Globe King 500C: Never enough radios

[N6YW]

Updates:
After purchasing and using the pole saw to remove the tangled mess that snagged my antenna, all went well after that. The Cobra Ultra-Lite Jr. does in fact work quite well considering it's limitations. I am very pleased. Locals and out to 500 miles have reported up to a 3 db increase in signal strength over the vertical which is not surprising. I am not an avid DXer, so arm chair rag chew QSO's are my thing on 75 & 40 meters. So far, it's worked wonderfully.
Now the sad part.
A couple of nights ago, I was using the Globe King 500C and I received reports of distortion on my audio.
I hadn't touched a thing since I had been previously given excellent audio reports. I investigated using the dummy load to re-tune, and modulate my signal while monitoring with headphones and receiver. Sure enough, it sounded like voice peaks were clipping badly with a nasty note, a brief one at that but consistent. Turning down the audio had little effect. I got to thinking audio tube path or bad mod iron.
Then, while retuning in CW mode to make sure it was loading correctly, The plate current shot up and blew the 20 amp fuse on the power supply. Okay. My bad. After replacing the fuse, I no longer get a dip on my plate current meter. I key up and it sits at 200 ma and doesn't move from there.
I pulled the RF deck out of the cabinet just to have a brief inspection of the top sides. Everything is fine looking but I need to remove the bottom cover and see whats up. I also need to measure the HV supply and screen supply to see if maybe I smoked one of the plug in SS rectifiers.
At this point, and because this rig is such a bitch to isolate and troubleshoot without making a custom test harness to spread the work outside of the cabinet, I am left to measuring components at the static level not energized. RF, Mod & supply decks individually. Grrrrrr.
So, what suggestions might the group have to direct me in a manner consistent with good practice concerning this radio?
Man this thing was sounding fine before all of this came about.
Thanks,
Billy N6YW

[Jim, W5JO]

Take a close look at the switch that is used to switch in fixed capacitance and if all the fingers are there, pull the caps and inspect those.  Be sure the antenna relay is closing, and check the bandswitch for melted contacts.

[WD5JKO]

In addition to what Jim (W5JO) said, I have a couple of comments.

You can set the three decks on the floor making a circle. That way each deck is accessible positioned upside down or upright. Make sure the chassis's are bonded together and grounded. You don't want any surprises.

I would rule out the modulator deck being an issue by routing the B+ straight from the power supply deck to the RF deck. Keep the modulator filaments off.

My bet is that something croaked in the RF deck. With no drive (CW Key UP), there should be no plate current. Tackle that DC condition first. Switch to linear mode (again no drive), then see if you get the usual idle current (50ma?). If the DC tests fail, look at that plate current meter. Is it wired into the cathode side? If so, consider the possibilities. One might be a blown plate choke, and that 200ma you were seeing was screen current! 

Have a plan, and be careful.

Jim
Wd5JKO

[N6YW]

Well looky here 
It seemed the screen parasitic suppressor (buffer plate) committed suicide. It's a 1 watt 56 ohm
carbon resistor. Can I use 47 ohm without much difference?

[N6YW]

I ended up finding a an exact replacement in my stash of vintage parts.
 

[N6YW]

Replacement has been completed. I added some rubber insulation tape to the outside of the small transformer next to the solder tab for the PS-1 I replaced. There was evidence of a possible arcing
on the lamination of that transformer. It was indeed very close!

[WD5JKO]

  Remember that the resistor that blew apart had to have seen significant VHF energy to get hot like that and break. This goes along with a VHF parasitic to the extent that the 20 Amp main fuse popped. You are finding peripheral damage, and not enough to explain all the symptoms.

  I have run a GK500B, and I recall specifically dealing with the 4-250 screen bypass capacitor where I replaced what I recall to be a single big mica-mold .002 uf with four 500 pf VHF button capacitors where I had two going to each screen pin, and returning to ground at each of the four socket hold down bolts. That kept the screen at RF ground, and a tendency for the big 4-250 to soar at VHF was dramatically reduced. That RF tube is not neutralized, and it really should be.

  Was that new antenna you put up having a different tuning solution of the GK-500 pi-net? I ask because that might have put the King at a different point such that stability was closer to the edge of carnage.

