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In Reply to: RE: Does any one feel vindicated by TAS' editorial? nt posted by HiFiOd on April 26, 2012 at 12:05:44
I think the measurements do show that SET's sound very good. It's just the lack of proper multidiscipline training on the part of electronic techs and engineers of that kind just shows their lack of a full understanding and interpretation of the measurements.
Nobody has all the answers yet, but clues are out there. The ear distorts in similar fashion to the SET amp, for example. But that is not enough. We need more answers. And it's really only one way to skin a cat.
If the ear distorts in a similar fashion to a SET, then phase should be easily audible since the evens will either sum if in phase to 2X the distortion or sum out of phase (I hate the word cancel) to create higher order odd harmonics.
dave
As long as triodes are used without NFB in a SET, we can only really talk about harmonics and polarities of the harmonics. Phase other than 0 and +/180 degrees is possible to design out over most of the audio band. With NFB, phase will come into play due to lack of ultra wide bandwidth to go through the entire audio range. And, only with NFB does an increase of higher order harmonics happen.
Even order distortion is built up by not cancelling it (PP does that). Even order distortion is partially cancelled with two cascaded similar tubes (like 6922 > 6922) as their polarities are inverted with each other. I never put two similar tubes (except for each different channel) in the same system once I listened to my system without tube duplication in one channel. Every time I tried it again, it was awful to my ears. It sounds better to try a different tube every time and let the blend mix itself, tuned by ear.
Still, the ear is not an even harmonic only distorter. It does have low order 3rd and 5th harmonics self generated (there is some ear symmetry). The 45 SET output stage as Mr. Cheaver's master's thesis wrote, "scored", in his viewpoint, as closest to the human ear for the similar distortion transfer function over all other amps outputs. At least, this was how I read it.
The general "good distortion" amp has only low order harmonics, and a natural to the ear magnitude progression for each harmonic, and a similar distortion tracking to the ear by volume.
This does not mean that PP is a worse option every time for everyone. That's just not true.
"Even order distortion is partially cancelled with two cascaded similar tubes (like 6922 > 6922) as their polarities are inverted with each other."Maybe you can explain this to me. No one else has been able to.
Let's say we have two hypothetical gain stages. Each with a gain of 10 and having only 2nd order harmonic distortion. Let's say the percentage of 2nd order distortion is 10% (each stage).
If we apply 1 volt of 1kHz to the first stage we get 10 volts of 1kHz and 1 volt of 2khz feeding the second stage.
The second stage amplifier the 10 volts of 1kHz to give us 100 volts of 1kHz.
The second stage amplifies the 1 volt of 2kHz to give us 10 volts of 2kHz.
The second stage distorts the 1Khz and gives us 10 volts MORE of 2khz for a total of 20 volts 2kHz.
The second stage also distorts the 1 volt of 2kHz input signal, to give us 1 volt of 4kHz.
What have I missed?
Thanks
Edit. If you just say "their polarities are inverted with each other" then why wouldn't the fundamental cancel as well?
Yes, each stage's output is inverted in relationship to it's own input but that's not germane to the question.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/30/12 04/30/12 04/30/12 04/30/12
Good idea to go through the calculations on this assertion.
You wrote:
"Let's say we have two hypothetical gain stages. Each with a gain of 10 and having only 2nd order harmonic distortion. Let's say the percentage of 2nd order distortion is 10% (each stage).
If we apply 1 volt of 1kHz to the first stage we get 10 volts of 1kHz and 1 volt of 2khz feeding the second stage.
The second stage amplifier the 10 volts of 1kHz to give us 100 volts of 1kHz.
The second stage amplifies the 1 volt of 2kHz to give us 10 volts of 2kHz.
The second stage distorts the 1Khz and gives us 10 volts MORE of 2khz for a total of 20 volts 2kHz.
The second stage also distorts the 1 volt of 2kHz input signal, to give us 1 volt of 4kHz.
What have I missed?"
I will now edit all those lines to correct the errors, one by one:
"If we apply 1 volt of 1kHz to the first stage we get 10 volts of 1kHz (polarity inversion needs a minus sign) and 1 volt MAGNITUDE of 2khz feeding the second stage, phase not found due to not knowing the distortion transfer function completely. It could be 180 degrees, where it is likely. That means we might assume it is actually 1 volts of 2khz midband.
The second stage amplifier with 10 volts of 1kHz then gives us (1)(10)(10) = +100 volts of 1kHz. Note two polarity inversions for inverting common cathode stages.
