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Author Topic: Globe King 500 OSC screen resistor not going to correct socket pin  (Read 3136 times)
K1YTG
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Posts: 225




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« on: August 11, 2012, 04:16:38 PM »

I have replaced switches and cord and checked some caps.  So the filament circuits and 400 volt supply are working.  I put in a 6V6 tube and crystal and tried to dip the 1st tuned plate circuit. I do have plate current.   No dip.  Looking into the wiring and measuring voltages I found I do have 300v on the plate and 0 on the screen.  I followed the wiring and found the screen resistor is not going to the screen lug on the socket.
Does anyone know of an alternate tube used for the osc in place of the 6V6?  One where the connection is different for the screen?  The set came with no tubes in the sockets and I assumed 6V6 was correct.
Thanks, Norm
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AC5UP
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« Reply #1 on: August 11, 2012, 04:48:04 PM »

Assuming you don't have an accurate schizmatic, and I would assume you've been here to download all four pieces of the King500 docs  http://bama.edebris.com/manuals/wrl/  the next step would be a visit here:   http://www.nj7p.org/Common/Tube/SQL/Tube_query.php?index=1

Look up the 6V6 you suspect belongs in the oscillator socket. Click on 'identical characteristics'. Go through the list to see what might be similar electrically but has the screen on the pin you're looking for. I know this sounds ass-backwards, but if you know your octal base tube numbers it won't take long to either find what you're looking for or realize you're out in the tall weeds on the 6V6 hunch..................   Wink
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K1YTG
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Posts: 225




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« Reply #2 on: August 11, 2012, 05:03:25 PM »

6V6 was on the schematic.  But your suggestion might lead me to what tube they did have in there.  Or I can change the wiring to what is on the schematic. 
Someone must have thought that another tube would do a better job than the 6V6.  And they changed the wiring. 
Which way to go from here?  Any opinions on 6V6 as an osc?  It does seem like an odd choice.
Norm
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AC5UP
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« Reply #3 on: August 11, 2012, 05:28:25 PM »

Path of least resistance would likely be to move the resistor back to where it ought to be while keeping in mind a previous comment that WRL schizmatics were prone to errors.

As for a 6V6 as an oscillator, Hell Yes. Plenty of one or two-tube CW rigs were built back in the day and the idea of a 6V6 as an oscillator was all about a robust output level using a very common (cheap) tube. RCA sold plenty of consumer radios that ran a single ended 6V6 and were good for maybe 3-5 watts at the speaker.

BTW: Good chance you'll make a few subs & mods while in the restoration process since it isn't 1955 any more, so consider keeping a log of deviations from the original. That way you won't need to rely on memory should you want to back out a change if and when you have a bigger & better idea.
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W5JO
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« Reply #4 on: August 11, 2012, 08:14:44 PM »

As I mentioned in your other thread, things may be mixed.  Your deck could have had a 6sj7 or a 6ag7.  Compare different diagrams.
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W9GB
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« Reply #5 on: August 11, 2012, 09:30:14 PM »

Globe King models.
Models 500 and 500A without VFO, later models 500B and 500C with built-in VFO.

VFO tubes: 6AU6 6CB6 0A2. Also the 6V6 was replaced by a 6AG7 and keyer 12AU7 added.
Latest 500C using 4-400A as final amplifier instead of 4-250A.

GE data sheet for the 6AG7 Pentrode, MT8 - Metal tube Octal socket. (EIA 8Y)
http://www.r-type.org/pdfs/6ag7.pdf

GE data sheet for 6V6 Beam Pentrode, GT - Glass tube.
http://www.r-type.org/pdfs/6v6gt.pdf

There are differences for the screen (grid 2) connection:  Pin 4 for 6V6, Pin 6 for 6AG7.
==
Globe King 500:  transmitter; Advertisement:  JUL 1954 QST; price $675.00 wired;
500 watts AM, 500 watts CW, 500 watts SSB (with external exciter);
160 thru 10 meters; bandswitching.  
http://amfone.net/Tech/wrl/king500.htm

RF Section: 6V6 crystal oscillator, 6146 buffer, 4-250A final, 6X5 and 5Y3 low voltage rectifiers.

Modulator:  6SJ7 speech amplifier, 6C5 amplifier, 6C5 amplifier, 6L6G driver,
push-pull 5514 modulators, 5Y3 low voltage rectifier, pair 816 high voltage rectifiers.  

