Globe King choked up

[W9RAN]

Just discovered that the low B+ choke (CH5) in the Globe King 500B I'm restoring is open.  It's rated at 7H 200ma, and while I dug up a couple from the iron collection that are the same core size (3" x 2.5" x 1") - they're both 12H which per the old catalog data makes me suspect they're only good for 80ma.   Hoping to find something that will fit the existing mounting U-frame holes and space if possible, but haven't been able to cross to a specific manufacturers part number yet.   

So if anyone would care to dig thru their musty rusty iron, I'd sure appreciate it, or suggestions where to look. 

Thanks and 73,
Bob W9RAN

[WA1HZK]

Give us the dimensions or post a link to the core drawing with dimensions.
There's a lot of those out there.
Keith
WA1HZK

[k4kyv]

It wouldn't hurt anything to use more inductance than the original, if the new choke is rated for 200 ma, or whatever the maximum current drain is through the power supply, including bleeder current.  But if you find one with substantially higher inductance, make sure the DC resistance isn't also higher, to the point of affecting power supply regulation and/or voltage output. 

[W9RAN]

Thanks guys.  It's a "U-frame" style, two mounting holes 3.56" centers or 4" across the outside of the mounting tabs.  2 1/4" thick by 2 1/2" high.  7 henrys at 200ma should translate to a DC resistance of 70-100 ohms. 

Hammond has some new ones that might work but they only rate them at "maximum DC voltage of 500V.   I've asked them if this is just a conservative rating, as you'd think with modern insulating materials they should be able to equal or beat the old 1500 V rms ratings.

73, Bob W9RAN

[The Slab Bacon]

So put the choke in the negative lead and the breakdown voltage becomes a lot less critical!! As long an you have enuff henryage to filter adequately, you'll be good to go!!

I have had a 2500v rated choke in the negative lead on my 4-1000 rig for years now with 3450V to the plate!! Its happy as a clam!

                                                                      the Slab Bacon

[WQ9E]

I can't remember if the 500B has the built in VFO, that might be only in later versions, but if it does have the built-in  VFO watch out for increased "hummmm" if you use negative lead filtering.  The earlier handbooks warn that the capacitance of the transformer secondary to ground is in parallel with the choke and this may lead to problems with excess ripple in the case of VFO or perhaps with low level audio stages.  If this "low" B+ is actually the "medium" B+ with a separate supply for the low level audio then you should have no issues with negative lead filtering.

I am surprised the hammond chokes carry such a low voltage rating.  I remember reading somewhere that an AM'er successfully used several of the hammond chokes in series for his audio reactor on a legal limit AM transmitter but I can't remember if that was a website posting or where I came across it.  It would be nice to find because I imagine those chokes were a lot higher than 500 volts above ground in his application.

Rodger WQ9E

[k7yoo]

The GK 500B has a built in VFO that has its own supply, unrelated to the low B+ supply, which supplies the screen of the 4-400/4-250 as well as the 6146 exciter section

[W9RAN]

Skip's right about the VFO, but in the 500B at least there's a separate screen supply on the RF deck, this supply just runs the exciter.   I did try putting my 12H choke in the negative lead and measured B+ right at 600 volts with only the bleeder load.  The schematic says 520V but I don't think the exciter will object.   Thanks Slab for reminding me of the virtues of negative lead filtering. 

Why didn't everyone do this?  It's hard to even find a reference to it except in the Orr handbook.  Can't see why this wouldn't be the way to go for any choke-input filter.

73, Bob W9RAN

[WQ9E]

Bob,

Orr points out two possible issues with negative lead filtering:

A.  Increased ripple due to the transformer secondary to ground capacitance partially bypassing the filter choke, again only a problem where ripple must be very minimal which generally does not apply to higher voltage supplies.

B.  Some transformers don't like having the center tap above ground (since the CT in this case would be effectively at ripple above ground) and it could breakdown internally.  This could be the deal breaker if you don't know about the particular transformer in a given rig-It probably isn't a common case but I would hate to lose an expensive plate transformer trying to protect a choke.  Of course if the choke were to open or if the power supply bleeder were to fail then the CT would rise to a much higher voltage above ground with no load.

As they say, your mileage may vary!
Rodger WQ9E

[The Slab Bacon]

Negative lead filtering is in a lot of my old handbooks. I'm not sure of all the pros & cons involved but I've never seen it used in any piece of manufactured gear. Seems to have been commonly done by HBers in the past to get around the cost of purchasing appropriately rated chokes for use in the B+ lead.