The second stage amplifies and inverts the 1 volt of 2kHz to give us +10 volts of 2kHz.
The second stage distorts the 1Khz and gives us 10 volts MORE of 2khz for a total of a magnitude of (+10)+(10) = 0 volts 2kHz!"
It gets complicated quickly when the order is increased to calculate % distortion of a single even order harmonic. But you can see that it's doing it quite well to the 2nd order in terms of cancellation.
"(polarity inversion needs a minus sign) and 1 volt MAGNITUDE of 2khz feeding the second stage, phase not found due to not knowing the distortion transfer function completely. It could be 180 degrees, where it is likely. "So you are saying that, at the plate, the 2nd harmonic distortion product of a grounded cathode triode stage is out of phase with the the fundamental?
Why would that be?
BTW The voltage in question is AC. We should not use a  sign. The phase is inverted but the voltage is neither  or +, it's AC.
In the studio we use a circle with a diagonal line through it. Ø
From Wikipedia "In audio engineering, the ø is used to represent a signal whose polarity (sometimes called phase) has been reversed."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/05/12 05/05/12
In EE circuit analysis texts, there are two components to describe the sinusoidal signal: either use cartesian, or Rectangular on your calculator (Real and Imaginary) or Polar (Magnitude and Phase). They are convertible using a scientific calculator with R> P and P> R keys. EE's prefer to use "j" as the imaginary number term instead of "i" since "i" is already taken to be AC current.
Real is on the Xaxis and Imaginary is on the Yaxis, something that is graphically representative of how the math works in this realm of "phasors".
If a Magnitude is 1 + j1 in Rectangular, then the Polar Magnitude is the length of the diagonal showing, SQRT(1^2 + 1^2) = SQRT(2), and the phase is +45 degrees, with respect to the input reference. Positive phase is lagging, negative is leading.
Now for the example of +1 Vrms vs. 10 Vrms. First we know that the input to the first tube is 1 Vrms Magnitude and is the reference, 0 degrees. That is a vector (arrow) pointing from 0,0 to 1,0 in Rectangular. It's on the real line, in positive direction.
The output is best seen as a real Magnitude of 10 Vrms, but in the opposite direction, +/180 degrees lagging or leading (it's the same). In Rectangular form, that shows up as a vector pointing from 0,0 to 10,0. The magnitude is ABS(10) = 10, but the vectors show an important inversion using vector arithmetic, and is represented as a negative number. The entire problem is one of vector addition, not just magnitude addition, because we have phase changes.
Another example for kicks: If you sum +10, zero phase reference, and a partial inverted 1 Magnitude, Phase = +/180 degrees, you get a net 10  1 = 9 Magnitude, zero phase output.
NEXT QUESTION:
How are the harmonics handled in vector arithmetic, not going to the old simple 'magnitude only' addition?
This is helpful if you have a solid understanding of FFT's. But this time I opt to go by observation instead of polarity signs of +/Acos(wt)+/Bcos(2wt)...+/Ksin(wt)+/Lsin(2wt)... in an infinite series.
Instead of this equation, we can determine by inspection what "polarity" the B coefficient in Bcos(wt) is.
The transfer function of the 2nd harmonic is determined from a simple graph you can create from measurements of such a common cathode stage for Vpk v. Vgk (it's a different special plate curve). We can think of it like this: At about 15 VDC at Vgk, the tube cuts off and we get all the B+ to the output (high voltage) to say +250 VDC. At +1 VDC Vgk (slightly into class A2 in this example), the plate voltage will go down as the tube drains current away from B+. Let's say to +30 VDC.
And then let's say normal bias offers us +140 VDC on the plate. So the cutoff swings up to +250 VDC, or +110 VDC more than quiescent bias point, and small class A2 pull offers +30 VDC, or 110 VDC less than quiescent bias point. Note the bias is centered.
But plot the graph of the output. It will have that 10% 2nd order distortion of asymmetry that is the feature of 2nd order distortion. Now observe the orientation of where the bend is. Since the tube can work slightly into A2, it is still pretty linear over in that part. BUT! The plate curves show that remote cutoffs and all triodes with imperfect linearity towards cutoff really distort more over on this side when in the signal operating range. The upshot for this side of distortion is an elongation of the sinusoidal negative going curve. For odd order distortion, it's about each end with symmetry.
