Globe King 500C: Never enough radios

[N6YW]

Gentlemen
On top of all of the other projects I have going in my radio collecting and restoration activities, this
"pup" came in out of the ether and right into my life, without benefit of warning.
A rather pristine Globe King 500C, complete with tubes and the original cabinet. It appears the SK
in which it came from had it for many years and took extremely good care of it. I will be picking it up
this weekend in Orange county some 30 minutes away. The paltry sum of 350 bucks seems fair considering I never see these things come up for sale. What is even more bizarre is I was the first to call the seller. This almost never happens for me. I can make room for it and will pair it up with a National NC-300 for now.
My preliminary inquiry about modifications only pertain to non invasive type enhancements to make it
perform better and if needed, safer. My initial reading suggests that many like to disable the frequency restricting "Couplates". Does anyone have any examples of what to put in place of them?
Are there any suggestions for improving the modulation characteristics, if needed? I realize this is not a BC rig nor do I intend to pretend that it's anything beyond what it is. Just curious about the "conversation" in being a great performer within certain design limits. I am still rather new to this world so bear with me.
The examples I have heard on the air sounded very good. A couple of guys go into the patch input with external audio processing which sound quite nice. However, I think it could sound just fine with a D-104 using the built in speech amp. There is something fun about grabbing the mic, and push to talk... like my ART-13 rig. I mean, I do have state of the art audio gear in use with my FLEX 5K/4-1000A rig which kicks butt BUT, the point is about using this as close old school as possible while using a modern approach to being "better.
What gives Jeeves?

[N6YW]

After reading over the schematics for the modulator/speech amp section, it appears that removal of the couplates will require installing the appropriate plate load resistors and coupling caps. I suppose I can start with published resistor specs for the 6SJ7 & 6C5 tubes. Being that the 6SJ7 is a pentode, I will have to strap the screen together with a 1 meg resistor to the B+ and seeing how I will not be using the clipper stage at all, I can just remove the 6AL5 from it's socket.

[WD5JKO]

  You will get many opinions on this topic.

There are two weaknesses with the GK 500 series on AM. This is my opinion, so take this with a grain of salt.

First is the audio driver to the 811's that uses a single 6L6 in beam power tube configuration. They use a push pull driver transformer with a single ended driver tube. The audio driving impedance when driving the 811 grids is high, so when modulating upward, the grid current of the 811's loads down the drive. This results in high distortion, and limits the peak modulator power.

There is a very simple solution to this, and it is to go back to the GK 400 driver that used the same interstage transformer, but instead of a 6L6 single driver, they used a pair of triode connected 6F6's. This solves a number of issues, and the design stays WRL. Why Leo switched is a mystery to me..

The other issue is the RF linearity of the 4-250/4-400 RF PA tube. Some optimization of grid current, grid voltage, and tweaks to the self modulated screen grid could go far to enhance RF linearity, and to boost the positive peaks.

You could go on and on, like adding audio NFB, boosting the power supply filter capacitors (Mod B+ and RF B+), but going down that road could be a long one.

My vote would be to add a plate over the existing modulator deck, and punch holes for the GK 400 modulator driver tubes..

http://bama.edebris.com/manuals/wrl/king400/
modulator schematic is drawing 'f'.

Jim
Wd5JKO

[KA0HCP]

Centralab couplate circuits from The AM Window!
http://www.amwindow.org/tech/htm/couplate.htm

Two other references
http://pacifictv.ca/schematics/centralabcouplates.pdf
http://anniversary.ipc.org/pdfs/Book112248.pdf

[N6YW]

You guys are the best!
Update:
I am picking up the GK-500C tomorrow morning. It seems my trip to Orange will be two fold.
Drop off a 30S-1 and pick up the GK. A fine trade off for space as far as I'm concerned.
I will post some photos to share. It appears from the Craigs List photo that both sets of
HV rectifiers were replaced with those SS plug in units. Fine by me.

http://losangeles.craigslist.org/lgb/ele/4489834184.html

I am likely picking up the other two items as the prices are hard to pass on. The GREEN cabinet
for the SX-28 is pretty hideous. Not sure what that unfortunate knob job is about on the KW MB,
bit it's in nice shape otherwise. What a shame.
Billy N6YW

[KA0HCP]

Looks like pretty fair deals, especially if the SX-28 is in good condition.  A sheet of sand paper and a can of spray paint will fix the cabinet quickly.  bill

[nq5t]

I owned, restored and operated a 500B for several years, before passing it on few years ago in my slogging quest to lighten the load around here.  Still suffering from seller's remorse, but it's in good hands.

The 500B/C is a great radio.  Pull the couplates and just use the standard audio RC values from a contemporaneous handbook.  I may still have the values I put in mine  .. if you need them, drop a line, and I'll send you my docs.  It got (and I hear still gets) excellent audio reports.

