[HBR] Another Receiver Project -- HBR-4, Part 17

[N2EY at aol.com]

In a message dated 11/12/2004 6:32:00 PM Eastern Standard Time, 
waltah at earthlink.net writes:


> So after a few months of pretty hard work, I measured the 
> interference-free dynamic range.  That's the range from the weakest 
> signal you can copy to the strength of two strong equally spaced 
> signals off to one side required to produce a same-frequency signal 
> of strength equal to the one you're copying.   On the lower bands --
> 80 and 40 at least -- IFDR is often the determining factor in what 
> you can copy and even more frequently determines when signals 
> sound 'clean' and when they don't.   
> 
> It was a rather disappointing 75 db.  Well above the early solid-
> state rigs, on a par with the better Collins ham sets, not too close 
> to the 90 db range of the R-390 and certainly not close to what I 
> expected with a beam tube mixer and no RF stage.
> 
> I took stock.   The mixer is a 6JH8 operated at 140 volts on the 
> plate; nearly all the published designs used 7360's.   Maybe that's 
> a better tube, but the 6JH8 and 6ME8 came later; one wouldn't 
> expect them to be markedly less good.   


All depends what they were intended to do. I don't think any of them were 
designed with the idea that they'd be rx mixers.

> 
> The manufacturer's data sheets 'typical operation' info is 150 plate 
> volts for the 7360 and 250 V for the 6JH8 and 6ME8 but I have seen 
> comments that the 7360 requires 300 volts or so -- that's the 
> manufacturers absolute maximum -- when used as a receiver 
> mixer.  No doubt higher plate voltage would improve the linearity.  
> 

In the Southgate Type 7, I use 250 volts on the 7360 mixer. W1KLK used 250 
volts too IIRC.


> I looked again for wiring errors and reconsidered test setup 
> problems.   Didn't find any.  
> 
> A partial check on the location of the problem was to reduce the 
> mixer cathode resistor.   I got more noise and less gain -- but the 
> IFDR seemed to go up a couple db.  That seemed to finger the 
> mixer.   
> 

Did you try different 6JH8s?


> Fork in the road time.   75 db isn't that bad an IFDR and it can 
> almost certainly be improved by raising the plate voltage to the 
> maximum -- 250V for the 6JH8.   Increasing the plate voltage would 
> require another transformer but it's certainly possible to do it.  But 
> these tubes were supposed to be not just 'good', but outstanding in 
> the linearity department.   Heck the R-390's get 90 db with 6C4 
> mixers.   Why the hoop-la over beam tubes if it's a struggle to 
> equal a 6C4?   Why not just duplicate the R-390 circuit with a far, 
> far cheaper tube?   Might a 12AU7 with the halves in parallel be 
> even better?   If we're doing that, what about a 5814?
> 
> Haven't I seen speculation about the properties of an 833A as a 
> receiver RF stage?   
> 

How about a 6ES8 Pullen mixer? 


> Such ideas certainly might be tried, but I still wanted to know why 
> the big deal about beam tubes.   The only tube anybody used for 
> the receiver mixer job was the 7360 and as it happened that was 
> the only tube for which I had never printed a manufacturer's data 
> sheet.   
> 
> I think that was a light that just went on above my head.   RCA's 
> data sheet shows two circuits using the 7360; one is a balanced 
> modulator, the other is a high-level SSB mixer as would be used in 
> a transmitter.   
> 
> Neither one of the two circuits was designed to give a rip about the 
> linearity of reproduction of the signal on the tube's G1.   The 
> balanced modulator used it for the carrier input, and the mixer, for 
> conversion oscillator injection.   
> 
> The sign illuminated by that light bulb up there says "seen from 
> G1, beam deflection tubes are just *pentodes*; there's no reason to 
> expect remarkable linearity there.  What's highly linear is the path 
> via the *deflection plate* input."
> 
> (From the curves in the tube data sheets the 7360 shows pretty 
> good linearity over about 10V p-p on the deflection plates, while the 
> 6JH8 looks good to around 40V.)
> 
> In a receiver, acheivement of the remarkable performance of which 
> these tubes are said to be capable seems to require driving one or 
> both deflection grids with the antenna signal and using G1 for the 
> local oscillator.   Since the single ended G1 signal is strongly 
> rejected at the push-pull output that should have the added 
> advantage of taking out most spurious responses, probably at the 
> price of requiring a 9 Mcs trap in the antenna circuit.
> 
> There could even be instability problems, though it seems unlikely 
> with the deflection plates having relatively low sensitivity and 
> always tuned fairly well away from 9 Mcs.
> 
> I don't know about the deflection plate gain.   With input signals of 
> 10V on G1 and 8V on the deflection plates, RCA got 40V p-p on 
> the ouput plates.  That's a conversion gain of >12db, but I probably 
> can't put a 10V single-ended signal on G1 and the deflection plates 
> draw a little bit of current -- maybe 10 uA for the 6JH8.   
> 
> However, unless the noise level is dramatically higher (why?) the 
> sensitivity problem seemingly could be easily made up with an RF 
> stage.  If the mixer input can truly reach levels of ~10V or more, 
> there's room for lots of gain before there's a mixer overload issue.   
> The 6EH7 is a great RF tube at these frequencies and running at 
> very low gain should give an outstanding performance.
> 
> My first reaction was to cringe at the thought of rearranging the 
> connections to that stage to reverse G1 and the deflection plates.   
> Maybe a different tube would be better?  But it turns out that 
> simply turning the 6JH8 socket 180 degrees will make it close 
> enough.  The single ended premixer drive can be obtained with a 
> link on the premixer plate coil.   The change shouldn't take over a 
> day.   Well, that's not counting the (almost certainly needed) 
> installation of an RF stage.   But at least I already have the socket 
> hole center punched.
> 
> The remaining question is, if this speculation turns out to be right, 
> why did everyone else get it wrong?   


