The AM Forum

I owned a few Military surplus receivers with two RF stages vs more standard one RF stage. Two stages are more sensitive at high frequencies (20 meters) than single stage. But on another forum a Ham states not true. I never owned a single 6K7 RF tube outperform two 6K7 RF tubes.

Define “outperformed”. 

Sensitivity/gain is a small part of receiver performance. Selectivity and resistance to overload from other signals are likely to be more important to overall performance. How often are you tuning in and working an S1 station? Band noise is much higher than the sensitivity limit of even a basic receiver…in other words, sensitivity doesn’t matter. Perhaps the designers used two tubes for other than maximizing gain??

Ed

Dual RF stages were typically used to increase front end selectivity, particularly in single conversion 455 Khz IF receivers.  Any decent single RF stage will be enough to establish the noise figure for the receiver with needed gain properly occurring later in the IF section.

Rodger WQ9E

Another reason some military receivers included two RF stages had nothing to do with receiver performance, but instead was intended to minimize or eliminate local oscillator leakage.   Especially important onboard ships, the goal was to prevent the enemy from performing radiolocation using the emitted local oscillator signal.  The additional RF stage provides increased isolation.  This was exemplified in many of the National receivers which were descendants of the NC-100 series.  Some of the receivers had the second RF stage added specifically for this purpose.  You may read more about this on the RadioBlvd site.

Always been one of my triggers but every time I hear about how “the enemy is able to radio locate by LO radiation” that I have always regarded this as an urban legend or at least a story told to prevent people from using unauthorized equipment. It always struck me as a theoretical possibility but technically impossible. The real issue is keeping LO radiation from interfering with other equipment onboard the aircraft or ship. Radio direction finding was often used to locate submarines, aircraft and ships when they transmitted but show me just one example where someone at sea was located by LO radiation.

To follow the poster's fantasy of receiver gain, receiver sensitivity, and RF front end receiver design,  here is the link to the QRZ thread he started:
https://forums.qrz.com/index.php?threads/receivers-with-two-tube-rf-amps.945514/

His claims were refuted several times but he continues to push on with the fantasy.

Thanks for the head's up, Pete!  As a little kid, I learned not to get into a urination competition in the middle of a hurricane.  So sorry I wasted internet bandwidth and storage on the AMfone server.  Clearly, the first post reveals the entire story! And to think this disease has propagated from one forum to another AGAIN!

Indeed.  OP insists on posting declarative and incorrect statements he's gleaned from various internet sites.

N8FVJ - you dragging a made-up controversy over here to AMFone from QRZ where you were wrong, you were shown you were wrong, yet persisted, is not appreciated.

I have a low tolerance for trolls and your obstinacy unfortunately leads me to conclude it's time to say 73.

W8KHK said:When the post was first made on that 'other forum, I advised Jim, N2EY not to respond because of what happened previously. Thus which lead to my 'clairvoyant' comment.

Anyone care to pass the popcorn please?

Trolls come and go. I appreciate all the rest of you here that make the AMfone worthwhile.

Giving the fellow the benefit of a doubt.... After looking for something else, ran across this wonderful piece in QST...

https://www.worldradiohistory.com/Archive-DX/QST/40s/QST-1947-09.pdf

See the paper by Goodman, page 13. How sensitive is your receiver.

W4AMV said:You know, those fellows back then were on top of their game! In most cases they took great care not to be dispensing BS, (unless it was the April issue!). To me, I state how sensitive a receiver is by how easily I can discern a signal from signal generator. If I can readily hear a signal under say, 0.5µV, then I call that good. I don't know what the commercial standard is but that's just my benchmark. No need to get into some kind of a 'southern-most' North American country-peeing contest.  ;)

back in my McIntosh audio days, we had a standard we went by to measure sensitivity of FM stereo receivers. I don't remember the details or the numbers but in a general sense to determine if a receivers sensitivity was within factory spec we dialed down the signal generator until either a certain distortion level was reached or S/N ratio was reached. It was a long time ago so things are fuzzy.

Yes sir Bob. The 12 dB SINAD (S+N+D)/(N+D) or (signal+noise+distortion)/(noise+distortion) ratio
is an excellent method of checking FM sensitivity and it correlated well with the 20 dB quieting method.

