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Author Topic: GE 102 AMBC radio tube problems  (Read 23946 times)
N4NYY
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« Reply #15 on: August 15, 2011, 09:56:56 AM »

Quote
If you don't already have an AC Isolation Transformer on your bench, get one.

Just got one
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AC5UP
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Posts: 4546




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« Reply #16 on: August 15, 2011, 12:30:10 PM »


HOLY CRAP ! I WAS WRONG !!!


Hard to imagine, but true... According to this 1935 advertisement (and adverts never lie) the metal tube was promoted by RCA but developed by General Electric... Although the advert does mention RCA patents. I thought sure the metal tube was initially an RCA thing since they dominated the new & replacement tube market, but I was worng.

Sealed in steel - Made like a fine watch

Meanwhile... Here's a trivia challenge for Mr. 'NYY: What's the difference between a 6J7 and a 6SJ7? A 6A7 and a 6SA7? Between a 6K7 and a 6SK7? There are more, but you get the idea. Many tubes are like this but only one answer is needed.

What could the difference be?
« Last Edit: August 15, 2011, 12:36:12 PM by AC5UP » Logged

G3RZP
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Posts: 1268




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« Reply #17 on: August 15, 2011, 12:34:21 PM »

It's also worth getting a variac. If I don't know how long a set has been standing, after examining for burnt compnonents and bulged or leaking electrolytics, and assuming the electrolytics look OK and measure OK on the VOM, I pull the tubes (valves to us Brits!) and gently run up the input with a voltmeter on the transformer. If that seems about right and it doesn't get hot, then I tack solder a silicon diode (or diodes if full wave) and very gently run up the voltage while monitoring the B+. Get it up to 50 volts and let it rest for ten minutes,  then up to a 100 and so. Watch that you don't go over the ratings of the electrolytics, though. If any of them have a resistor in series, measuring the drop across the resistor at each step will tell you how well they are reforming. If any start to get warm or the volts won't come up or the fuse blows, stop. If you can run the B+ up OK like that, then switch off and remove the silicon diodes - after letting the capacitors discharge. Then fit the tubes and proceed...


AC5UP - what is special about a 6SJ7Y?
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AC5UP
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« Reply #18 on: August 15, 2011, 12:52:34 PM »

Best I can tell the 'Y' version had a low-loss Micanol base.

The NJ7P tube database on the web mentions that, but not a word in my RCA or Sylvania tube manuals............
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N4NYY
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Posts: 5224




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« Reply #19 on: August 15, 2011, 01:47:46 PM »

AC5UP, I did not get the "high alert" ringtone to notify me that you were wrong!

Quote
It's also worth getting a variac. If I don't know how long a set has been standing, after examining for burnt compnonents and bulged or leaking electrolytics, and assuming the electrolytics look OK and measure OK on the VOM

G3RZP, I do have a variac. The same ham that gave me the tube tester also gave me the variac. I replaced the caps. Personally, there is not look all that good. The safety cap across the line, was leaking some yellow goop. I am assuming that was dielectric, but it looked closer to something out of a zit! I did not want to fire up before I replaced those!

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NB3O
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Posts: 53




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« Reply #20 on: August 15, 2011, 05:56:34 PM »

"basic GO / NO GO test device useful in weeding out the dead, the dying, the shorted and gassy."

If they only made one of these for people........
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AC5UP
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« Reply #21 on: August 15, 2011, 06:18:26 PM »

For those of you who lived in a cave 80 years ago... You could bring music, news and entertainment into your life through the miracle of radio!

http://library.duke.edu/digitalcollections/media/jpg/adaccess/lrg/R0606.jpg

If the radio thing didn't work out, there was always this: http://library.duke.edu/digitalcollections/media/jpg/adaccess/lrg/R0192.jpg

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G3RZP
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Posts: 1268




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« Reply #22 on: August 16, 2011, 12:55:44 AM »

Strange thing about the 'Y' versions. As far as I know, there was only the 6SJ7Y, and the only place I've seen that was in some models of the BC221. You would have thought that there would have been a 6SG7Y, a 6AC7Y and a 6SH7Y, considering their applications in HF receivers and radar IF strips. Another funny is that a 6SJ7 is single ended 6J7, but a 6SL7 is not a single ended 6L7!
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N4NYY
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« Reply #23 on: August 16, 2011, 03:30:14 PM »

5UP,

No way I know the difference as I just learned about black metal tubes.

