The AM Forum

I know there are proponents/fanatics of each, but I am looking for for info/impressions.  In the absence of real money sometimes all I can do is read, and lately I have been doing a bit of reading about the Drake R-7A.  The Drake gets a lot of love from quite a few (ignoring the drift issues), but some say it's just intollerable for AM.  I always end up looking where a receiver I am reading about ends up on the Sherwood Engineering Receiver Test Table (http://www.sherweng.com/table.html, granted, it is sorted by close in dynamic range, but it also lists other interesting info like sensativity and MDS), where it seems to rank favorably.  But what surprised me was that another receiver I saw on that table that does not get such a favorable ranking, the Racal 6790/GM (I assume, sans pre-amp.  I know there are a number of people using the Racal for AM duty, so it is at least working well for them.

Perhaps this is a naive question, but I was wondering, how is it possible that the 'mil-spec' Racal 6790/GM can rank so far below the Drake R-7A in so many ways (spec-wise)?

Drop the Drake from six feet, put it in the trunk of your car over winter and hook the power up backwards a few times....  the Racal will probably laugh at yew....  klc

What specs?

Preamps are a definite liability on HF.  They decrease needed dynamic range while providing absolutely no benefit, except perhaps on 10 meters.  The atmospheric noise level on most of the HF spectrum is MUCH greater than the noise figure of the Racal.  It's best to get by with as little gain as possible before the first mixer.

The R-7 has a fairly noisy synthesizer as well, although the VFO is nice and quiet.  I've never had one but a close friend did.  It did OK, wasn't great in the audio department.  I'm not a huge fan of the 6790/GM either - while it's definitely in the top 5% of receivers ever made, it too has a somewhat noisy synthesizer.

In my experiece, the best professional receivers by far ar the W/J 8716 or 8718, and the Harris RF-590.  I think Frank GFZ might chime in with his RA6830s though!

73 John

Well John the 6830 is a different synthesizer than the 6790. I ran the 6790 factory noise test on the 6830 and it passed by 30 dB so the 6790 must be pretty noisy.
I like the Racal AM detector a lot. But it has to be set up correctly.
Dallas Lankford made a 6790 pretty clean after some TLC.

Many times guys go out and buy an RX with problems and end up calling it a bad design. Most times it just needs some repair or a proper tune up. I would dig into the 3030 if I had some spare time since it is so well built.

I have measured the 2050, 3030, 6830 and 8718. I trust the source of my 590 numbers. Close in phase numbers are hard to measure and most receivers have a wide roofing filter in the first IF that lets too much crud into the second mixer.
Look at the width of the Sherwood filter in his R4X, which is why it dose so well. 
I do trust his numbers though

In a shoot out the the RF590 and 6830 are very close in performance.
I am impressed with the performance of the older 8718 but close in the above RXs measured better numbers.
The Cubic R3030 makes a fair amount of noise within 10 KHz. dBC but outside of than it falls off quickly.
I have a TR7 and fairly noisy.
TCI 8174 roofing filter blow by made it hard to measure but looks like a clean design.
Nothing beats my HP8640B for phase noise.
I would take parts of the 590 and 6830 designs to build the ultimate analog machine.
The next step up would be the Cubic R3150, 95S1, 8711/8712 
BTW the 6830 and 590 will take about 30 watts input without damage. The 6830 input
module is an active limiter with gas discharge tube across the input.  

I guess I'll be looking more into these 2 when I consider a premium rx upgrade.


These should make for some interesting reading.

Thanks.

Yep, cavity based oscillators are about as clean as you can get. How about an RX design using the 8640 as the LO?

Steve,
I have considered putting a 8640B cavity in my homebrew RX. I have a hanger queen chassis. Then a divide by 8 or 10 to really send the noise into the mud.
The HPSDR guys are talking DDS but might use the cavity instead.
I would think it could be left on all the time to keep it stable.
My HB RX display actually counts the LO frequency so it would work. The synthesizer was never mounted in the chassis so easy to delete.

David;

I bought a 6790 a few years ago not expecting much... but I was surprised how good it sounded on AM. I've found it makes a great broadcast band dx'ing machine, great AM recovery. Not sure you would see that on a "specsmenship" table.  As John mentioned, noise isn't really an issue on HF unless you're planning on using it as an IF for some high end VHF/UHF converters. It's rock stable, available pretty cheap these days, parts are plentiful and if you don't care to work on your own receiver, there's a guy in MD who will do a great job resoring/repairing your 6790. I would definately take the 6790 over a Drake R-7A any day!... regardless of Mr Sherwood's findings.

Cool beanz. Sounds like a kick a$$ system. If you can find a HP 8663 without spending a ton of money, you could have a low noise synth. Seems like overkill though.

Steve the cavity tunes 250 to 500 MHz so divide by 16 would be perfect for my 41 MHz. first IF. I'm sure the cavity frequency can be extended. A switch on the divider chain or multiple front ends would extend the range to well into VHF. The homebrew only has 96 db of dynamic range so would need to rebuild a few stages to bring it into the the present century. But then there is HPSDR so maybe want to marry it into the radio.  fc