Trust me, in 99.9% of the cases it works just OK-FINE!!  However many many people just cant comprehend the application of a negative lead filter. I have given up trying to explain it to people.
But it has always worked for me!
                                                                                          the Slab Bacon

[W9RAN]

Thanks for the comments, I've posted a scan of the Orr handbook pages here:  http://picasaweb.google.com/RANickels/GlobeKingPS if anyone is interested.   It was also common for the field coil of old BC radios with dynamic speakers to be wired on the negative side.

Rodger,  except for the most critical applications, I think the effect of shunt capacitance from putting the choke in the negative lead will be negligible.  I agree with your second point, which I've seen this mentioned elsewhere.  I  tested the circuit using an 80ma choke I have, and with bleeder load the ripple voltage raises the centertap about 170 volts.   I'm confident this would be in the safe range, but would prefer to keep the original design if a suitable choke can be found.

I will relay what Hammond has to say about their max voltage rating, as it could be of value to others.   

73, Bob W9RAN

[WQ9E]

Hi Bob,

I imagine 170 volts would be safe; many of the vintage receivers open the center tap connection for standby so there is a lot more voltage there.  My concern would be with the higher voltage transformers that were never intended to have the center tap above ground, particularly if the choke were to fail open.  I also agree that the additional ripple won't be a problem in most circuits and certainly not in your Globe King supply.

All of this Globe talk reminds me that I need to get to work on the "little" Globe Champion (165 or 175) that is sitting under one of the radio tables.

Is the Globe King sufficiently healthy that we will hear it on Rob's net in the morning???

73, Rodger WQ9E

[The Slab Bacon]

Oh it indeed does work Frank but I've never been in a situation where I needed to resort to using it. I think Rodger's posted A & B points need to be considered also.

Mack, (and others),
                              Like I said before there is a very small percentage of applications where it isnt usable. But that percentage is very small. I dont advocate using it for all applications, but sometimes it comes in handy when you have to "beat the square peg into a round hole". Lets face it, chokes with 3, 4, or 5kv breakdown ratings arent all that easy to come by. But chokes of the required inductance and current rating but lower voltage rating come by a lot easier (and more cheaply). This is is a scenario where one may want to consider negative lead filtering.

I would much rather put the choke in the negative lead than trying to float it on stand-off insulators. As this is also a pathway to a possible deathtrap.  Either way will get the job done electrically, but which would you feel more comfortable with?? Not a whole lot of thought process here!! 

As far as the above ground potential on the transformer center tap it shouldnt be any more than the ripple voltage plus the DC drop across the chokes internal resistance. What could you be looking at a few hundred volts?? If the center tap on ANY high voltage plate transformer wont take that, it's probably ready to crap out anyway.

As I said before it is not always the answer, but it could be a savior most of the time. It could save you from shelling out mega bucks for a choke when you allready have something on the shelf that will work for you.

                                                                                     The Slab Bacon

[WB2RJR]

Just a note, the 1800 volt supply in the 500B uses a choke off of the center tap.

Years ago when my B+ transformer in my 500B went south, and I had no money to replace it or even actually find one (about 1976), I replaced it with a couple of Navy surplus transformers in series with a full wave bridge with the original choke in the bridge to ground. It has worked well for 30 years.

It filters the AC fine. I don't really see a reason to go with a hi voltage choke in the + lead. If you want to waste money for something no one can hear, be my guest.

73

Marty

[WA3VJB]

several of the hammond chokes in series

For the purposes of breakdown voltage, doesn't the potential divide across all of them?  Maybe do the same thing, use a couple of chokes.

I had this situation in wanting to self-modulate the screens in my homebrew 7527A rig. The chokes I had were of marginal breakdown voltage, but since I wanted to add up the inductance anyway (series connected) it all worked out.

[The Slab Bacon]

Paul,
         If you were putting the chokes in the negative (grounded) lead, and the choke's mounting frames were bonded to ground, then the breakdown potential would divide between them. But...........if you have the chokes in the positive lead, other than voltage drop from the internal DC resistance the breakdown potential would be the same on all 3.
You are just lucky, or your chokes were conservatively rated.
 
                                                                                   The Slab Bacon