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 Post subject: R-274 Sensitivity
PostPosted: Feb Wed 28, 2018 2:09 pm 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 34
Location: Warminster, PA
I have my 274 up and running on all bands now. A couple tube swaps (good for good but one works better the thye other....) and a couple capacitor changes.

In checking the sensitivity I find that while it seems to receive well, the sensitivity is somewhat lacking. A quick check based on the RF stage gain chart in the manual confirmed my suspicions. For example, I need 12uV input to get 10 (-10v actually) across R107 on band 1. This is after alignment per the manual. Walking through the other section confirms that each stage needs more input then shown to get the -10v specified.

So I decided to back up and check the other sections per the manual. I found just the opposite when checking the 2nd and first 455 IF stage gaines in that I needed less, much less, input to get the specified output.

Normal Broad, V10-1, calls for 30,000uV to get -10V. I only need to input 12,500uV to get -10v
V9-1 calls for 3200uV and I input 1100uV to get -10v
V3-7 calls for 700uV and I only need 260uV.

These values seem to be way off..... Too much stage gain?


Going through the second mixer starts to show a possible issue:

T26-7 calls for 1500uV, but 500uV will get me to -10v
V6-1 calls for 170uV, but 120uV will get me to -10v

V3-7 calls for 120uV but I need 220uV to get to -10v. So it looks like this stage has some issue. I went over all of the resistance and voltage checks around V3 and all are well within spec. Most all of the resistors have been replaced through the entire radio as have the capacitors except for the ceramic ones and a couple of the micas that seem to measure fine.

There are a couple things I'm wondering about (as I bounce around subject areas here), I did replace a couple of the 33pf coupling capacitors between stages. I used mica caps vs the ceramic called for in the parts list. Should I go back to a ceramic? Maybe the mica is messing with the coupling between circuits?

I did verify that the output of my HP signal generator is correct at the various frequencies and voltages levels specified using my spectrum analyzer.

Looking for some clues.......

Tom
W3TA


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 Post subject: Re: R-274 Sensitivity
PostPosted: Feb Wed 28, 2018 2:24 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5420
It is very common to find a 6BE6 tube with low conversion gain and I would not be surprised if that is the issue. A tube tester will NOT catch this issue because a 6BE6 with poor conversion gain will often test fine in a tube tester where it is not being tested at RF frequencies. I bought several 7036 tubes that were designed for high reliability in early vacuum tube computers and these make very good long-lived substitutes for the 6BE6 EXCEPT they require a tall shield. Both of my SP-600 receivers and one of my R-274/SX-73 are running these in the first mixer position.

Another high failure item and often the cause of low conversion gain is the 6C4 HFO tube which may not be producing sufficient drive to the first mixer. Try a substitute here if a different 6BE6 doesn't fix the problem.

Your mica capacitor replacements should be just fine because they have excellent RF performance.

Rodger WQ9E


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 Post subject: Re: R-274 Sensitivity
PostPosted: Mar Thu 01, 2018 12:42 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 674
Location: Tokyo
Rodger, how about the 5750 in place of the 6BE6?

Rob


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 Post subject: Re: R-274 Sensitivity
PostPosted: Mar Thu 01, 2018 1:05 am 
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Joined: Jun Sun 19, 2011 2:31 pm
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Rob,

The 5750 would be a good choice and should be more reliable than the regular 6BE6. In my experience the 6BE6 is the single most replaced tube type in gear I have worked on and although it rarely completely dies it is prone to becoming microphonic, losing gain at higher frequencies, and suffering heater to cathode leakage. It isn't horribly failure prone but it does seem to experience more issues than other commonly used tubes.

I suspect that the 5750 and 6BE6W are built to the same standards. A 5915 (another logic oriented version of the 6BE6) would probably also work fine.

Normally mixer tubes aren't tied to the AGC bus but if you have the oddball receiver where it is then you have to be careful substituting because the cutoff characteristics will differ across some of these otherwise equivalent tube types. That isn't a concern with the R-274/SX-73.

Rodger WQ9E


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 Post subject: Re: R-274 Sensitivity
PostPosted: Mar Thu 01, 2018 2:11 am 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 34
Location: Warminster, PA
rsingl wrote:
It is very common to find a 6BE6 tube with low conversion gain and I would not be surprised if that is the issue. A tube tester will NOT catch this issue because a 6BE6 with poor conversion gain will often test fine in a tube tester where it is not being tested at RF frequencies. I bought several 7036 tubes that were designed for high reliability in early vacuum tube computers and these make very good long-lived substitutes for the 6BE6 EXCEPT they require a tall shield. Both of my SP-600 receivers and one of my R-274/SX-73 are running these in the first mixer position.

Another high failure item and often the cause of low conversion gain is the 6C4 HFO tube which may not be producing sufficient drive to the first mixer. Try a substitute here if a different 6BE6 doesn't fix the problem.

Your mica capacitor replacements should be just fine because they have excellent RF performance.

Rodger WQ9E



I tried several 6C4's when trying to get band 6 working. Settled on one that gave me the best results across the bands. Some would be low on the ends and peak in the center, others were just the opposite - but as you stated, they all tested good on my Hickok tester.