But did they?

Squires-Sanders measured dynamic range differently. They set up the rx to 
receive a weak signal (say, 2uV) then saw how strong a signal they could park a 
few kHz away and still not desense. 

Such things happen -- there 
> 
> were whole decades in which we all told each other that good 
> VFOs were wired with #8 wire.   But the reason for that error was 
> simple -- we were looking at the excellent VFOs of WW-II which 
> were expected to keep working in bombers with an engine shot 
> away or ships providing shore fire at Omaha beach.  Taking the 
> best VFOs of that period and simply rewiring them without all those 
> internal struts (#8 wire is a pretty strong structural member) and 
> heat pipes (#8 is pretty good at moving heat, too) would 
> dramatically improve their performance under the more favorable 
> conditions of our hamshack desks.

Maybe! There's also the issue of mechanical stability.



> 
> The gain's almost certainly higher with the input on G1 and when 
> the HB-65, HB-67, Junior Miser's Dream, and the W1OMX designs 
> were developed, nobody that I know of in the ham world was doing 
> intermod testing, though obviously engineers like Ernie Pappenfus 
> did it.   There were a couple of other QST articles in the 50s-60s 
> timeframe by people who were aware of the issue, I believe -- By 
> Goodman, right?   But weren't the designers of the SS-1R in the 
> same league?  I don't know.

I think they were. But they may have used different standards of measurement.

There's also the RSGB work.

> 
> With a new tube type, there were no prexisting designs to follow.  
> Once someone stuck the antenna signal in the logical place and 
> got good results, why wouldn't others do the same?   All those 
> designs were built in the last five years (1965-70, roughly) of the 
> vacuum tube's ham radio lifespan; there wasn't the time for the 
> hashing things over that occured with issues like VFO design, let 
> alone linear amps.  

The SS-1R and MD came earlier - definitely before 1965.


> 
> I'll know more in a couple of days.
> 
> Does anyone know of any receiver design using a 7360 (or other 
> beam deflection tube) mixer with the input signal on the deflection 
> plate(s)?
> 


Never seen one.

Remember that RCA meant the 7360 for highlevel appications - they listed it 
in the transmitting tube manual! Its main advantage was that one tube could do 
the work of several in balanced modulator and mixer service. It was 
Squires-Sanders that thought of using it as a receiver tube if my info is correct.

One problem I have had with premixer designs is that *any* impurity in the 
premixer chain results in mixer degradation. I suspect this is one reason 
premixer designs didn't get used more - only Drake used them extensively.  Collins 
and many others (including Squires Sanders) used dual conversion rather than 
premixing.

Back about 1990 I came across the 1400 kc. filters that are the heart of the 
Type 7. An early attempt to use them ended in dismal failure. In that design, 
which was never assigned a number, the VFO tuned 5.1 to 5.4 MHz, and was 
premixed with either a 7.5 or 13.8 MHz xtal.

7.5 minus 5.1 to 5.4 equals 2.4 to 2.1 (3.8 to 3.5 tuned range)
13.8 minus 5.1 to 5.4 equals 8.7 to 8.4 (7.3 to 7.0)
7.5 plus 5.1 to 5.4 equals 12.6 to 12.9 (14.0 to 14.3

Looked nice on paper but worked terrible. Maybe somebody else could do 
better.

73 de Jim, N2EY