Power Supply:  5U4G low voltage rectifier, pair 866A high voltage rectifiers.

Crated shipping weight 270 lbs.
« Last Edit: August 11, 2012, 09:48:41 PM by W9GB » Logged
W5JO
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Posts: 59




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« Reply #6 on: August 12, 2012, 06:24:34 AM »

What futher complicates things is people mixed decks from different models to make a complete transmitter.  They would find a 500A missing the modulaton deck and then locate a B modulator deck to go with it.  You may have a 500 power supply with a 500C modulator then a 500A RF deck or any combination thereof.  Given the popularity of these things it happened quite frequently in the past 10 years.  Add to that an RF deck from the factory marked 500A that has a B model oscillaor.
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K1YTG
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Posts: 225




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« Reply #7 on: August 12, 2012, 12:47:04 PM »

I have the osc working after many hours and testing every component.  I replaced a lot or resistors as some were out of tolerance and a lot just looked bad with short leads that had been spliced.  And every attachment point had a lot of old cut wires still attached that needed to go.  So it looks a lot cleaner now and is laid out in a much more orderly fashion. 
It turned out that the 500 schematic did not have pin numbers.  So I looked at the 500b schematic to get the pin numbers.  That was a bad idea.   Because the 500b has a different tube.  the 500b has the screen on pin 6.  The 500 was wired correctly having the screen resistor going to pin 4. 
So after rewiring the osc with many new parts and attaching a new screen resistor  to pin 4 it works well.  The 6146 is dipping at resonance also and there is grid drive to the 4-400a.
So the next step is to connect some HV and see how the PI-network and final tank circuit work.
Thank You for all the pointers offered. 
It really does give me confidence to take on a project like this knowing that I help is available if I get stuck.
Norm

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KA5N
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Posts: 4380




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« Reply #8 on: August 12, 2012, 02:05:47 PM »

You need to invest a couple of bucks for an old ARRL Handbook of mid 1950's  to 1960's
version they are pretty cheap on Ebay and usually real cheap at hamfests.  They have the
base layout and tube specs for most all tubes used in older ham gear.  Newer ARRL Handbooks
have done away with this info and cost close to $40 or so.  An RCA transmitting tube manual
is also a good deal for a couple of bucks.  You might find some good stuff at  used book stores
(the older and dirtier the store the better the bargain).  Terman and Orr are good authors.

Allen KA5N
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W5JO
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Posts: 59




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« Reply #9 on: August 12, 2012, 02:50:09 PM »

Not only that Allen but the individual circuits for every stage in the 500 is in the old handbooks.  Norm you should find one before you begin to work on the modulator deck, the build may very well be different than the diagram you have and what is there is straight out of the handbook.  The pin numbers will be correct there.
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AC5UP
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Posts: 3956




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« Reply #10 on: August 12, 2012, 03:26:33 PM »

Bookstore?   We don't need no stinking bookstore......   You can download everything you need here:   http://tubebooks.org/

ARRL Handbooks, Bill Orr (W6SAI) Radio Books, plus tube manuals from RCA, Sylvania, GE, Eimac, Western Electric and more.
Take a little time going through the various pages. The site navigation doesn't offer instant gratification but there is a buttload of material to be had if you're willing to open your eyes and Seymour.

BTW: Take a look at the Frank Jones antenna book from 19 and 37. You'd think 75 years would make a night & day difference in the theory and design of antennas, but you'd be wrong.......   Cool
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K1YTG
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Posts: 225




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« Reply #11 on: August 12, 2012, 03:52:04 PM »

Thanks for pointing me to all the good resources.  I will look for old Radio Amateur Handbooks.  As well as the circuits, tube data, there are the old ads trying to sell the gear.
Norm
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K1YTG
Member

Posts: 225




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« Reply #12 on: August 13, 2012, 05:15:39 PM »

Looking at the table I have been working at I have a schematic for a Globe King 500A that clearly says 6V6 for the osc tube but has the pin connections for the later tube with the screen resistor going to pin 6.
So I was not in error in thinking that the resistor going to pin 4 was wrong. 
It did not occur to me that the schematic could be wrong.  I will learn to double check such details and not jump to incorrect conclusions.
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