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Old 28th April 2005, 01:30 PM   #11
CV is offline CV
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Yo Dave,
Here's my take - connecting 2 electrode systems together forces the elements to be at the same potential, which gives you one composite device. No blurring.

Where this model breaks down is that there is inductance/capacitance associated with those connections, which enables 1 device to fight the other til they eventually settle into steady state [do we ever get there with music?].

In a single envelope, those parasitics are minimised. 2 separate tubes, more lead length.

The way to test this theory is to take a dual triode and parallel the halves up with short and very long leads? Or even just insert an RF choke.

Just my theory anyway
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Old 3rd May 2005, 09:00 AM   #12
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Hi all
Quote:
Is it the complexity of getting two tubes to operate at
the same point (matching), or is there more to it than that
There is more, Casey. And there is a precise physical reason.

Quote:
Whether you have multiple tubes in parallel in a single envelope,
or in multiple envelops, you still end up with a single set of
average chatacteristics,
It is not so. Paralleling tubes does not give us tubes with averaged
characteristics but a different animal. I'll try to explain why.

Suppose to have two tubes (triodes) which perfectly adhere to the Child's law
but with different gain, for instance.

Now, immagine the tubes being completely paralleled. Suppose one tube
has a gain of 3 and the other a gain of 4, for example, and suppose
that the currents of the two triodes are the same for vg= 0 Volt.
Then, for a given value of grid voltage, say -20 Volt, the tubes
will have different currents and, if we change the anode to cathode
voltage, different anodic curves.

If we make an orizzontal average of the curves we end with a
curve with average characteristic, that is average gain in
this case.
But the action of paralleling tubes make a vertical average, since
the total current is the sum of the current of each tube.

so we end with a curve with the following equation

i=k[V+m1*vg]^1.5+ k[V+m2*vg]^1.5

and this is not the Child law.
But..., maybe it is a better one..not so.
It will result clear from the following graph.
You can see the curve corresponding to
m=3 and m=4 ( in blue)
the orizzontal average (green) that has m=3.5
and the the vertical average (red)
which appears very distorted in the small current region.
in this region, in fact, the mean is done betweeen only
one curve and the zero line (because one tube is OFF).

So, paralleled tubes have anodic curves different from
single tube. In particular, the curves result more
distorted in the region of small currents.
Note that it is a geometrical effect, it is not important
that the law is the Child's one. Only in case of
a linear device the orizzontal and vertical average will coincide,
and only at high currents.

I don't know if it is the cause of sound difference nor if this is a good explanation. surely, it is a possible explanation.

bye
Federico
Attached Images
File Type: gif parall.gif (2.6 KB, 349 views)
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Old 3rd May 2005, 10:22 AM   #13
dhaen is offline dhaen  Europe
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Frederico,

Your's is the first explanation that makes sense to me.

Thank you
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Old 3rd May 2005, 03:13 PM   #14
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Quote:
Originally posted by CV
Y
In a single envelope, those parasitics are minimised. 2 separate tubes, more lead length.
seems like a good distinction between the two.

Quote:
Originally posted by fscarpa58
[B]

It is not so. Paralleling tubes does not give us tubes with averaged
characteristics but a different animal. I'll try to explain why.
I agree with your assessment, but think you miss the idea that this same thing happens with in a single tube also.

Lets stick with the planar model of a tube and consider the Single plate 2A3. I chose this tube because the harp shaped filament most closely resembles a planar emitting surface. This gives us a single plane cathode with two individual grid and plate planes on either side which can be considered two tubes in parallel. Even if we assume the elements are parallel, its a pretty fair guess that the distances will have some difference between them which will translate to different gains for each plate-grid structure.

I fail to see how this conceptually is any different than two discrete tubes in parallel and if on the surface the concept is not consistent, then any conclusions drawn from the concept seem suspect to me.

If we look at the "single" tube construction and consider what happens under a number of situations the "large number of tubes in parallel" view becomes more clear.

Lets stick with the single plate 2A3 and see what happens if we remove one half of the plate structure. Since the gain is determined by the spacing and all we did was cut the plate area in half, the gain will remain unchanged. The reduction of the plate area does change the Gm (cuts it in half) and if we hold the mu constant and halve the Gm we get double the Rp. This remains consistent with what one would expect with the parallel connection of tubes.

If we keep the parallel plane model with the same grid and plate structure and spacings and switch from a harp filament to a single V filament, we end up with a tube with the same gain, but a smaller area of the plate is used so Gm will go down. Everyone seems ot think that the harp filament of the 2A3 was done to attempt to mimmic the planar cathode surface, but I wonder if it was done to make full use of the plate area allowing the 700 ohm Rp from a tube that would have a 1500 ohm Rp with a VV filament.

If we move to a tube like a 45 with a VV filament and pretend one of the V's does not light again we get a different tube. Since the Gm is related to the area of the plate and its distance from the cathode, when you remove 1/2 the cathode you essentially remove the plate area adjacent to it from the picture and again end up with 1/2 the Gm. Now imagine what happens if we take this tube with 1/2 of the filament lit and remove the plate adjacent to it leaving the plate by the unused filament connected. The plate area remains the same, but the distance from cathode to plate has increased so again the Gm goes down AND the cathode-grid-plate spacings have also changed so the mu will be different.

