Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently May Mon 25, 2020 5:48 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 74 posts ]  Go to page Previous  1, 2, 3, 4
Author Message
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 24, 2020 3:33 am 
Member

Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
OK Brad, I checked the link you gave me and the NC-98 schematic has it wrong. It is a CRL Midget Couplate PC-71. The Sam's Photofact is indeed correct.

Thank you Brad!

Scott, N6CIC


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 24, 2020 4:57 am 
Member

Joined: Sep Thu 01, 2016 3:56 am
Posts: 345
Location: San Jose, Ca.
This post is in reference to why the Noise Limiter tube filament has a current reduction resistor.

Brad, Thanks for finding that link to the thread and providing it in this thread. The explanation provided by Ed Engelken there is very good, but only talks to the technical operation of the tube elements, not how it would affect the operation of a tube circuit, like a limiter or detector. In that thread, Ed wrote:
"Lowering the cathode temperature in a vacuum tube diode reduces the contact bias that occurs when electrons emitted from the cathode contact the plate. That bias must be overcome before rectification can take place. The effect of contact bias in a vacuum tube diode is similar to the forward drop in a silicon diode.

Both National and Hallicrafters used filament dropping resistors in the 4 to 7 ohm range for the 6H6 and 6AL5 tubes used for detectors and noise limiters in many of their receivers." Here's the link for those interested:

https://www.antiqueradios.com/forums/vi ... 5&t=213140

My post in this thread where Steve asked why the resistor was in the Limiter filament line, I said that I believed it was to reduce hum in the limiter. The reason I said that is based on this post in the AMFone blog on this subject. Here's a link to it:

http://amfone.net/Amforum/index.php?topic=15898.0;wap2 .

In that thread, k4kyv (Don) asks why is there a resistor in the filament line of the limiter in my 75A4? W3JN (John) replies with: "That's typical with detector circuits as well; many Hallicrafters and National 6H6s, 6AL5s, etc use this arrangement.. Reduces the contact potential of the diode, and decreases hum in small signal circuits." And, AB2EZ (Stu) responds with a much more detailed and indepth explanation of the tube operation. The first 4 entries in this thread are very worth while reading.

Both of these men are highly qualified to talk to this subject. John has a lot of experience as can be seen from this article he wrote in 12-7-2004:

http://www.amwindow.org/tech/htm/jnreceiverguide.htm

about what is the best and worst receivers and why. Stu has been an electrical engineer for a long time with serious interest in tube equipment.

Now, here's my understanding of how the full filament current and cathode temperature (filament resistor not being used) causes a low level of hum in the tube noise limiter circuit. With no plate voltage applied to the limiter (cathode to plate equals zero volts), there is a very small amount of current flowing to the plate (as explained by Stu). Because the filament is being heated by an AC current, the electron cloud on the cathode (which causes this small current flow) is fluctuating at that frequency, so the very small current flow to the plate is also fluctuating at that frequency, 60 cycles. Hence, we have AC hum in the limiter plate circuit. This, of course, will go directly to the audio input. Normally we will not notice the hum because there is usually sufficient audio signal level to obscure it (an audio signal equals voltage on the NL plate). But, when there is no audio, then it is noticeable. Zero audio happens often enough that it is a problem.

So, today we do not see that many resistors in the detector diode filament lines because the RF output of the IF amplifier is high enough that there never is zero signal input to the detector. In order for the hum to be there, there needs to be zero input.

The reason I'm interested in this subject is that I've been working on electronic equipment for 59 years and specialize in Collins R-390As (which has a very good audio noise limiter circuit, also with a dropping resistor in it's filament line). As you may know, Collins does some quite nice designs and work (of course they have also had their share of blunders), but they usually only do things for a very good reason. So, I have fixed numerous noise limiter problems over the years and am very familiar with how they work and have concluded that this lower cathode temperature also does two other things: 1. reduces low level audio distortion. 2. reduces low level audio signal loss going through the limiter.

I think a disclaimer is in order - I do not know and have never talked with any of the people mentioned in this article.

Regards, Larry Haney


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 24, 2020 7:44 pm 
Member

Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Larry,
I read your post and the links and have found it to be very informative. There are some very knowledgeable people who post on this forum.