The measurement is conducted with 3 kHz FM deviation, at 1 kHz audio, 5 kHz max for NBFM. A distortion meter is used to null the 1 kHz audio. What is left is N+D. A good FM receiver was
capable of achieving -123 dBm (0.15uV) or better for this measurement.

 

Most receivers have too much sensitivity via front-end gain. And it limits the dynamic gain of the receiver. The received signal-to-noise ratio (SNR) in most situations are limited by external noise (ambient, atmospheric, etc) and not the noise floor of the receiver.  IIRC, a noise figure of about 14 is good enough for most HF applications.

Excellent Steve.

At 14 dB NF sensitivity is at:
-174 dBm/Hz + 14 + 10 log (10kHz_AM? BW) + 10 dB S/N = -110 dBm (sensitivity)...

To get to say 0.5 uV (-113 dBm) just reduce the AM BW a tad...

Hi all, been lurking in the background on this thread.

I attach the schematic of the receiver portion of the Retro75 transceiver, a 2 watt 75 or 40m AM transceiver.

The receiver with a front end mixer/oscillator , a SA602, is dead quite without an antenna connected. Hooking up my antenna, and I hear the band with good volume capability, usually about S5-S7 on my better receivers (Flex 5000, Icom R8500). The noise floor is defined by the band, and not by the fact that there are no RF gain stages ahead of the mixer stage.

For the simplicity, and cost, Dale Benson, K1SWL did a good job on this design. The main drawback is the lack of AGC in the RF stages, but that is addressed by the AF gain pot, and audio AGC that will kick in occasionally when a tall ship starts modulating. Sensitivity, selectivity, and stability are all good enough.

I think back about the RCA AR88 I had. There were two 6SG7 RF1, RF2 pentodes in there. With all that gain, intermod was likely when a station was off frequency enough such that it did not get through the IF bandwidth skirts. IMHO, too much gain upfront is more of a problem that not enough gain. The dynamic range can get squished downstream when the signal is too high, or cause intermod when there is an off frequency signal that is within the RF bandpass, but outside the IF bandpass.

Jim
Wd5JKO

Jim, what a coincidence.  this thread and the other thread on QRZ got me thinking about the NE602/SA602 and subsequently the Retro 75 and how sensitive it is and is OK on selectivity.  BTW, I still have my Retro 75 and 40. Both have your audio mods. I added an RF stage (MPF102) to the 75 because I wanted to improve on the AGC function. Some of the big guns here in the NE can easily overload the 602.

In my main vacuum tube receiver. it doesn't have a front end RF stage. The receiver comes to life, when you tune in a signal.

Recently I had a nice short discussion with a gentleman I know who works on/restores both the R390 & R390A. Mainly it was me asking about the differences - that is the R390 has two RF stages, the -R390A has one, and IIRC the R390 has two more IF stages than the R390A.

There are a couple other smallet things but these differences had me pondering which I would prefer, were I to opt for yet another. While his comments are way more detailed than this post and enlightened me along lines I had either suspected or knew not of, it was a private email so respect of that.

I could summarize that the 390A uses a better tube in the RF front end, thereby making do very well with one, PLUS the reason for the RF stage in the first place (for my uses) is rejection of unwanted signals - so if that is met and all else is equal, it's even-steven as far as "my hobbyist listening results" are concerned.

As far as the IF stages, it goes at least partly back to the LC tuned circuits needing be being more complex than those sharp crystal filters circuits - again other things being equal in the performance of both fine receivers.

Been avoiding commenting on this subject as to not appear as a complete knob. But here goes! Whit the atmospheric noise being quite high on the HF bands and with all the man maid noise out there today what’s the point of having stupid high gain figures on a HF radio anyway? You don’t get really good or low floor levels until you are into the VHF range and any additional gain also brings with it higher internal noise or overload issues with strong signals.
I am not smart enough to do all the math, bandwidth and signal to noise calculations but have always gone with the test of just cranking down the output level of a signal generator and doing A/B test with other boxes that I have. Sensitivity between different modes like AM, CW and sideband are all different and somehow think the width of the signal also has some effect on the numbers you get so will leave that up to all the smart people.
My test for a HF receiver, crank up the AF and RF gain all the way with a termination acrost the input and how much noise comes out. The lower the noise floor in the radio the better and if the radio can produce a clean audio output with a one hundred percent modulated signal (1kc) at a microvolt that’s good.
Ok, so VHF/UHF radios can receive clean FM signals down to a fraction of a microvolt but the noise floor at those frequencies let you get away with that.
Looking at the chart I stole from WiKi it appears at 40 and 80 meters the noise floor is so high as to make the super high sensitivity sets a waste of effort.
I figure multiple RF stages are for selectivity and image rejection only, and additional RF or IF stages start to appear like all the adds in the back of the radio magazines in the thirties and forties said the quality of the radio was a factor of how many tubes they had and a fourteen tube receiver was far superior to a ten tube set.
Somehow think SDR receivers exist in some other form or realm, by direct sampling they can somehow perceive signals that if not at the noise floor may be below but don’t know if that true or not.
 

