1 Mar
2010
1 Mar
'10
9:13 p.m.
On Monday 01 March 2010, Folderol wrote:
On Sat, 27 Feb 2010 22:50:02 -0500
Gene Heskett <gene.heskett(a)verizon.net> wrote:
The 6BN6 was what was called a gated beam tube, with the #1 grid being driven
by the somewhat am limited carrier (4.5, 6 or 10.7mhz, I believe it worked
well at all those freqs) Half a volt to a volt of signal IIRC although its
now been 50 years since I probed the circuit with a scope. The 3rd grid,
after the screen grid, was also quite closely spaced so that it had decent
gain also, and a tuned circuit with a Q selected according to the expected
bandwidth was tied from this grid to ground. Space charges drove this grid
by the coupling effect of the electron beam going by, and natural resonance
effects saw to it that its voltage lagged (or led, its been years) the driven
grids voltage by about 90 degrees electrically. The end result was a pulse
duration modulation of the plate current because the tuned circuit lagged the
input signal by an amount dependent on the instantaneous frequency.
Actual plate current in this tube was only 1 to 1.5 milliamps, so there
wasn't really a large amount of power being handled. Screen (grid #2)
current was maybe 3 milliamps, so the developed audio voltage wasn't too
large and they usually had another medium gain small triode to bring it up to
a level suitable for driving a 6V6 or similar output stage that gave about 2
watts of reasonably clean audio to drive the speaker(s).
In solid state, RCA made a CA3089E for several decades that worked on a
similar principle, but mopped the floor with a 6BN6 because of its superior
performance.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
The future is a race between education and catastrophe.
-- H.G. Wells
On Saturday 27 February 2010, Folderol wrote:
< snip .. very interesting stuff actually :) >
That must have been in the USA I guess, over here the intercarrier is
6MHz
Us young 'uns laughed on two levels.
The first was that most of us didn't have a clue how the things worked,
due to pathetic instruction from our employers (I'm looking at you
Radio Rentals). When asked what, exactly, quadrature demodulation was,
there was a pregnant pause after which the instructor said 'Well it's
demod in quadrature of course' then swiftly moved on to the next topic.
Our next cause of merriment was the horrendous EH90 heptode (yes I did
say heptode) implementation. See, it scarred as all so badly I can
still remember the precise details!
The screen feed was via an 18k - 5k6 potential divider, only there was
something very strange about the composition resistors used for this.
In the first place they were under-rated (especially the 18k. If you did
the math you could prove it. Secondly, where resistors normally go
high if they overheat these went low, so a nice little thermal runaway
ensued. This actually set fire to the (paxolin) PCB, burning a hole
through it. If you were very unlucky it also cooked up a wirewound
resistor further along the board, which then de-soldered itself, arced
and burned another hole in the board.
In those days we were expected to fix these things on site, so after
scraping out all the carbon, you were left with point-to-point wiring
across the gaping holes, supporting the replacement components as best
you could.
Such fun we had!
On Sun, 28 Feb 2010 00:50:07 +0000
Gordon JC Pearce <gordonjcp(a)gjcp.net> wrote:
Why were you laughing? Zenith did this, with a self excited circuit,
using a type 6BN6 gated beam tube to recover the audio directly from the
4.5 mhz inter carrier frequency, starting in the fall of '51 with the
intro of the '52 model year. It worked fairly well too. Stable, not
sensitive to the fine tuning setting, so folks out in the fringes could
tune for a slightly better if not as sharp a pix. On Sun, 2010-02-28 at 01:41 +0100,
fons(a)kokkinizita.net wrote:
> On Sun, Feb 28, 2010 at 12:40:00AM +0000, Gordon JC Pearce wrote:
> > I wanted a very simple SDR with jack inputs and outputs for a
> > demonstration I was doing. I had a look at the DSP guts of dttsp
> > and quisk, and sat down to code.
>
> Forgive my ignorance, but what is an SDR ?
Software-Defined Radio. Basically you downmix incoming RF to the
audio range with two mixers fed 90 degrees out of phase. You can then
mung this in various different ways to tune and demodulate various
different radio signals.
Gordon MM0YEQ
Sounds suspiciously like some form of quadrature demodulator. Rather
like GEC/Sobel introduced with their 1018 TV chassis in the 1960s.
Oh how we laughed ...