From: "James Moritz" <J.R.Moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Fri, 12 Sep 2003 15:30:06 +0100
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Subject: LF: RE: Inverted tube amplifier
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-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Stewart Bryant
Sent: 12 September 2003 09:20
To: rsgb_lf_group@blacksheep.org
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.

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 - 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. 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.

Cheers, Jim Moritz
73 de M0BMU