Thank you for your post.

Scott N6CIC


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 24, 2020 8:45 pm 
Member

Joined: Aug Sun 28, 2016 2:53 pm
Posts: 140
Location: Falls Church VA 22180
Quote:
I think a disclaimer is in order - I do not know and have never talked with any of the people mentioned in this article.

Regards, Larry Haney


Don't believe everything you read on a ham forum without being sure of accuracy. All it does is live forever in cyberspace to find new "believers" when repeated over and over.

The reduced filament voltage on those diodes was primarily done to reduce tube failures when the equipment was left on for long times with no signal. This caused open filaments. I am sorry Eds accurate explanation as well as one from a former National engineer confuses/upsets you.

The rest of the explanations you referenced were added by outsiders over the years including from one engineer who is not always correct since being an academic in a completely different discipline does not always result into a clean flow into another world. I have no intention of prolonging this thread drift by further commenting on them.

Brad


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Wed 25, 2020 12:50 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2848
Location: Mt. Airy, Maryland
WB1BGB wrote:
Don't believe everything you read on a ham forum without being sure of accuracy. All it does is live forever in cyberspace to find new "believers" when repeated over and over.


True.

Quote:
The reduced filament voltage on those diodes was primarily done to reduce tube failures when the equipment was left on for long times with no signal. This caused open filaments. I am sorry Eds accurate explanation as well as one from a former National engineer confuses/upsets you.


If that was true, why not put resistors in series with all filaments? Or run the filaments on less voltage? A 6AL5 is a cheap tube - why would an engineer put resistors only in the 6AL5 if this were the concern?

You see, contact potential in a diode - especially used as a detector - is an important factor. Since you don't believe me, here's an article that explains why: https://www.americanradiohistory.com/hd ... e-0041.pdf


Quote:
The rest of the explanations you referenced were added by outsiders over the years including from one engineer who is not always correct since being an academic in a completely different discipline does not always result into a clean flow into another world. I have no intention of prolonging this thread drift by further commenting on them.

Brad


Nice!


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Wed 25, 2020 1:15 am 
Member

Joined: Aug Sun 28, 2016 2:53 pm
Posts: 140
Location: Falls Church VA 22180
Quote:
If that was true, why not put resistors in series with all filaments? Or run the filaments on less voltage? A 6AL5 is a cheap tube - why would an engineer put resistors only in the 6AL5 if this were the concern?


Because the 6AL5 and 6H6 were high failure tubes compared to the rest in those receivers. Owners do not appreciate a sudden dead radio when a few pennies are the fix.

Quote:
You see, contact potential in a diode - especially used as a detector - is an important factor. Since you don't believe me, here's an article that explains why: https://www.americanradiohistory.com/hd ... e-0041.pdf


Interesting article but who is H P Manly? He appears to be an author of many books and subjects of the distant past but was he a master of any?

I really see no reason to question the engineers of the past at various companies just to take up forum space over and over many decades later. Were you an engineer in those days or now?


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Wed 25, 2020 9:34 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2848
Location: Mt. Airy, Maryland
Quote:
Were you an engineer in those days or now?


I see you are going to cling to your position and call into question my credentials as well. Not that I have anything to prove to you, but iIt's the latter, 31 years as an electronics engineer.

Here's an excerpt from the R-274 (Hallicrafters SX-73) manual. Says nothing at all about 6AL5s being high failure items - but does mention hum. Curiously, the ONLY 6AL5 I've ever found bad and had to replace was in a SX-73.


Attachments:
sx73fil.JPG
sx73fil.JPG [ 131.49 KiB | Viewed 347 times ]
Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Wed 25, 2020 10:21 pm 
Member

