Dave,
Many receivers did just that.
I don't remember if the NC300/303 used the rf amp on the low bands
or not.
The NC300/303 was one of the best AM receivers I ever had.
I think it needed better audio stages, but I always used a detector
output.
Frequency resolution and stability were very good, bandwidth choices
were good, although I don't remember how sharp the filters were.
The looks of the thing were quite Art Deco, and did not turn
everyone on, but they are very good receivers in my book.
Hard to mount one in a rack though, with the rounded corners.
I am not sure how much the RF amp adds to the noise level.
A well designed rf amp section can actually reduce noise levels.
Noise mostly comes from mixers, and overall tube counts.
All mixers add some noise, some designs are much better than others,
and the more there are, the more noise you get.
I used single conversion, with a quiet mixer setup, and
used two tuned circuits of very high Q in the input,
along with resonant dipole antennas for 80 and 40 meters,
so I don't get any images or other problems, as signals
out of band are attenuated very much before making it to
the mixer. This is not always the case in the general receiver
setup, as many bands are covered, low Q broad band coils are used,
and who knows what antenna will be used.
As far as the NC303/300 goes, if you can get past the style,
the only big improvement to be done would be the audio output
(if used), and maybe the addition of a KIWI filter module.
These are quite like mechanical filters, very sharp.
One of those would be very easy to add, without hacking
up the receiver.
Brett
N2DTS
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED] Behalf Of David Knepper
> Sent: Thursday, October 16, 2003 4:31 AM
> To: amradio@mailman.qth.net
> Subject: Re: [AMRadio] FW: Homebrew receiver
>
>
> Brett and others, I wonder if we could somewhat emulate the
> same results as
> you had by eliminating the RF stage from let us say a NC-300
> receiver and go
> directly into the mixer stage.
>
> I am sure that this has been tried before for operation on 160 and 80
> meters.
>
> Just a thought that is not so original.
>
> Thank you.
>
> Dave, W3ST
> Secretary to the Collins Radio Association
> Publisher of the Collins Journal
> www.collinsra.com
> ----- Original Message -----
> From: Brett Gazdzinski <[EMAIL PROTECTED]>
> To: <amradio@mailman.qth.net>
> Sent: Wednesday, October 15, 2003 8:46 PM
> Subject: [AMRadio] FW: Homebrew receiver
>
>
> > This is what I plan on sending to Electric Radio, along with
> > pictures.
> > What do you guys think?
> >
> > Brett
> > N2DTS
> >
> >
> >
> > I wanted a complete home brew station, and since I have
> > a homebrew pair of 813,s, modulated by one of two modulator
> > decks, push pull parallel 100TH,s, or a pair of 4sc250b,s,
> > and a classic push pull rig with link coupling, using 812,s
> > modulated by
> > a pair of 811,s, only a receiver was needed.
> > At first, I thought I would build something simple that worked
> > just well enough to be able to copy AM under good conditions, just
> > so I could say I had a home brew station.
> > But I wanted something a little better than the regen receiver
> > type of radio, maybe a simple superhetrodyne.
> > I did loads of research, looked in Bill Orr, and all my old
> > ARRL handbooks, looking for simple receivers.
> > All the circuits had some sort of problem, complex tapped coils,
> > hard to get parts, poor designs, etc.
> > I also looked at the diagrams for things like my Gonset
> G76, the Scott
> > model SLRM I have, the Hallicrafters sx17, and the R390.
> >
> > I decided to base the receiver on the Scott SLRM, since it works
> > very well, has good fidelity, uses 8 pin tubes and a 455Khz IF.
> > I ran into problems though, as the Scott was built to
> reduce emissions
> > out the antenna, with loads of shielding and an rf amp with
> > tuned circuits.
> >
> > I accumulated parts, and started construction with the basic layout
> > of two tuned circuits on the antenna input, an RF amp, a separate
> > local oscillator and mixer, two stages of IF amplification, hifi
> > detector, s meter circuit, agc circuit, and power supply.
> >
> > Since it was to be experimental, I used octal sockets for
> everything,
> > the antenna coils, the local oscillator coils, and the IF
> > transformers.
> > The receiver started out with plug in coils to change bands.
> >
> > I laid out all the parts, leaving room between things to allow
> > room for experimentation, and mounted the basic parts.