I have a PDF of a 40 page article by Kusunose on triode design, and while the math gets a bit deep, when looking at it it quickly becomes clear that the design of a triode can be looked at as the averaging of a number of different triodes in parallel. To then enclose all of those averages in a single envelope and assume what is inside is 100% the mean rather than the average of a bunch of different dimensions (tubes) is conceptually flawed imo.

If anybody wants to look at the kusunose, I'll put it up. (its an 8 meg pdf)

dave
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Old 3rd May 2005, 03:24 PM   #15
dhaen is offline dhaen  Europe
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Dave said:
Quote:
I agree with your assessment, but think you miss the idea that this same thing happens with in a single tube also.
That was my immediate thought too. Then I realised that those single tubes produced where the linearity was bad at the bottom end, would have been rejected at test.
How does that stand up?
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Old 4th May 2005, 12:04 AM   #16
jlsem is offline jlsem  United States
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Quote:
Everyone seems ot think that the harp filament of the 2A3 was done to attempt to mimmic the planar cathode surface, but I wonder if it was done to make full use of the plate area allowing the 700 ohm Rp from a tube that would have a 1500 ohm Rp with a VV filament.
It may have also been an (successful) attempt at reducing distortion. In directly-heated triodes, distortion decreases as the diameter of the filament wire decreases. A row of many very thin filaments results in lower distortion while supplying copious emission necessary for a power tube.

John
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Old 4th May 2005, 10:43 AM   #17
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Hi Dave, All
Quote:
I agree with your assessment, but think you miss the idea that this same thing happens with in a single tube also.
...
Even if we assume the elements are parallel, its a pretty
fair guess that the distances will have some difference between
them which will translate to different gains for
each plate-grid structure.
You are completely right, Dave. And no, I do not miss
that idea. I left it for this post...

Have you never asked to yourself why, in the small current
region, the anodic curves of the triode diverge from the
child law? The cause is, for the main part, the
phenomenon described by you.

Many are the reasons, for the most technological, that produce
differences between predicted and actual behavior of a triode:
the not uniform distribution of temperature of the cathode, chiefly in
direct heated tubes, the less than perfectly uniform step of the grid
spiral, in signal triodes, and in general, all those differences
between ideal and real geometry that cause a one-dimensional
idealized structure, like the Child’s one, to diverge from the complex
three-dimensional reality of a triode.

Now, think at the electronic flow (or the current) existing
between anode and cathode as the sum of elementary flows
standing in zones characterized by slightly different values of
the properties (say gain, for instance). The total current will
result from the integral of these contributions.

The effect is of the same kind, but more smoothed, of that
described in my previous post (one can see it in the attached
picture).

The models proposed during the years by some researchers
represent an attempt to mathematically describe the phenomenon.

Unfortunately they (Koren's one, etc.) are not based on physical considerations so often resulting in a great number of parameters.

Bye
Federico


Fitting of Child’s low (6SN7 curves from
http://www.audiomatica.com/tubes/6sn7.htm )
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Old 4th May 2005, 03:36 PM   #18
protos is offline protos  Greece
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If parallel tubes ``smeared`` sound then all those otl amps would sound horrible.
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Old 4th May 2005, 04:53 PM   #19
Radames is offline Radames  United States
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Default So trace curves in parallel...

I have been following this post with attention to try to understand a bit more about PSE vs SE.

If Federico is right, the decrement in sound performance of PSE vs similarly designed SE versions using the same tube is a matter of erroneous calculation of the parameters for operating thre tubes.

Following the same rationale, using a curve tracer to obtain the combined behavior of paralleled tubes, will produce a different Vp vs Ip graph for every Vg. Using that graph to obtain the operating conditions of the paralleled tubes instead of the graph from the datasheet (for one tube), would allow a better estimation of the circuit behavior.

Am I right? Do those expensive commercial PSE amps are designed in such fasion?

Cheers,

Rada.
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Old 4th May 2005, 05:36 PM   #20
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I have seen this explanation posted before, can't recall where or by whom, but sounds good to me.

For a single tube, the design is sufficiently "coherent" ie. same geometry thru-out, that the sound is OK. For a small number of tubes in parallel, the chances are that manufacturing alignment differences between the tubes will produce a distorted summation (the different Mu effect, etc), but for a large # of tubes in parallel, the variations average out unless one has a truly "bad" tube in the lot. So on this hypothesis, one should be able to parallel a small number of tubes successfully if one takes considerable care in matching their characteristics on a curve tracer.

Even the difference between well liked tube types and poor sounding tube types may be explainable by similar reasoning. Good sounding tubes tend to have a nice flat Mu curve versus plate current and plate voltage. Tubes with badly curving Mu curves are composed of non consistent geometry sections acting like different tube types mixed together.

I think to get a nice flat Mu curve, the geometry needs to be such that the grid falls on an equipotential between the plate and cathode. The grid wires need to be sufficiently close together to not cause shadows or "inselbildung" or "island effect", on the cathode. The cathode also needs to be of constant geometry/ temperature etc.

Hi Dave,
Is that "kusunose" article on your web site? I would like to read it. There are also some tube design articles here:
http://frank.pocnet.net/other/RCA/VTD_RCA/index.html
http://frank.pocnet.net/other/RCA/ETD62/index.html

Don
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