Pullen mixer or gated beam mixer?

[WA1GFZ]

How about 4 triodes in an h mode mixer

[W1VD]

Carl, Frank

Sounds like we're all pretty well set up for the task. Have 8970B/346A setups here as well. Could be a very interesting project to finally put some hard numbers to previously subjective test results. Fall/winter project? 4 triodes in H mode? Single & double balanced 7360s? Pullen? What else? Would be nice to have multiple 'labs' working on this project to spread out the workload.   

 

[k4kyv]

Has anyone compared the performance of the 7360 vs the (then) cheaper 6AR8's and similar TV beam-deflection tubes?

The last time I worked with the 7360 (late 70's) they made about the best front-end mixer available in terms of noise and dynamic range.

I have several of the tubes on hand that I bought brand new back then, plus a handful of 6AR8's.  Plans were to build a receiver with no rf stage ahead of the mixer, just passive tuned circuits, feeding the 7360 which would work directly into the selectivity filter.  Got sidetracked from the project, but may get back "round tooit" some day.

One thing I recall was that the 7360 was very sensitive to a.c. hum induced by the filament current.  I would recommend running the filament off DC, just as I do my 12AX7 microphone pre-amp stage.

[WA1GFZ]

Sorry guys I'm stuck in the solid state world and SDR. H mode mixers are hitting iP3 of 50 dB these days. Gain distribution is the bigger deal after phase noise.
.....But it is interesting reading..
My next big project is the PDM modulator for the 75 meter final I spent the spring making the chassis with a drilled and tapped heat sinks.

[KM1H]

Anybody can play with SS and SDR, just plug in the numbers   It takes real men to do it with hollow state

Having worked up to 75 GHz before retiring Im happy to be actually seeing what Im working on again.

Memory tells me that the 6AR8 was a real dog and pulled off the market. Of course all the NIB ones are still available since there is no replacement market. Perhaps someone has more info.

Jay, Id keep it to the two types for now as Im sure there will be a lot of interesting discussions, "arguments", changes, and whatever. Save other circuits for the next stage.

I like to prototype dead bug style using PC and perf board stock. Im also a big fan of toroids and use a military TS-671C Q meter to evaluate and optimize. It is very much equal to the HP-4342A as far as comparable results.

Winter sounds like a good time, its not that far off for NH.

Meanwhile we need a list of all the candidate tubes. The Pullen will have a rather large list and I suspect the 7 and 9 pin miniature will generate the most interest. Ive been playing around with many older styles as a matter of curiosity as to how they perform using modern circuits and components.

Carl
KM1H

[WA1GFZ]

Hey Carl,
If you can't see the spark it is magic at any frequency

[K1ZJH]

I got this reply on a usenet forum, and figured I'd share it. Anyone care to comment on how DC balance could have been achieved with this scheme?  Anyone have a copy of this RSGB publication??

Pedro
---------------------------------------------------------------------------------------------------------

Back in the 1960's, Pat Hawker, G3VA reported (Amateur Radio
Techniques, Pat Hawker G3VA, 3rd ed, RSGB, p83) that Brian Booth,
G3SYC (SK ca. 2003), had used the 6JH8 beam deflection valve instead
of the 7360. Brian compared the two devices:
i) 6JH8 has a good deal more linear transfer characterisitic
ii) 6JH8 operates with the deflector plates at 0 volts, ie unnecessary
to bias to plus 25 Volts as per the 7360
iii) roughly similar transconductance 4400micromhos vs 5400micromhos
for the 7360
iv) 6JH8 has double the anode dissipation (3 watts)
v) then as now the 6JH8 is significantly cheaper.

The final comment was that Brian had used the 6JH8  as a receiver
mixer and also successfully as a mixer in transmitter circuits.

HTH and 73

John  KC0G

[W1VD]

Carl

Guess I'll head off in the sheet beam direction since I've already been testing the 7360. Maybe you can  take the Pullen? Started pulling parts last night for the 'test bed' ... but that doesn't mean it will get underway anytime soon.

Pete

Couldn't find the curves for the 6JH8. Did see curves for the 6ME8 and it looks like that tube is set up for 75 VDC on the deflectors...and appears to handle bigger signals. Guess if one of the tube variants was designed for 0 VDC you could adjust one deflection plate slightly above or below 0 VDC.

So far my list of tubes to test includes: 7360, 6AR8, 6HW8, 6JH8 and 6ME8. What others need to be on the list?

 

[Pete, WA2CWA]

Maybe this is it Pete:
See attachment
Info came from a German forum:
http://www.circuitsonline.net/forum/view/48065/2

[KM1H]

That will be fine Jay as Im already involved with the Pullen.

Here is a pass at some suitable tubes. Im listing the older versions as possible easy retrofits to existing radios.

7 pin miniature: 6J6

9 pin miniature: 6ES8, 12AT7, 6DJ8/6922, 6BK7/6BQ7/6BZ7, 6JK8, 6JL8

Octal: 6SN7, 6SC7

Loctal: 7F8

Other suggestions welcome, I know there are lots of other 9 pin out there.

