the same: you can use a rugged 12AU7 or 12AT7 instead of the 6DJ8 or other too-hot tube. The impedance then depends mostly on Mu, not Gm or the ratio of Gm to cathode bias resistance.
When you get to Pentodes: the effective Mu is very high, but you must drive the Screen to track the Cathode. The obvious way is a capacitor from Cathode to Screen. You also need a path to supply Screen current, usually a resistor. When you do all that, the Pentode's very-high Mu is swamped by loading effects. I put far less trust in the available Pentode models than the triode models, but I think Runeight is right when he shows only modest or even no improvement from a ballasted triode. You could bootstrap the screen with yet another tube, but where are we going?
As Frank says "There's a lot of clever things you can do with tubes...there really is no end to it." Recall the words of the preacher in the Bible: "Of the making of many things there is no end... there is no new thing under the sun."
I think the simulation should have a load: say the typical grid resistance and capacitance, or the IHF standard Line Out load. If you cascode and bootstrap and CCS a tube to death, you make an ideal infinite object that can't interface with any real world.
For anybody who really wants to dig: just after WWII the MIT Radiation Labs (RADAR) released a 10-volume summary of their research. One volume is general amplifiers, and talks about such things. In general, as you reduce current while keeping supply voltage high, gain rises (but bandwidth decreases). They did try near-CCS topologies. In real life, the improvement from using another tube as a CCS is not as good as using an extra tube some other way, such as another amplifier stage or a buffer. You never see such frills on commercial gear from the heyday of tubes: they knew better.
There is no remembrance of former things... Ecclesiastes 1:11
(because runeight and I smashed them with hammers....) |
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| runeight |
| quote: | | And what if I tell you that some penthodes can be used to replace a choke? |
Frank, you knew I would have to ask how? So I could simulate it. :)
PRR, thanks for the additional illumination. As always you managed to do in a few paragraphs what took me a couple of pages and several thousand drawings.
| quote: | | (because runeight and I smashed them with hammers....) |
Et tu, Brute?
. . . for the former things are passed away. . . . Behold I make all things new. . . . Revelation 21:4,5
I trust we can be forgiven. :) |
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| fdegrove |
Hi,
| quote: | | Frank, you knew I would have to ask how? So I could simulate it. |
Sure....
Vb = 400V
V out = 335V
Va = 60V
Vg2 = 100V
Vg1 = -1.0V
Ik = 110mA
Rg2 = 60K
Rg1 = 470K
Rk = 50R
C in = 8.0µF
C out = 20µF
Cg2 = 8.0µF
Cg1 = 50µF
The tube used here is a 6216 as made by CBS-Hytron.
They claim this circuit emulates a 12H choke at 150mA..
Other than this application note, I fail to see anything extraordinary about this tube tube so I think it's safe to assume other penthodes can be made to work in a similar way.
Cheers,;) |
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| PRR |
> do in a few paragraphs what took me a couple of pages and several thousand drawings
I'm just slapping words. Broskie has a much better article. - SRPP Deconstructed - PDF
He starts with "the 1940 patent (US 2,310,342) for the circuit", moves on to Millman-Taub, and its use in video line drivers. Aside from driving some of the early computer "core" memory, that was the end of the line until its rediscovery by audio tube-heads.
- "Today, the prevailing view (a new view) is that the SRPP is an SE circuit made up of a grounded-cathode amplifier that is loaded by a cathode follower that also functions as a constant current source; (a nice trick: providing a low and an infinite impedance at once);...."
- "The alternate view (the old view) sees the SRPP as an ingenious way to do push-pull on the cheap, as the SRPP is simply a two-tube push-pull amplifier that comprises its own phase splitter in the form of a single resistor."
He then tries to "prove which view is most useful" and shows that neither view is a good foundation for understanding. Also he shows that it can be improved in several ways: in the next 20 pages he re-invents nearly every totem-pole topology and compares them.
"So what insights can we take away from these circuits? First that the load counts. The load to be driven defines the starting point. - If the load is a punishingly low 32 to 10kohms, then abandon any notions of constant current sources and single-ended operation. - On the other hand, if the load is 1-meg to infinity, then high-current push-pull operation make no sense and single-ended operation is closer to the actual functioning of the circuit. But as the load is so high why not move the connection.... - If the load impedance is not specified, as in the case in a line amplifier, do not use any type of SRPP circuit, as the SRPP should be designed around one specific load impedance. - In sum, the SRPP circuit has its uses, but those uses are limited. Where money and space allow, better more complicated circuits can be used to better effect than the SRPP; but where dirty and cheap are needed and where the load impedance is fixed and purely resistive, the SRPP is a good choice."
The end of that refers back to an earlier article SRPP Decoded. On page 3 is a big box-quote "Unlike the White Cathode Follower, the limitation to the SRPP is that it can only be optimized for one specific load impedance."