There are a host of changes you could make, but IMHO, just use the rig.  Even with the couplates, until you get around to replacing them.  I fed line level audio to the phone patch input and the radio sounded great.  No,  it's not a 20V or a KW-1, but it's a good sounding and reliable transmitter.  The only issue I ever had with mine was the occasional buzzing relay, which can be solved by converting to DC.  For the price you paid, you did very very well .... I'm envious  

Grant NQ5T

[nq5t]

One other thought about this radio, if you aren't familiar with it.  The key to tuning it up is to use screen current, and not plate current.  Rather than the usual dip and load on the plate, what you want to do is PEAK and load on screen current.  As you increase loading, peak screen current will reduce, which is what you're looking for -- target around 30-40 mils of screen current.  Obviously, the specs on plate and grid current nominal values need attention paid, but tune it up on screen current.

You're really making me wish I still had my 500B

Grant NQ5T

[W3GMS]

Billy,

Congrats on your latest find!  As you said, the GK looks to be in excellent condition.  They do come up from time to time but I have not seen one in that kind of condition for a long time.  I am sure you will give it a good home!  The price that you posted shows that it was a fantastic deal as well.  If I ever found one in that kind of condition, I would pick it up as well. 

I have several pieces of WRL gear around here.  My oldest WRL is a Globe Trotter.  Then I have a Globe Scout and a Globe Chief Deluxe.  Unfortunately no heavy metal WRL pieces though.  My mentor Harry, W3FDY (sk) had a Globe Champion that he used to drive a pair of 250TH's modulated by a pair of 450TL's.  I can still see that Globe Champion sitting in his shack.  They are also somewhat hard to fine in great shape. 

Much has been published on upgrades for the GK-500 so you will have to see in time what you want to do.  Personally, I would keep it in its original physical condition and just do some soft mod's with component changes!  Keep the drill far away from the rig and especially those panels!!

73,
Joe, W3GMS       

[N6YW]

Looks like pretty fair deals, especially if the SX-28 is in good condition.  A sheet of sand paper and a can of spray paint will fix the cabinet quickly.  bill

Yes indeed. I would likely strip it and have it powder coated. The last one I did came out perfect and is nearly bullet proof. I decided to pick up the KW MB for a friend who needs one, as I already own a
pristine example in use here. I hope he has the shield cage for the RF deck. Maybe there are more items lurking at the SK QTH. I will ask the seller about this.
Getting ready to leave now. Will post later after I get back from the "OC"
73 de Billy

[Jim, W5JO]

Something to consider about the couplates.  There is little in those things to go bad.  You can check them with an ohm meter and a capacitance checker if you want.  As for replacing them to gain bandwidth, all you have to do is parallel the coupling capacitor leads with another and leave the copulate in place.  Choose the size from the aforementioned tables in the handbook.

That radio was designed for narrow bandwidth so the power supplies and all parts of the set reflect that philosophy.  Without changing a lot you probably won't get 100% modulation, so you will have to decide whether you want to make a lot of changes.  I am with Joe, be sure the carrier and audio is clean and keep the mods to a minimum.  I have an A model and it will modulate with stock iron to right about 95% positive and negative, keeping the modulation deck needle where the manual says. 

But all that is up to you, just remember the manual may differ from the actual set.  There were a lot of changes along the way with WRL equipment and each set could have different tubes or mis-marked diagrams.  Enjoy the set, it works well and sounds good when things are right.  I have owned this one near 20 years and had only one problem.

I do suggest you get the plug in SS rectifiers for it.  Then make sure to disconnect the secondary of the filament transformers and run the HV to it's destination. 

[N6YW]

Well looky here...

It works. Getting 325 watts into the bird dummy load. Peak outpoot is around 950 peak with good clean modulation before the onset of noticeable distortion. I am very pleased and am inclined to keep it
as stock now. All of the low voltage caps have been replaced before now. I had to install two new
bat switches on the power supply deck. They were broken off when the guy moved it from the SK house.
Fortunately, I had some very heavy duty types that fit in place of them.
Unfortunately, someone had drilled two holes into the front panel of the RF deck. No biggie, I can fix them.
I ended up getting the SX-28 & the KW MB for 150 bucks... silly good price. The D-104 is a little raspy
sounding and the EV 664 sounds better. Both modulate about the same.
Here the pics I promised. All of your comments are extremely appreciated guys, thank you very much!

[N6YW]

Some more...

[N6YW]

More still...

[N6YW]

And finally, put together and installed ready to accept a receiver! 
I need to get a dimmer lamp for the VFO, it's REALLY bright!

[N6YW]

Fully wired and ready for a QSO. I am in vintage bliss right now... this is so much fun!

[nq5t]

Nice!!

[N6YW]

Here is the on air recording courtesy of KJ7USA in Oxnard Ca. This was recorded tonight with his TS-590 transceiver. Not bad at all, communications grade audio that leaves me without any regrets.
What do you think?
Billy N6YW

[Todd, KA1KAQ]

Yeah, my only regret would be that I didn't find it ten years sooner! Seriously Billy, even if you had a roached plate transformer and other, more serious issues than the toggle switches, you made a great find and got a great deal. Those decks look like they were made last week - not covered with the usual decades of crud, corrosion, and other damage often seen on radios of that age.

And I'd say your audio is considerably better than communications-grade. It sounds at least as good as a 32V which is one of the best sounding transmitters of the era out of the box. Your GK sounds pretty darned good! I don't think you need to do much to it.