Carl
KM1H

[Rob K2CU]

HI Carl,

It's a great idea to come up with a list, but it could easily become overwhelming. One thing you might consider is to have multiple samples of each type part number decided to be tested. We have all had experiences with individuals that perform better than others of the same type.

May I also suggest that you consider picking a representative from each of the four triode types, Low mu, medium mu, high mu, and variable mu, and compare them on that basis? The 6J6 is a case to also consider as it has the common cathode. You might also select from a common base diagram, such as the 12AT7/12AU7 base diagram. Then you can make quick comparisons with minimal circuit changes. One would expect the low mu types to have the better dynamic range.

If you have access to a transistor curve tracer, you might consider using it to get some transfer curve data from tested tubes. I have a unit that provides steps in gate voltage for FET testing. Such a tester could easily be used with for tubes as it goes to at least 150 V. I am feeling inspired to make up a static test rig for this.
My curve tracer works with an external scope, so I could use ti with the Linksys PC scope. Hmmmm.


If I understand it correctly, the circuit description of the Pullen mixer explains that it depends upon different mu at different plate voltages, and the plate of the mixer stage is run at low voltage, around 60 VDC. This would suggest that the variable mu type might be best, though rarer to find.

You might also consider adding a pentagrid converter tube, like the good ole' 6BE6 for comparison. As I remember, the RCA Radiotron Designers handbook had a good section on mixers. It was concerned with noise performance of mixers and used a derived input resistance to compare noise performance. Tubes like the 6BE6 had high resistance and thus high noise. Triode mixers were described as having low resistance and lower noise.

The study could be expanded to include pentodes, and consider remote cut off vs. sharp cutoff, screen injection with low screen DC voltage, such as how a screen voice modulator works.

Another technique form the past is reducing filament voltage of the mixer tube. This could be tried out with the 12AT7 family types. Starve the mixer section filament.

Frank, the H bridge idea is interesting. The upper pair would need a separate filament transformer. One problem is that triodes are not a good switches as enhancement FETs. There were some dual triodes developed for computer work where they were used in switching applications.

[WA1GFZ]

Rob,
All 4 switches in an H mode are referenced to ground.
H mode isn't new. I saw a SSB generator in the west coast handbook using 4 triodes. No they won't saturate like a FET but have a large voltage swing.fc

[K1ZJH]

Octal: 6SN7, 6SC7

Loctal: 7F8

Other suggestions welcome, I know there are lots of other 9 pin out there.

Carl
KM1H

Might as well add the 6SL7 to the list. Finding a use for those orphan TV
tubes is always worthy

[K1ZJH]

Carl, Frank

Sounds like we're all pretty well set up for the task. Have 8970B/346A setups here as well. Could be a very interesting project to finally put some hard numbers to previously subjective test results. Fall/winter project? 4 triodes in H mode? Single & double balanced 7360s? Pullen? What else? Would be nice to have multiple 'labs' working on this project to spread out the workload.   

 

Jay, some reports on what you find to be the best parameters for setting up those tubes would
be helpful as well. The anecdotal information on the web is all over the place, especially for
the operating voltages for the deflection electrodes!

Pete

[W1VD]

Pete

Yes, the information in print and on the internet is somewhat lacking.

Planning a wide range of tests. The 'test bed' will be set up with all voltages adjustable as well as plate load impedance, cathode resistance, LO level etc, etc. The 6XX8 tubes appear to have significantly different characteristics than the 7360. Should prove interesting to look at just the mixer by itself on the spectrum analyzer and noise figure meter. Identifying peak performance when changing parameters or tube type should be much easier this way...as opposed to measuring them through an entire receiver with the fairly cumbersome sensitivity and dynamic range measurements.

Going to try to get to the project sooner than later and will note results here. Please post progress on your receiver as well!

 

[KM1H]

Should prove interesting to look at just the mixer by itself on the spectrum analyzer and noise figure meter. Identifying peak performance when changing parameters or tube type should be much easier this way...

Thats how Ive been doing it with Pullen and other mixers. Once Im satisfied with the circuit and record the data I then hook up the mixer as an ordinary converter but with an output attenuator for zero gain to a reciever with known IMD. The R-390A makes a nice shack warmer for this

6SL7 added Pete. Im sure those audio octals will make it well up into the HF range and maybe even 10M. Many, myself included, prefer to keep some of the more collectible BA's at least looking original. For now Ive been using the 6SB7Y in place of the 6K8 and 6SA7 in some sets to bring the noise down about fourfold. Its certainly helped my BC-312N on 17M!

Carl
KM1H

[WU2D]

Yeah the 6SL7 was the original tube for the Pullen that was suggested in the Souping the SuperPro articles. I only used the 6SC7 cause it was metal and I would not have to shield it. For miniatures, you have a great list. I would say the 6J6 would be a good baseline to test from since it might warrant an R390 spin. How would a 6J6 Pullen compare to a 6C4?

Mike WU2D