(FWIW, my studies say the WCF gets best performance if you DO optimize for the specific load to be driven; but the difference in component values and performance is VERY small from short to infinity. One set of values does work good for any sane load. With the SRPP, you have to change values every time you change the load {to get "best" performance}.)
The fixed-impedance restriction on an optimum SRPP is a killer. You usually have both resistance and capacitiance. Cable capacitance, grid capacitance, whatever. Therefore the load impedance changes over the audio range (including supersonics which may be intermodulated into the audible band). Now, maybe the SRPP can handle "some" change of load: obviously it does not "stop working" when the load is changed a little. But does it handle varying load even as well as a grounded cathode followed by a cathode follower? I suspect the GC-CF is generally better: the GC stage is well isolated from load variation and the CF stage output can drive most loads with ease. With the GC stage essentially unloaded, it can be working in a highly linear range, without trying to "tuck it under" another tube. The biggest advantage of SRPP then is the fact that two tubes take B+ current in series. Over a small range of loads, the SRPP may be more power-thrifty than a GC-CF design. Over a larger range of loads, either the SRPP fails to drive low-Z as well as a CF, or it is eating more power current than a GC-CF amp could be designed for. |
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| PRR |
> penthodes can be used to replace a choke?
With gain, you can emulate anything.
The most basic emulated choke, using a perfect voltage follower:
The 1K resistor acts a lot like the copper resistance of a real coil. The 1Meg resistor acts a lot like any shunt resistance in a real coil (load impedance and core loss). The capacitor working against (R1*R2)^0.5 sets the "inductance".
Impedance plot:
Where it says "KV", read "KΩ" (since the driving source is 1Amp).
At DC it is 1K, at high-AC it is 1Meg. With these values, the -3dB point is about 32Hz.
You can use a cathode follower. In fact the cathode impedance can be part of R1. It can "be" R1, but then it is fairly non-linear and as much a "tube" as a "choke". The slight loss of gain in a real CF means a non-trivial shift in the computed inductance. Also a cathode follower needs a DC load, which is an additional shunt on the choke. However that DC load could be the plate of another tube, a very interesting plan. However it then becomes a form of SRPP (the kind with a grid capacitor) and Broskie's essay applies. |
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| fdegrove |
Hi,
Agreed on most counts.
The SRPP looks like a misnomer indeed, it's hardly a shunt regulator and it's even less PP.
Still, it does have a number of merits and when the load conditions as mentioned by PRR are met it can be be a handy solution to some problems.
As with most things electronic nothing's ever perfect, it's always a delicate balance of making the right decision for the job at hand.
One thing about the SRPP and its derivatives is that it's not well suited to low Rp, low µ tubes.
This should be obvious from John Broskies analysis or if you think for your own for that matter.
I've said it before, the SRPP and other stacked tubes deserve a well regulated PS on their roof to give of their best, if those conditions aren't met they'll just disappoint.
The major drawback of the WCF is its having no gain. As a tubed buffer it's hard to beat and add a stiffly regulated SU and all is bliss bar the silliest load.
So, Runeight if you want more work thrown your way; there you have some.
PRR,
Thanks for the liitle analysis on the penthode choke. It confirms what I suspected: any penthode with good transconductance could qualify.
This opens up a lot of perspectives; penthodes, at least some of them are dirt cheap and cost way less than an actual choke. Moreover they also only weigh a fraction of a choke and take up much less real estate.
| quote: | | However that DC load could be the plate of another tube, a very interesting plan. However it then becomes a form of SRPP (the kind with a grid capacitor) and Broskie's essay applies. |
Precisely, nothing that can't be overcome and not really a big deal if you follow me.
So, in a way we can have a penthode serving as replacement in a CCS ( thank you Gary Pimm), it mimmicks our classic anode choke, but also can we now use a penthode to replace a choke as a reactor in a PS (thank you CBS-Hytron).
And we can push those ideas further still, Dr. Evil still has a few aces up his sleeve...:D
Your call Runeight ,;) |
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| runeight |
Thanks PRR for reminding me of Broskie's article. I remember reading it quite a while ago. I should go back and read it again.
| quote: | | I suspect the GC-CF is generally better: the GC stage is well isolated from load variation and the CF stage output can drive most loads with ease. With the GC stage essentially unloaded, it can be working in a highly linear range, without trying to "tuck it under" another tube. The biggest advantage of SRPP then is the fact that two tubes take B+ current in series. Over a small range of loads, the SRPP may be more power-thrifty than a GC-CF design. Over a larger range of loads, either the SRPP fails to drive low-Z as well as a CF, or it is eating more power current than a GC-CF amp could be designed for. |
I agree with you on this one. If I were planning a line stage to drive an arbitrary set of loads, I would use a constant current draw GC-CF pair. If the operating region and gain are reasonably chosen, I believe the audio quality will be very high.
| quote: | | So, Runeight if you want more work thrown your way; there you have some. |
But, I'm already reeling from rolling!! Yes, there could be some more work to do. Where would you suggest we go? :) I am becoming more and more convinced that CCS loaded devices are not all that useful or "sonically" different. But, I am willing to listen and try more stuff.
| quote: | | And we can push those ideas further still, Dr. Evil still has a few aces up his sleeve... |
Now you're making me really nervous. :D |
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| fdegrove |
Hi,
| quote: | | But, I'm already reeling from rolling!! Yes, there could be some more work to do. Where would you suggest we go? I am becoming more and more convinced that CCS loaded devices are not all that useful or "sonically" different. But, I am willing to listen and try more stuff. |
And what if I tell you that most of this exercise is just not necessary for all things audio?