On the other side, it just hit hard clipping at the end of the power supply voltage. Let's ignore the part about overload.
So by observation, we have the polarity of the 2nd harmonic set to distort on the low voltage side every time when not clipping. But let's say the input was predistorted where the low voltage output would amplify that distorted end linearly, from a previous stage. What you can picture is that one signal is distorted on one side once, with one stage; but another signal is distorted by a previous stage on one side once, and then let go of linearly the second time.
What we have now is distortion on both ends, moving the distortion to a more odd order problem of distortion. If one end distorted: even order; if two ends distorted: odd order. That was with no math to see this happening.
But the math using vectors and polarity changed to plus and minus Rectangular coordinates offers a way to get a calculation. The harmonics are being played out in different mathematical phases through different polarity orientations.
That's the best I can do to try to explain this. I hope you got something from this.
"But plot the graph of the output. It will have that 10% 2nd order distortion of asymmetry that is the feature of 2nd order distortion. Now observe the orientation of where the bend is. Since the tube can work slightly into A2, it is still pretty linear over in that part. BUT! The plate curves show that remote cutoffs and all triodes with imperfect linearity towards cutoff really distort more over on this side when in the signal operating range. The upshot for this side of distortion is an elongation of the sinusoidal negative going curve. For odd order distortion, it's about each end with symmetry."
Should read:
But plot the graph of the output. It will have that 10% 2nd order distortion of asymmetry that is the feature of 2nd order distortion. Now observe the orientation of where the bend is. Since the tube can work slightly into A2, it is still pretty linear over in that part. BUT! The plate curves show that remote cutoffs and all triodes with imperfect linearity towards cutoff really distort more over on this side when in the signal operating range. The upshot for this side of distortion is a compression at the positive end of the output swing and an elongation of the sinusoidal negative going swing.
For odd order distortion, this will be symmetrical on each swing sides, which is what will happen in cascaded identical stages. Cancellation of evens plus creation of odds is happening here, I believe.
"But plot the graph of the output. It will have that 10% 2nd order distortion of asymmetry that is the feature of 2nd order distortion. Now observe the orientation of where the bend is. Since the tube can work slightly into A2, it is still pretty linear over in that part. BUT! The plate curves show that remote cutoffs and all triodes with imperfect linearity towards cutoff really distort more over on this side when in the signal operating range. The upshot for this side of distortion is a compression of the sinusoidal positive going curve AND and elongation. For odd order distortion, it's about each end with symmetry."
The fundamental passes with inversion on each stage, 10 being the gain of each stage.
The 2nd harmonic passes with DOUBLE INVERSION on each stage, thereby it is not inverted in effect through the stage generating that distortion. Let me explain. The first inversion is the polarity flip of the circuit stage itself, the next one comes from the polarities of what constitutes "positive 2nd HD" and "negative 2nd HD."
The triode common cathode circuit produces an inversion of a "negative 2nd HD." To illustrate this, a normal expected noninverted 2nd harmonic distortion product has the positive cosine peaks of the fundamental coinciding with the positive cosine peaks of the 2nd harmonic. An inversion in the triode stage's distortion curve in the transfer function causes the opposite inverted distortion within it: the positive cosine peaks coincide with an inverted 2nd harmonic signal where the peaks of the positive fundamentals coincide with the peaks of the most negative 2nd harmonic signal output.
When you start with the fundamental of the first stage, the fundamental proceeds obviously as: +1 V => 10 V => +100 V.
The harmonic generated from the 2 triodes is thus: 0 => (1)(0.1)(10)(10) = +10 and an addditional (10)(0.1)(10) just generated by the fundamental on the last triode stage alone = 10. Add the two and you get a complete cancellation.
It can be done by graphical calculations to show this to you better. The graph of Vpk vs. Vgk shows negative gain with the flat top of the bending curve for a straight line to appear flattened on the positive signal end and elongated at the negative going end. This one curve is mainly a 2nd order distortion.
Do this again a second time, doing fundamental gain and curvature of distortion from 2nd order, and this time the anode flattens the opposite polarity of the signal on the positive side. This is converting the 2nd to an odd order "square wave" type distortion. It takes away some harmonic richness for people who like it.
The effect can be taken to extreme with a strong signal generator and see it with a scope. Take a look at each stage. The input will be a nice sinewave, we hope. The input to the second stage will be a cutoff on the positive end of the scope (highest voltage) while an elongated sinewave feature will show on the negative going halfcycle. Finally backing off on amplitude, the output of the second stage will show cutoffs at a certain signal level input to have only one cutoff still, the final tube. Small signal distortion is less noticeable in normal use, but it counted to me.