Joined: Sep Thu 01, 2016 3:56 am
Posts: 345
Location: San Jose, Ca.
Since the position from Brad for using the filament dropping resistor in 6H6 and 6AL5 limiter and detector filaments was premature tube failure, I thought I'd check and see how many receivers use them today or what else they use. I knew that the R-390A did not, as it uses 5814/12au7s dual triodes connected in a diode configuration (the grid tied directly to the plate). Well, to my surprise, I find that the 51J-3 (R388) and 51J-4 use a 12AX7 (dual triode) for the audio and avc detectors and 1/2 of a 12AX7 for the noise limiter. These are of course connected in the diode configuration as just described. And, guess what, only the half of the 12AX7 used for the noise limiter has the filament dropping resistor. The Hallicrafters SX-111A uses a 6BJ7 (triple diode) for audio and avc detector and the ANL, and uses a filament dropping resistor in it's line. The 75A4 uses four 6AL5s for a few functions and one is for the noise limiter (V12), but it is the only tube that has a filament dropping resistor. The other three 6AL5s (V10, V16, and V19) do NOT. If the 6AL5 had a premature failure problem, they would have put dropping resistors in all of the 6AL5 filaments, NOT just the limiter.

I am obviously not agreeing that the 6H6 and 6AL5 tubes had premature failure (as I have seen them in many circuits and they have lasted for a very good life) without dropping resistors. So, if I was seeing only 6H6 and 6AL5 tubes with filament dropping resistors, then I might consider that the reason for the dropping resistors was to extend tube life, but that is NOT the case at all. You can see from the above that three different tube types are used in noise limiter circuits (besides the 6H6 and 6AL5) with the filament dropping resistor. I am convinced now more than ever that the reason for using the dropping resistor in the filament line of the noise limiter circuits is to improve the operation of the noise limiter and NOT to extend it's tube life.

I am quite sure that if I were to spend some more time looking for receiver noise limiter circuits that are not 6H6 or 6AL5 tubes with dropping resistors, I would find them. But, I think that what I have found already should be facts enough to demonstrate that tube life was NOT the issue.

Regards, Larry Haney


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 26, 2020 5:48 am 
Member

Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
OK-I replaced the couplate (CRL Midget Couplate PC-71) with discrete components per the Sam's Photofact schematic, not the schematic in the NC-98 manual which is incorrect.

http://pacifictv.ca/schematics/centralabcouplates.pdf

My NC-98 performs very well now even without a realignment, but I will do that next as I have replaced so many components.

Thanks for everyone's suggestions in this thread!

Scott, N6CIC


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 26, 2020 7:42 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 4320
Location: Lincoln City, OR 97367
Greetings to the Forum:

Just a note on tube longevity:

I have been playing with ham radio and exposed to tube type gear of all kinds since I was 13... that would be some 58 years. In all that time, I think I have encountered at most two bad 6AL5's (maybe only one... they sure don't stand out in my memory as high-failure items); I don't recall ever having encountered a bad 6H6.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 26, 2020 4:11 pm 
Member

Joined: Jan Tue 07, 2020 1:41 am
Posts: 1195
Location: Fenton, MI 48430
The 6AL5 is a miniature version of the 6H6. It is rated at 9ma out per plate. The 6H6 is rated at 8 ma per plate. It was used in Johnson Viking transmitters as a bias rectifier and no issues were reported. A bias supply would likely be more stressful vs other 6AL5 applications.


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 26, 2020 5:13 pm 
Member

Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1451
Location: Champaign IL 61822
I have seen many 6AL5s and 6H6s and only only one was ever bad: the 6AL5
FM detector in my RCA CT-100 color TV had one of the two filaments go open.


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 26, 2020 5:54 pm 
Member

Joined: Aug Sun 28, 2016 2:53 pm
Posts: 140
Location: Falls Church VA 22180
It saddens me to read so much from those who apparently did not bother to read and digest what I posted earlier.

The reduced filament voltage on those diodes was primarily done to reduce tube failures when the equipment was left on for long times with no signal. This caused open filaments. I am sorry Eds accurate explanation as well as one from a former National engineer confuses/upsets you.

Next thing we will see is Ban The Resistor posters on here and at hamfests. :roll: Time to move on folks Steve has a working radio now.


Top
 Profile  
 
 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 26, 2020 11:23 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2848
Location: Mt. Airy, Maryland
Source? I've never read that anywhere, no experienced it.


Top
 Profile  
 
Post New Topic Post Reply  [ 74 posts ]  Moderator: Sandy Go to page Previous  1, 2, 3, 4

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 11 guests



Search for:
Jump to:  


































-->


Privacy Policy :: Powered by phpBB