Jim

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James Moritz wrote:

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Stewart Bryant
Sent: 12 September 2003 09:20
To: [email protected]
Subject: LF: Inverted tube amplifier


I came across an interesting concept yesterday that I thought
worthy of further consideration for an LF active antenna. The
so called inverted vacuum tube amplifier...

Dear Stewart, LF Group,

From what I could figure out over lunch - As well as reducing the value

of mu, the value of transconductance gm will also be reduced by a factor
equal to the "normal" mu of the valve. For a preamp, this has serious
consequences for the noise. The noise voltage referred to the input of a
triode is proportional to the square root of the "effective noise
resistance"; as I recall, the ENR of a triode is approximated by about
2/gm. Therefore, low gm will result in high noise, as well as low gain -
or if you prefer, the same noise but less signal at the output compared
to normal operation.

I have no (recent) experience with valves, but with the amplifiers
we use at the moment we are not device noise limited at LF, so I
am not sure how much of a problem the increased excess noise would be.

The linearity is good because a given signal input only drives the valve
over a small part of the characteristic curve due to the large bias

voltage and small gain -

Actually I think that it is a bit more subtle than that. The collegue that
pointed this out has spent a long time studying the behaviour of valves
and claims that the physics causes the transfer characteristic to be more
linear, rather than simply just relying on a smaller part of the
transfer curve.

but this leaves you back at square one, because
you then need to follow this input stage with enough gain to bring the
signal level up sufficiently to drive the receiver, while maintaining

linearity.

But once you have done the impeadance transformation you can then put
in a filter and reduce the linearity problem to more managable proportions.

If you could make an amplifier which would do this
satisfactorily, you could just replace the "inverted tube" stage with an
attenuator, with the advantage of lower noise, and even better
linearity. The low gain and noise could be overcome by increasing the
size of the antenna, but the increased input would then increase the
distortion products again, cancelling the linearity advantage.

It is certainly an interesting circuit, if for no other reason than it
enables useful output from the valve with extremely low supply voltage,
thus allowing a low-impedance load to be driven relatively efficiently,
as in the audio amp at the URL you gave - but I think the low gain
negates any advantage as a preamp.

The thing that I find interesting is that it was invented by Fred Terman
who was the leading light in his field, but was never used for any
application. I think that it would be instructive to read his paper.
One reason to cast this aside in the past would be the difficulty of
getting power out of the grid structure, but we don't need any real
power, we just need to do the impedance transform into a selective
small signal stage.

73

Stewart G3YSX