> >
> > I tried various circuits for the local oscillator, using coils
> > wound on ceramic forms, B+W coil stock, and slug tuned ceramic
> > coil forms.
> > This step would have been very difficult without the aid of a
> > very nice spectrum analyzer I have through work. It allowed me
> > to look at the frequency output, harmonics, hash, drift,
> > frequency range, amplitude, all at the same time.
> >
> > At first, I went with plug in coils in the local oscillator,
> > used the rf amp, using the spectrum analyzer to peak things
> > and check gain. The mixer was easy, then to a filter.
> > I planed on using a mechanical filter, but they are
> > expensive, and a little tricky to put in the circuit.
> > I found a company on the web, kiwi, who makes various filters, and
> > went with one that has an op amp input, three filters of slightly
> > different center frequencies (sets bandwidth) and an op amp output,
> > and runs off 10 to 30 volts dc.
> > There is no loss through the filter, and its quite similar
> in results
> > to a mechanical filter. I used a 5.5kc model.
> > It mounts on Velcro, and has pig tail shielded wires to hook up
> > to the IF system.
> > This filter is easy to add to any receiver using 455 KHz as an IF,
> > and really works fantastic.
> >
> > I copied the IF system out of the Scott, and used a hifi detector
> > on one of the AM web pages.
> > It took some experimentation to get the agc takeoff and IF gain
> > control systems working well, then I added the S meter
> circuit I stole
> > out of the Bill Orr handbook using a 6SN7.
> >
> > Taking the receiver for a test drive revealed problems.
> > Startup drift was excessive, muting the receiver seemed impossible,
> > the RF amp caused all sorts of problems, and the if amps were
> > unstable.
> >
> > As a test, I hooked the antenna up to the mixer input, and bypassed
> > the rf amp, and had very good results, so I removed the rf amp
> > completely, and went with two tuned circuits then into the mixer.
> > Some experimentation with the antenna link on the input
> coil boosted
> > gain quite a bit.
> > I ordered a selection of NPO caps, and did weeks of experimentation
> > on the local oscillator stability, changing components,
> > design, putting the coil in a metal plug in can to shield it, and
> > got the stability much better, but still have startup drift for
> > the first 5 minutes.
> >
> > Careful shielding and reducing the gain of the IF
> eliminated the odd
> > oscillations I got at times, and the receiver was working
> quite well.
> >
> > I did not like the tuning dials I had, marking the
> frequency was hard
> > with the drift, and I have a real problem marking the frequency
> > so it looks nice on the dial.
> > I needed something better, and found the almost all digital
> > electronics digital frequency readouts, basically a
> frequency counter
> > with a selectable frequency offset.
> > You program the thing to offset the IF frequency, in my
> case, 455Khz
> > lower, and all you need to do is get the pickup close to the
> > local oscillator tube, and the display reads the exact
> > receive frequency down to 1000 Hz.
> > I used their backlit display, which looks nice, and a real accurate
> > frequency readout is very nice to have.
> >
> > The performance of the receiver was astounding!
> > With the transmitting antenna used, sensitivity was very
> > good, fidelity was great, I use a marantz amp on all the receivers
> > in the shack, to a big three way speaker, and the homebrew sounds
> > the best, because of the low distortion fi fi detector I guess.
> > The biggest surprise is the noise level.
> > Since the tube count is low, and the mixer design is a quiet one,
> > the receiver is incredibly quiet.
> > Its MUCH quieter than anything else I have, or have ever had.
> > Forget the modern rigs, the IC chips just can not run quiet, and
> > there are so many of them in modern rigs that the noise
> and distortion
> > in any modern rig I ever used is way high.
> > Comparison to my very well working r390a was dramatic, I
> could CLEARLY
> > hear signals that were well under the hash level of the r390a,
> > the signals were unreadable on the R390a, but very good
> > comfortable copy on the homebrew receiver.
> >
> > After the results I got out of the homebrew, the plan changed from
> > something I could use sometimes, just to have a complete home
> > brew station, to the receiver of choice.
> > This caused problems.
> > I had the receiver mounted in a rack cabinet, and had to run around
> > back to change the plug in coils, a real pain in the butt
> over time.