Don't get me wrong, I like high PSR and all that when it comes to audio circuits but it's oh so circuit dependent you see.
When it comes to linear loads, it's hard to beat a good old resistor...
Anything else is horses for courses IMO...
I should catch some sleep...3.16 PM
The seemingly contradiction isn't as contradictary as it seems at first look...
Think about it...
| quote: | | Now you're making me really nervous. |
Don't be...back to the future!!!;)
Cheers,;) |
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| runeight |
Say, I went back and re-read Broskie's SRPP articles. And now I'm asking myself, why did I even crank up this thread? :) I should have just pointed folks to the TCJ.
Well, not exactly, it was worth the time to look at it from a simulation perspective. And I have noticed that since becoming part of Beige Bag SW, Mr. Broskie has taken a real liking to simulations because they really do work when you know how to use them. :angel: I have had some very good correspondence with him and he has been quite generous in giving me some of his time and expertise. |
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| Antonio Tucci |
Runeight,
I think you have made a very interesting analysis, and I thank you very much for it.
However, I'd like to submit two variants to your analysis.
1) Un-bypassed tube diode in the cathode circuit
2) High impedance plate choke loading.
Specifically, I'd like to ask how much the presence of a high impedance plate choke loading and/or a tube diode in the cathode may improve the performance of your basic grounded cathode amplifier.
My questions spring from a recent schematic proposed by Dennys Boyle (http://home.att.net/~chimeraone/axiom300bschematic.html). In this project he utilizes a tube diode in the cathode (eliminating both resistor and capacitor) and a high impedance plate choke loading, obaining amazing results ( http://home.att.net/~chimeraone/axiom300btestdata.html)!
I'd be very grateful if you could furnish your considerations and/or analysis about this variation. |
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| PRR |
> I'd like to submit two variants to your analysis.
Looks like food for a whole new thread.
I will comment that the "a well regarded traditional design 300B amp" is clearly flawed. A 300B should not be making 1%THD at 1 watt, 2%THD at 2 watts. This is driver-stage distortion. That amp needs to either justify its THD as euphonious, or clean up its act. |
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| runeight |
Antonio, here is a preliminary sim. I tried the drive stage. I don't have a 6y6 or a 6gc3, so I used a 6kg6 and a 6d22s damper diode.
This is the circuit:
Bias voltages a little different from the axioim schematic and the idle current is around 73mA.
I put a 5Vp 1KHz signal into the pentode. Here is the FFT of the output at the plate:
The peak output is about 19V so the distorion from 2nd harmonic at 160mV is about 0.84%
Here it is with 10Vp at the input:
About 39V output, 650mV at 2KHz, or about 1.7%.
Doesn't seem particularly good to me. But, you guys have to tell me. |
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| runeight |
Oh yeah, here's the response curve in the form of DB of gain.
This looks very good to me. |
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| Antonio Tucci |
Thank you very much for your interest.
I believed that high inductance in the plate could perform as a current source... and that a diode in the cathode could improve the behaviour of the tube.
On the contrary, your preliminary results demonstrated that this is not the true. Moreover, they seems to be in contrast with Boyle's considerations.
Can you comment this discrepancies ? |
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| PRR |
> High impedance plate choke loading.
For a given supply voltage, a good choke will always beat a resistor load.
In commercial design, it is sometimes a better plan to use a higher supply voltage, a low-cost resistor instead of a high-cost choke, and put the money saved into something else: better output transformed, gold knobs, or the maker's profit.
> 1) Un-bypassed tube diode in the cathode circuit
I wanted to see Runeight's comment. I'm not sure I understand what it does. But my feeling is: it is a little better than fixed-bias or cathode bypass, but not as clean as an unbypassed cathode resistor. It adds a nonlinear resistance in series with the nonlinearity of the cathode, and does not even work in phase opposition which would tend to cancel distortion (see long tail differential pair, where one of the triode can be replaced with a diode and still cancel 2nd harmonic). It may be a good in-between technique for some situations. I sure do not see how it can be a "Miracle Cure For Distortion", despite Axiom's numbers.
It leads to an amazing number of hot bottles on the chassis. Maybe that is a Good Thing. |
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