Peter wrote:
"(why only half? - the idea of the cascode is that the low load impedance on
the first stage minimises the voltage swing and thus the Miller effect. So a
lower impedance with the two halves of the 6J6 in parallel would seem to be
indicated)"
This was the subject of considerable discussion in the VHF community in the
50's.
It appears that the outcome was that the increase of interelectrode
capacitances by paralleling two sections offset the higher gm of using two 6J6
sections.
I have a 6AK5/6J6 cascode 2m converter (SM5VL design) hidden somewhere in the
more ancient layers of the "junk-box". It may be interesting to evaluate this
converter using modern instrumentation and methods and verify the difference
between one or two sections.
73/
Karl-Arne
SM0AOM
----- Original Message -----
From: "Peter Chadwick" <g3rzp@g3rzp.wanadoo.co.uk>
To: <craxd1@verizon.net>; <amps@contesting.com>
Sent: Tuesday, July 25, 2006 11:41 AM
Subject: Re: [Amps] "Tubes 201" - How Vacuum Tubes Really Work
> Two things make grids emit. One is heat. You try to minimise that by flashing
> of gold etc, although if you boil that off, it's not there, so you're left
> with the basic material. Unfortunately, in oxide coated cathodes, barium and
> strontium can boil off the cathode, and land on the grid. Then you have a
> nice low work function emitter. The other is secondary emission, and for
> that, a carbon coating is useful. (all this from care and feeding and the RCA
> tube manual)The first tube that I know of with a gold flashed grid was the
> 6J4, gg recieving triode of around 1943/44. That has a gold flashed grid
> because the grid - cathode spacing was pulled down, and gold flashing helped.
> Often used in Wallman cascode with a triode connected 6AK5, although a lot of
> people used 1/2 a 6J6 for the gg stage (why only half? - the idea of the
> cascode is that the low load impedance on the first stage minimises the
> voltage swing and thus the Miller effect. So a lower impedance with the two
> halves
o
> f the 6J6 in parallel would seem to be indicated) However, the gold flashing
> wasn't considered necessary for the frame grid pentodes such as the
> EF183/6EH7 and EF184/6EJ7, and they ahd very close grid cathode spacings,
> giving a very high gm.
>
> 73
> Peter G3RZP
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