I have several 6BE6's so I'm off to try them now.

Another thing I noticed is that while the schematics I have show a 6C4 being used for the VFO, the resistance / voltage chart shows a 6BE6 there......

Tom
W3TA


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 Post subject: Re: R-274 Sensitivity
PostPosted: Mar Thu 01, 2018 7:09 pm 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 34
Location: Warminster, PA
So last night I made some good headway. I found out that I was causing some of my own problems (not the first time and won't be the last) and Trying different 6BE6's makes a difference.

First my self induced issues - I was using a tube extender to facilitate the injection of the signals at various points in the RF stages. Come to find that removing the extenders and injecting directly on the tube socket pins made a big difference - lesson learned......

Once I got that sorted out I started swapping 6BE6's in the first mixer and selected one that gave me better results then the others I had.

So here are the measurements (summarized) from last night. All look good except two areas that still need some TLC.

Band 1 - spec says 3uV to get -10v, I was able to get -10v using only 1.5uV

Band 2 - spec says 1.5 - 2 uV, I'm at 4.4uV

Band 3 - spec says 3 - 5uV, I' m at 2.5uV

Band 4 - spec says 3 - 4uV, I'm at 50uV - have some work to do here............

Band 5 - spec says 2.5 - 4, I'm at 2.8 to 5uV

Band 6 - Spec says it is not measured, I am at 200uV at 32 mhz and 18uV at 54 mhz. Interestign to note that the sensitivity drastically increases around 42mhz. I need to look into this a bit also.

So other then Band 4 and Band 6, I'm happy so far. Hopefully tonight I can figure out what is wrong on those bands and button this radio up and move it from the bench to the shack!

I have several others waiting for some attention.... an SP-600, a CR-88, a SX-117, a HT-32...... the list goes on

Tom
W3TA


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 Post subject: Re: R-274 Sensitivity
PostPosted: Mar Fri 02, 2018 9:09 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 674
Location: Tokyo
Tom, there's also the 6BY6 for the 6BE6 for receivers without AGC on the mixer or converter. Carl, KM1H ('burnt fingers') recommended this substitution, 'quieter and less prone to overloading'. I have a few basic receivers with 6BE6 front ends and the 6BY6 does seem to give a small, but consistent improvement. It's also a one dollar tube.

As for the 6BE6, Carl once wrote something like, the best thing to do with this tube is to throw it against a wall. That appraisal mirrors Rodger's. I wonder if the 6BE6 troubles are manufacturer related or due to some inherent design flaw.

Rob


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 Post subject: Re: R-274 Sensitivity
PostPosted: Mar Fri 02, 2018 4:51 pm 
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Joined: Jul Tue 21, 2009 1:38 pm
Posts: 815
Location: SW WA state
Rob,

I suspect the 6BE6 issue could be any number of things: It's always possible that unanticipated effects pop up in a product's design and manufacture.
I think that the likelihood of manufacturing problems goes up with the complexity of a device.
The 6BE6 certainly is more complex and has more components than say, an '01-A... Spacing between electrodes is much closer, it's a smaller tube... If there is slop in dimensions, or a substandard or slightly different material is used, this can introduce errors.
And that's not even taking into consideration human error! We all know that some people do better or more thorough job, or pay closer attention to the manufacturing process.
If the 6BE6 were an inherently bad design, I don't think it would have had such a long production run, or be as ubiquitous as it is.

-Tom


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 Post subject: Re: R-274 Sensitivity
PostPosted: Mar Sat 03, 2018 2:34 pm 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 674
Location: Tokyo
Tom, perhaps given that this tube, in the form of the 12BE6, even in considerably degraded condition, gave acceptable performance in millions of AA5 radios, perhaps that allowed manufacturers to tolerate a higher level of slop in construction.

The 6BE6 in HF receivers does seem to cause trouble, more so than other tubes. First step is obvious: start with a military or industrial version, avoid the common 6BE6.

Rob


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 Post subject: Re: R-274 Sensitivity
PostPosted: Mar Sun 04, 2018 3:33 am 
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Joined: Jan Fri 20, 2012 10:25 pm
Posts: 34
Location: Warminster, PA
So I Finally got this beast to the point where I'm happy with the operation. Took some final pictures and put the covers on. It cleaned up real nice and I think it works pretty well. Now to find a place in the shack.....

I ended up swapping around a few tubes, reworking the alignment a bit and rewinding one of the antenna input coils. My sensitivity measurements are as follows, I'm just going to list the band average, not points - too much typingat the end of the day........

Band 1, 3.5uV
Band 2, 8uV (I could tinker a little more here but it seems to receive well so I'm leaving it for now....)
Band 3, 5uV
Band 4, 4uV
Band 5, 4uV
Band 6, 20uV (they have no spec in the manual for band 6. I'm guessing it should be better, but....)

Here is a link to some pics. At the end is a video of it playing.

http://www.thdesignsinc.com/R-274.html

Thanks for the help.

Tom
W3TA


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