One thing I found on Google when I was doing a tube cross reference, was these websites that give "supercharge you AA5" info. I did not know that there were hacks doing AA5 radios. some suggested replacing certain tubes with ones that have double the gain. Doesn't that double the noise?
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KE3WD
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Posts: 5688




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« Reply #24 on: August 16, 2011, 04:34:58 PM »

Noise is always defined as a ratio.

When doubling the gain, sure the noise figure goes up as well, but when looking at it as the Signal-to-Noise Ratio, one can see why the increased noise floor becomes more and more negligible as the gain increases like that. 


73
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N4NYY
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« Reply #25 on: August 16, 2011, 04:55:40 PM »

This is the site I found with a search for a 12SA7. Personally, as a tech, I have never been a "mods" guy. Unless the design engineer made a change, or there was a tech service bulletin, I try not to hack up radios. The only exception is a fix for a known flaw. If there isn't any flaw, I am very skeptical (more afraid), of making any design changes for so call improvements.

http://www.wa2ise.com/radios/12ba7.html
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AC5UP
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« Reply #26 on: August 16, 2011, 06:10:40 PM »

No way I know the difference as I just learned about black metal tubes.

If you looked at the 1935 advert extolling the virtues of metal tubes that I linked in this thread you should have noticed the tube shown has a grid cap. Could have been a 6K7, 6A7, 6J7 or any of a dozen other 6?7 numbers that were replaced by a 6S?7 version with NO grid cap. Instead of the grid lead coming out the top of the tube the newer version puts the grid on the bottom with everything else. From a manufacturing perspective the grid cap adds a bit to the cost, but from a repair guy perspective the grid cap made signal tracing a breeze. In some cases you could buzz through the converter and IF stages without taking the chassis out of the cabinet and give your retail customer a repair quote in five minutes or less. On the downside, a customer with wandering fingers could discover there is a bit of DC on some grids. Surprise!

So... Here's your lesson for this week: There are gazillions of tube types but relatively few unique tubes. (?)

Example: The venerable 807 Beam Power Pentode developed in the early 30's spawned a plethora of power tubes electrically similar but differing mainly in the envelope style, heater voltage and base configuration. The 1625 tube is an exact clone of the 807 except it has a 12 volt heater and a seven pin base. The 807 has five pins. The 6L6 is another clone of the 807 in an 8 pin Octal base with glass that can be coke-bottle (ST) shaped, straight sided, or metal. Another classic example is the 6K6, 6F6, 6V6, etc. group of medium power beam pentodes. They all derived from the 41 or 42 tube and in the case of the 6K6 the lineage is 41 > 6K6 > 6AQ5. Or, the 41 was the 30's tube, 6K6 in the 40's, 6AQ5 from the 50's and up.

As for hot-rodding an AM radio, the limiting factor in weak signal AM reception is almost always local noise. Adding more gain in the IF section won't change the S/N ratio of a noisy location because that's determined outside of the radio. Best you can do is move the set near a window and in the case of the GE would probably be a waste of time. Strong stations may overload a hotter converter stage and the AVC line might need revision. The RCA you have now could be modworthy as it's a more sophisticated box, but instead of looking at higher mu tubes I think it better you should look at newer models of the same radio. I can guarantee you not every 'new' radio was 100% new and RCA did running design tweaks from one model year to the next on essentially the same line of radios... Find the same basic set five to ten years later and it could be a better design. Or it might use Novar base tubes instead of Octals and be essentially the same radio.