Now, you got a nice deal on the other two pieces too, but inquiring minds want to know: what about the old wooden 3 dial TRFs in the background? Were there more old radios or were those three and the Radiola speaker it? 

[Jim, W5JO]

That is a nice set.  One thing you might want to know.  That fan has oil holes for the bearings ,they are on the bottom side and you have to remove it to get to them.  It might be good to put a drop of machine oil in the bearings just for grins.

[N6YW]

@Todd
I wish!!!! He had a bunch of wooden radios and all of them are PRISTINE!!!!
Thank you for the compliments. I still cannot believe my good fortune and the biggest thrill is that it fired right up without a single issue. Check this out: We have a small group that we jokingly call "The Venice Gang" because the three of us live basically on the West side of LA and all three of us are heavily into vintage radio. Jay NY6L owns a GK-400 & 500A, Juan KB6KMU who owns a GK-500B and I with the 500C. I say that's pretty remarkable. All three of us got on the air last night to "Christen" my first transmission with it's fellow brethren. Little stuff like this in radio might sound a little corny to some but I get a massive kick out of it. Then John KJ7USA came on and recorded the clip for me. The West Coast AMI guys are always very into it and much like the East Coast folks, are like family. Very supportive of each other.

@Jim
I will do that. The little blower is the smallest of that type I have ever seen and it seems to do an adequate job of keeping the final cool. Of particular interest is that I am getting 325 watts of carrier and RF peak output of 1000 watts on average. The 811-A modulators are pulls I had on hand and the 4-400A is also a pull that came out of a homebrew linear given to me in non working condition. It seems I had all of the parts on hand waiting for this GK to arrive. Serendipity indeed! I even had the DPDT switches to replace the one's that were broken.

For the time being, I am going to enjoy it just as it is. I finally own a piece of gear that I don't have to restore in order to get it on the air. I also got to put the NC-300 through it's paces and it does a good job. One aspect I enjoy about having a SSB receiver for AM use is to be able to make the moronic "Southern Moss" in Texas disappear from my passband. These are SSB mental midgets who refuse to co-operate with us AM er's on 3870. Fine by me. They no longer bother me during the summer months anyway but when propagation is in their favor, I slice them up like Beets. I like that 

What an extraordinary hobby we share!

[Jim, W5JO]

I can't tell if you removed the filament transformers on the SS plug in rectifiers or not, but if you haven't, I suggest you do so.  That will stiffen the supply a bit and help.  If you have, ignore my bad eyes.  The power reading you are seeing out is probably about right but that is dependent on the meter you are using.

I don't know if you plan to try to use it on 160 or not, but if you do, remember it is designed for 300 ohm output.  If you try to load into a 50 or 75 ohm load, you will burn that coil on top of the RF deck.  I made a modification to mine to allow it to load directly into 50 ohms without a tuner to match the 300 ohms and I don't recommend trying to use a balun between the output and the antenna.

[N6YW]

I can't tell if you removed the filament transformers on the SS plug in rectifiers or not, but if you haven't, I suggest you do so.  That will stiffen the supply a bit and help.  If you have, ignore my bad eyes.  The power reading you are seeing out is probably about right but that is dependent on the meter you are using.

I don't know if you plan to try to use it on 160 or not, but if you do, remember it is designed for 300 ohm output.  If you try to load into a 50 or 75 ohm load, you will burn that coil on top of the RF deck.  I made a modification to mine to allow it to load directly into 50 ohms without a tuner to match the 300 ohms and I don't recommend trying to use a balun between the output and the antenna.

I won't be removing the filament transformers and it's unlikely that operation on 160 meters is in my immediate future without the needed room for the antenna. I wish.

[N6YW]

If I removed the filament transformers, you mean just removing the primaries from circuit, and lifting the center tap leads and attaching them to the outputs of the SS rectifiers, correct?
Explain.
Thank you.

[Todd, KA1KAQ]

I think that's what he means, Billy. I'm a little confused as I thought the plug ins just open the filament circuit anyhow, but it would still be good to remove or otherwise isolate the leads. Not familiar with the circuit so I'm probably missing something [obvious].

Glad to hear you're not going to perform major surgery on the old girl. A lot of us AMers get wrapped up in sounding good and lose sight at times of the fact that we're using amateur, not professional gear designed for HiFi. I'm always willing to make some 'soft mods' as Joe calls them that don't require drilling, blasting, or major surgery. Beyond that, it's easier (and a good excuse) to get something else more purposely designed for the job. 

Those old wooden sets are near and dear to my heart. It's always interesting to see what turns up. And it was a great way to divert my GK-envy. I've wanted a Globe King for years (even have a once-NOS cabinet in the garage)since seeing Brian/W1LYD's highly-modified 'Glob Queen' 400 back in the 90s. Manage to talk myself out of it each time due to downsizing, too many projects, and all that. Sure, Leo's crew cut some corners and used surplus parts or whatever else, but to my eye they are the perfect table-top transmitter and very well laid out visually. Even the Collins KW-1 cut some corners, especially in the audio dept.

Maybe some day I'll find a set of grubby chassis that need a home.