Hear you go, the ad states that because of its eighteen tubes it is more powerful and will outperform other radios along with pulling in those distant stations. Imagine this has two or three RF stages!

Not only do those eighteen tubes make the receiver more sensitive, they allow it to provide six "wave bands," rather than just six ordinary bands!

My favorite - National's mighty AN/FRR-24

A separate RF unit for each band, each with
    3 RF amp stages 6BA6 + 2x9003
    1st mixer (1750 kc IF out) 6BE6
5 IF amp stages 5x9003
2nd mixer (50 kc IF out) 6BE6
1 IF amp stage 6C4
Bandwidth selects 1 of 8 filters (125 cycles to 18 kc) with 0-2 IF amp stages 9003 or 6AG5
7 IF amp stages 3x9003 + 6C4 + 3x6AK6
detector 6AL5
AF amp  6BA6
AF output 6AK6

http://www.navy-radio.com/rcvrs/frr24.htm

Only one RF stage:
https://antiqueradio.org/art/MidwestOwnerManual6.jpg
But it must sound enlightening during a thunderstorm with four 6F6's (parallel/push-pull) in the audio output.

Given some of those older Rxs were single conversion, front-end selectivity and gain were important.

Hi All!

This easily can become a very deep subject as receiver design has advanced over the decades, from cat's whiskers to SDRs.

I enjoy the National HRO line of tube receivers, which have two RF stages. The earliest HRO had 6D6 tubes for the RF stages. This tube has a tranconductance of 1600 micro-mho's. The later HRO-60 had 6BA6's that have a transconductance of 4,400 micro-mhos. The dual-gate MOSFET that was used in many receivers in the 1970s has a transcunductance of 12,000 micro-mhos. There are related MOSFET transistors used today. Happily device gain has increased over the years, giving us more sensitive receivers with less overall noise.

Any amplifying device will have decreasing gain as the frequency is increased. This is due to interstage capacitance, usually grid or gate to plate or drain. When you have two RF stages in series, the loss at the higher frequencies will be double the loss of a single stage.

In the 1930s when the HRO was designed, mixer tubes were noisy, and signals were weaker (the majority of hams and SW broadcasters used less power). Combined with less knowledge or interest in receiver dynamic range, the natural thing to do back then was to amplify the antenna signal as much as possible before mixing to an IF frequency. This seemed good until you tuned off of a strong signal, where the AGC would bring the gain up and the front end and mixer would saturate.

If you look at the bandwidth of an RF front-end filter, it is not that narrow and will not reduce nearby strong in-band signals. Its main purpose back then was to reduce images (for a 455KC IF, the image would be 910KC away) and the HRO was good at this until about 14MHz (20M). Around 10M the images were only down about 20dB.

If using this topology it would be best here to have the first stage have low gain and low noise to provide some image rejection, then the second RF stage would do most of the amplification and have a good deal of dynamic range.

Today the better analog solution is to up-mix the HF range of interest to above 40MHz. A wide filter here gets rid of the images. Then this first IF signal is down-mixed to 9MHz or 455KHz where tight filtering can easily be provided. Of course, an SDR, being a direct-sampling receiver, has hopefully enough dynamic range, no images and the selectivity is created in math.

Ulrich Rohde N1UL, Wes Hayward W7ZOI, Doug DeMaw W1FB have excellent books on receiver design from the 1980s and 1990s.

Cheerio,
Dan
W1DAN

IIRC, many of the tubes used for the first mixer were noisy. So more RF gain was needed.