> > So out came the receiver, and a new front panel and band switching
> > was added, along with 160 meters.
> > Tuning was changed to a system using TWO back to back vernier
> > drives, the tuning range was changed to cover only part of the
> > ham bands, giving very nice slow tuning range.
> > A bfo was needed for zero beating AM signals, so I found and built
> > a 455Khz crystal oscillator circuit, with a variable output
> > level by way of a pot in the screen voltage.
> > The level control is on the front panel.
> > The bfo also allows me to copy cw and ssb quite well, without
> > a product detector, so I can listen to the ssb guys complain about
> > AM.
> > The receiver moved into a cabinet on the operating desk, and
> > was integrated in the shack with muting and so on, and is
> > the main receiver now, the others are almost never used...
> > The only problem the receiver has, and it does not bother me, is
> > the startup drift. From a cold start, it drifts about 1kc
> > over about 5 minutes, then is rock stable.
> > This might be due to the choice of octal tubes, the actual tube
> > used effects the drift quite a bit.
> > Experimentation with npo caps can reduce the drift, but it starts
> > drifting the other way over longer periods of time, and I think its
> > better to have 5 minutes of drift and stop, rather than drift less
> > but over longer periods of time.
> >
> >
> >
> > I was quite surprised about how easy it was to build, and how much
> > raw fun it was to design the thing, and do all the testing
> > and development.
> > You sure do learn a lot when you build something step by
> step, without
> > any overall design to start with.
> > Every system must be analyzed, built, tested, changed, other
> > things tried, etc.
> >
> > The end result looks a little rough inside, as it was
> > changed quite a bit, deleting the RF amp, adding band switching,
> > etc, but it still looks ok.
> > Its been totally reliable and stable for about a year now, with
> > quite a lot of use.
> >
> > My next project is a superhet receiver using 7 and 9 pin
> tubes, using
> > things I learned from the first one:
> >
> > Start off with band switching,
> > Forget the RF amp, its not needed on the low bands at all,
> > Do NOT leave a lot of space between things, but put the
> tube sockets
> > and IF cans close together as possible, along with the local
> > oscillator parts and band switch.
> >
> > It will also include two filters, 4.5Kc, and 5.5Kc.
> > The 5.5 was a great overall choice, but a 4.5 will help
> > when things get crowded on the bands.
> >
> > Building a good receiver for AM reception is not as hard as most
> > people think, and I encourage people to give it a try.
> >
> > I have no formal electronics background, all I know I got out
> > of books and by playing around, so if I can do it, almost
> > anyone can.
> >
> > Parts are not a limitation, although it may take some time
> to assemble
> > all you need at a reasonable cost.
> > Things like IF cans can be got out of old tube radios, old table
> > top AM radios are a good source of parts, as well as mouser
> > electronics, Antique electronic supply, ham fests, even
> radio shack.
> >
> >
> > You may find you can build something better than anything you can
> > buy for almost any price, as YOU pick what is important,
> > I only wanted part of 80 and 40 meter coverage, low noise, and hi
> > fidelity, along with reasonable frequency resolution.
> >
> > Old tube receivers like the Scott SLRM, SX17, SX28 can be quiet and
> > hi fidelity, but lack frequency resolution, good filters, and cover
> > more bands than I need.
> > Newer tube receivers like the R390 series, the Collins 75a series,
> > the National nc300/303, Drake and others have some good points,
> > but lack fidelity, bandwidth choices, look ugly as stink,
> or have some
> > other drawback.
> > All new ham equipment seems to be very high in noise and
> distortion,
> > and you may THINK some of that stuff sounds good, until you
> > compare it to a good AM signal through an old tube hi fidelity
> > receiver like the Scott SLRM or the SX17.
> > Even with output from the detector into a good hi fidelity
> > amp and speaker, there is no comparison between the new and
> > old stuff on AM.
> > I integrate all the receivers into the Marantz amp, and I can
> > jump between various receivers quickly, all tuned to the same
> > signal, and the difference is dramatic.
> > I have tested many, Kenwood ts440, icom 735, Kennwood r1000,
> > IC 756pro, and others, and they are all poor receivers for AM
> > if you want fidelity.
> >
> > Brett
> > N2DTS
> >
> >
> >
> >
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>
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