Which means you're usually better off by keeping it stock.  Tongue
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G3RZP
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Posts: 1268




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« Reply #27 on: August 17, 2011, 12:43:05 AM »

Funny thing about S versions of octal tubes. Not all of them are the same thing as the non S versions.  And there was a 6SB7Y.

6SB7 is not a version of 6B7. Same goes for 6SG7, 6SL7, 6SN7.

BTW, 6AQ5 is nearer to a 6V6 than a 6K6, although the difference is minimal.

One has to be a bit wary of changes. Not all radios were that well designed, and many more suffered when production 'cost reduced' them. A good example of poor design can be found in the Yaesu FT102, where relays are used for switching the low level RF signals. The contacts oxidise, and the receiver goes dead. Any old fashioned telephone maintenance guy from the 1940s could have told them to run a bot of DC through the contacts to avoid the problem. A further example is the very bad receive spurious on 3500, which can really kill CW DXing on the low end of 80. Again, a design fault.
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W8JI
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« Reply #28 on: August 17, 2011, 03:21:26 AM »

The 6L6 was the first USA beam power tube. There were many later copies of the technology, and some tubes use similar construction styles or shared tooling for some elements of the 6L6, but that hardly qualifies them as true clones. The 6L6 is the real grand daddy of USA beam power tubes.

The 807 is a slightly later production that was very similar to the 6L6 in most element ratings, but not quite the same transfer, and different shielding, wiring, and insulation. The 807 was obviously run from much of the 6L6 tooling with modifications to improve RF performance and peak voltage performance.

This isn't uncommon. The 3CX1200A7 shared grid and filament tooling with the 3-1000A. Sharing tooling with minor changes is manufacturing and engineering economics at work.
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AC5UP
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« Reply #29 on: August 17, 2011, 07:46:35 AM »

Well, Well, Well... This is interesting...

Just spent the past half hour refreshing some of the stuff I "know". Tom is correct that the 6L6 was released before the 807. I thought sure the 807 came out first as it has an older style five pin base and I've always seen them with an ST envelope. Just looks like the older of the two. Also discovered what I'm calling a miniature 7 or 9 pin Novar base is actually a Noval base. Off by one letter but otherwise so close...

Not that there's a competition in play, but it turns out I'm not the only one who can mis-remember a detail or two...

Check this out: http://www.nj7p.org/Tube4.php?tube=6BQ5 Mechanical data section says miniature button 7 pin base. Tube outline shows 9 pins. It's 9.

http://www.kbapps.com/audio/tubemanual/153.html Scanned from the RCA tube manual, we learn the 6L6 is a Beam Power Amplifier.

http://www.kbapps.com/audio/tubemanual/008.html and http://www.kbapps.com/audio/tubemanual/009.html These two pages describe the Beam Power concept as used in the 6L6, and that beam forming is commonly used with Pentodes and Tetrodes. If you look at the 6L6 page linked above you'll note the tube outline is drawn as a Pentode.

Look up a 6L6 at NJ7P and there's no question it's a Beam Power Pentode: http://www.nj7p.org/Tube4.php?tube=6L6 Says so right up top.

However, at Wikipedia, the 6L6 http://en.wikipedia.org/wiki/6L6 and 807 http://en.wikipedia.org/wiki/807/ATS25 are described as Beam Power Tetrodes.

Apparently in some quarters the beam forming plates of the 6L6 / 807 are not counted as an internal element like a screen or grid wire would be.

Go Figure.  Cheesy

BTW: In case you've ever wondered where the "KT" designation came from in the KT-66 and KT-88 Hi-Fi audio output tubes, a common Tetrode has a minor ripple in its response curve. This was referred to as the "Tetrode Kink" in tech slang but the KT-66 and KT-88's design eliminated the kink... Which makes them a "Kinkless Tetrode" and merits the KT- prefix. Found that on Wikipedia so it's probably true..............
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