The most problematic amp area with even order cancellation will probably be between driver and final output triodes. The signals start to get large and both these tubes can generate a great deal about the overall tone.
I run a two stage amp that shares a nondecoupled power supply. The power supply noise will cancel to some extent because it is shared bewteen the two stages (which are out of phase). The signal does not know or care what absolute phase it is in as it travels between stages so I agree that you should not be getting any type of cancellation.
I have always had a simplistic view of thermonic devices compared to their SS brothers. The valve amplifier simply provides Harmonic distortion closer to the root than other methods of amplification.
Triodes, off the shelf, are more linear and Pentodes can be tamed to provide a marvelous sound with a little feedback particularly if the feedback is limited to the output section.
SE and PP are a matter of taste. I use both because I like both.
No argument is needed. Everyone has their own ears. You can't convince someone of anything with measurements.
Stuben
Hi Stuben,
I saw the photo of one of your 50 monos. I was wondering if you tell a little about them and if you have a photo you could post of both of them in action. Thanks. BTW they look great my friend !
Thanks
These amps started as 45s. Because my speakers are not quite efficient enough to really let he magic out, I found some 50s to play with. I was starting up a foods plant and had some over time money in my pocket at the time...they were a bit pricey...
All of the power iron is Hammond while the audio transformers are all Lundahl. Kevin at K&K had a couple of rev A LL1692a Interstage trannys that he let me have cheaper because they had a weird pin out.The output iron is a couple of LL1663 gaped for 50 mA. One mono is AC heated and one is DC heated. I did this so I could compare the sound while I tweeked the DC supply...worked well.I'm running the 50s gently at around 50mA and 380 VDC on the plates. I'm driving the ITs with a single 5842/417a..amazing little tube. The 50s really have a special sound..glorious mids, good thump and sweet high end.I have used these puppies as a reference for my other projects. (other amps in the picture)...
Stuben
What rectifier are you using ? Thanks.
Mullard GZ34s...
I worked with many but the Mullard variants are the best.
When you're working with such a simple circuit, the rectifier has a serious influence on the sound...you can definitely hear the rectifier.
Stuben
Thanks for the photos and the scoop on them. Wonderful system ! The amps are beautiful and I am sure you must be pleased. I noticed the EML put out a 50 not long ago but PRICEY ! I wish I was more of a diy guy so maybe I will have someone build me a pair down the road. Thanks again for the photo and info my friend.
Hey 1954,
My non work email address is michelestew@roadrunner.com.
Stuben
Thanks Stuben. Mighty nice of you. Would be real nice to have a pair. At the moment I am running a type 46 tube amp which is incredible.
BTW what rectifier tube are you using ?
I tried a few and Ultimately the Mullard GZ34 was my choice. I run this rectifier in all of my amps. Expensive, but with simple designs, the rectifier has a real influence on the presentation.
Stuben
Nice amps !!
You just gotta love the sound of 50's, puts a 300B to shame...
IMHO !
Have fun!
Willie
Thanks...We had a DIY get together at my place a few years ago and I think it was Larry Moore who said " A 50 is what a 300B wants to be" :> )
They are hard to come by... I have (2) spare globe 50s. I have heard that Shuguang makes a new stock 50, I haven't seen any around though.
Cheers
Stuben
Cool shot. I just put up this one on my blog and still love the two below by F stop schroder.
and finally a picture of a dead soldier properly laid to rest.
Recently Released EML 50. See the photo above and link below. Their 45, 2A3 and 300B are excellent, so I would imagine the 50 is too.
Gerry
Gerry,
Thanks..very cool..more flexibility in the future. I had considered installing some different OPT and filament trannys and changing the amps to 2a3s because of the 50s becoming silly rare...
Stuben
Stuben,
Could you give more details on your amp? A schematic or a link to a site would be nice. I have a few globe 50's I would like to try.
Regards,
David
Sidewinder,
All of my schematics are in Visio. I will pdf and get you a copy.
Stuben
Hello world
Sidewinder,
I've tweeked this schematic several times. The regulator on C+ especially.
The LL1692 wire with all of the primaries in series and the secondaries in series.
Have fun
Stuben
If it sounds good it is good. Little else matters.
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