Jim
Wd5JKO

[N6YW]

Jim
You bring up a good point.
First, I inspected and checked all of the components inside the RF deck. No shorts or opens other than the PS-1 I replaced. Having said that, I will put it back into service to see if there is an existing problem not
found through static checks. I have the feeling that the resistor may have been cracked from previous use prior to my ownership... just a hunch.
The new antenna loads up fine. When I had the blow out, I was tuning into a Bird 2KW load, purely resistive, and I used no loading in the tank circuit. I was getting roughly 300 ma. with 345 watts output before all of this occurred. I will advise in a little while what my findings are.
BTW, I oiled the blower motor bearings too following an earlier posting advisement.

I am going to pull the Modulator deck to have a closer look at things to see if maybe there was an arc, loose connection or anything that might point to that distortion issue. It's using an old metal case 6L6 tube which I think should be replaced with something that sounds better. I will see if the bypass caps in the audio chain need to be replaced also.
Thanks again, and more to come.
73 de Billy N6YW

[KA0HCP]

If you did not go thru the radio, checking the values on all the resistors it is more likely that it simply was out of tolerance.

[Jim, W5JO]

The problem you experience is not totally unknown, I have seen it before.  I would loosen that nut and rotate that terminal board away from potential problem.  Beyond that, check the components around the 6146 to see if you see anything amiss.  

There should be a choke in the plate lead of the 6146, look at it for evidence of heating and problems.  The diagram shows an RF choke feeding bias to that 22 ohm resistor, is it there and what is its condition?  From what I can tell work has been done in that area, be sure all bypass and blocking caps plus other components around the 6146 are good and placed correctly.

[N6YW]

The problem you experience is not totally unknown, I have seen it before.  I would loosen that nut and rotate that terminal board away from potential problem.  Beyond that, check the components around the 6146 to see if you see anything amiss.  

There should be a choke in the plate lead of the 6146, look at it for evidence of heating and problems.  The diagram shows an RF choke feeding bias to that 22 ohm resistor, is it there and what is its condition?  From what I can tell work has been done in that area, be sure all bypass and blocking caps plus other components around the 6146 are good and placed correctly.

Done & done.
I found that way back in the day, someone had replaced the bypass capacitors in the audio stages of the modulator deck (servicing now, removing couplates) and low and behold, the ground of one cap wasn't even soldered... it wasn't even making good contact with the ground solder lug. All of that has been sorted out. I added a 1.5k swamping resistor to the control grid of the 6L6 right on pin pin 5. Even though there is a 500k grid stopper to ground prior to this, that alone isn't enough to prevent audio parasites. Interesting how the 6SJ7 Pentode is being used, and with just a 47K input resistor from the mic input, shouldn't I consider adding like 4 megs for using the D-104? Let me know before I close it up.

[N6YW]

Back to the RF deck. No output and the screen meter reading pegs to the right.
On the bench she goes.

[N6YW]

I overlooked the obvious...
However, I have an identical spare. Now, should I try repairing the burned one and install a parasitic choke on the anode, or use the new one with a PC?

[N6YW]

RF deck fixed. 300 ma. @ 325 watts of carrier.
Back to the modulator deck. It's a comin.

[N6YW]

It's fixed.
There was a problem with the print of my schematic that I corrected. Back on the air and it sounds
excellent without the distortion! Maybe I should try one of my EL-37's or KT-66's for the audio driver.
Hmmm.

[WD5JKO]

Maybe I should try one of my EL-37's or KT-66's for the audio driver.

   It has been a long time ago, but I did something similar to one of Ozana Bob's (W5PYT SK) Globe Kings's.
I used a 6CA7 in place of the 6L6, and added a negative feedback loop from one of the 811 grids back to the driver tube grid using a couple of resistors in a voltage divider.

   If I was doing this again, I'd look at that driver transformer primary. Is there a third wire there that is unused? If so, this is the centertap left over when WRL used a push pull driver with the GK 275's, and the GK400's. I'd connect that center tap to feed the driver tube screen grid. This will act as an "ultra linear" of the beam power tube such that it will act more like a triode (low plate resistance) with the almost the full gain of a beam power tube. Worth a try.

