Hi all

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.

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