Jim
Wd5JKO

[N6YW]

Maybe I should try one of my EL-37's or KT-66's for the audio driver.

   It has been a long time ago, but I did something similar to one of Ozana Bob's (W5PYT SK) Globe Kings's.
I used a 6CA7 in place of the 6L6, and added a negative feedback loop from one of the 811 grids back to the driver tube grid using a couple of resistors in a voltage divider.

   If I was doing this again, I'd look at that driver transformer primary. Is there a third wire there that is unused? If so, this is the centertap left over when WRL used a push pull driver with the GK 275's, and the GK400's. I'd connect that center tap to feed the driver tube screen grid. This will act as an "ultra linear" of the beam power tube such that it will act more like a triode (low plate resistance) with the almost the full gain of a beam power tube. Worth a try.

Jim
Wd5JKO

Jim
While I was inspecting the modulator deck, I indeed saw a clipped wire exiting the driver transformer.
It's a rather easy job to remove the transformer & end bells as to extend a new lead for such an application. I will place a bookmark in my brain on this one.
Single ended Class A ultra linear is something I have read very little about, and because I build and restore guitar amps for a living, it's a topology that I rarely consider but have heard of others doing.
I appreciate these ideas and it provokes a lot of thought of maybe doing some extra work to make this beast rock a bit better. However, right now I am just glad I have this wonderful and simple giant working.
One of the aspects of these types of forums that I love a lot, is the way in which the discussions push an individual to use their mind to work through problems, which ultimately result in a fix and sometimes a refinement.
It's quite easy for anyone to assume that just because so and so did this and that to their rig, will equate with the same results for mine or anyone else's equipment. What I am learning a lot about these old pieces, is they all have a character all their own and because their own unique history, (sometimes unknown) and inconsistencies in manufacturing, we end up finding out that we have to learn the particular if not peculiar "personality" of said equipment. This transmitter is certainly no different but I'll tell you what! After the little bugs are scraped away the design "center" starts to become more pedestrian. In other words, a little more predictable.
This learning process is HUGE! Many of the more intelligent naturally gifted electronic guru types might take a lot of this stuff for granted but to us less gifted but equally capable types like me are grateful for the experience.
Having said all of that, my troubleshooting chops got a gigantic boost today and the dividend is large.
I fixed this hairy, heavy old bastard without resorting to a lot of spaghetti. Just a lot heave and hoe.
Thank you very much. This project will continue.
73 de Billy N6YW

[N6YW]

The rig is running great today. I am now getting 345 watts of carrier at 320 ma. The audio is nice and clean, free of distortion. The particular D-104 I am using does not exhibit much energy below 200hz, but gives a very full account of itself through 4khz and gently rolling off around 6khz.
Using the display on the Flex 5000 shows a very nice envelope and frequency content.
Jim's comments about the audio driver stage really has me curious.
I could easily do the UL mod but it got me to thinking about using a better grade of SE transformer altogether. Perhaps along with that, exploring the idea of doing away with the 5Y3 rectifier and going SS for the entire audio stage HV rail. I should measure the voltage drop of the 5Y3 under peak conditions.
Anyway, just thinking out loud again but more importantly I want to thank you guys for all of the comments. Getting this rig back on the air has been quite an experience and I happy to report that my body wasn't injured in any way due to repeated moving of the decks during service.
I shall relax with a cold drink tonight and converse with a close dear friend who has made a tremendous recovery from a stroke... on AM of course.
Thank you.
Billy N6YW

[W2VW]

Great stuff.

[W6DRZ]

Billy,

It sounded really great on 3870 last night.  Congrats!

Craig, W6DRZ

[N6YW]

Billy,

It sounded really great on 3870 last night.  Congrats!

Craig, W6DRZ

Thank you Craig! It was good to hear you back on the air. Glad you repaired the antenna problem.
We had 4 Globe King transmitters on the air last night!
K6ZSR, GK-500C
KB6KMU, GK-500B
NY6L, GK-400
N6YW, GK-500C
What a hoot! I hated to go to bed. It was like having out own King Biscuit Flour Hour without the music.
We'll do it again tonight if the conditions are right.
73 de Billy N6YW