Minimum choke size?

[KA9EGW]

My brain's just a-spinnin' in info-overload.  Got me a transformer rated 2KVA, one 240V winding and one 2400V winding, so I expect I'm going to *have* to go choke-input for my pair-of-813's project.  Trying to find a choke good for half an amp at anything measured in Henries is proving challenging...but I'm finding lots of 100 mH, 250 mH, etc...what I'm not getting clearly from the Handbook is a good feel for how small a choke I can get away with...or should I just look for a deal on a 240V variac and go capacitive-only?  73, Brian KA9EGW

[Opcom]

There is a handbook formula for critical inductance which is a minimum value for good regulation. Using a choke at least that size will be good. Its based on the minimum amount of current to be drawn, maybe the bleeder current plus the tubes idling current.

L henries= load ohms / 1000

so the example.
if the B+ is left on all the time:
If a pair of 813's draw 50mA at 2KV idling, then the resistance is 40K.
If the bleeder is also 40K then the load ohms is 20K.
20,000 / 1000 = 20. L=20 henries.

If the B+ is keyed with carrier:
pair of 813's drawing 150mA, plus the 50mA bleeder = 200mA. 2KV/200mA is 10K Ohms.
10,000 / 1000 =10H.

A smaller choke may not keep the HV from climbing too much at the lower current levels.

Is that the situation you are asking about?

A 500mA choke is plenty large for a pair of 813's, maybe a 300-400mA one would be OK and easier to find, but I always go full on ratings myself.

[N2DTS]

The Bill Orr handbooks had a lot of great circuits for power supplies, with diagrams for every kind of power supply for every size rig.
I find the info in those handbooks much more usefull and easy to fine then the ARRL handbooks.

Besides power supplies, pie net coil/Q sizes, modulator diagrams, all kind of great info in the books....

I never go light on power supplies, if I need 400 ma, I use a choke rated at 500 ma CCS, and rated for higher voltage.
Things get large, but I have rigs I have built that have not had a crap out in 35 years!

[WQ9E]

Opcom covered the calculations and if your transformer rating (secondary current) is based upon a choke input filter you either have to use a choke input filter or de-rate the secondary for the higher recurrent peak current and resulting transformer heating that comes with using a capacitor input filter.  The ratings for most vintage plate transformers assumes a choke input filter, some have an additional lower rating for capacitor input filters.

Some commercial rigs, like the Johnson Desk KW, biased the modulator tubes to draw a little current in some non-AM modes to provide sufficient current draw to avoid an excessive large bleed resistor to meet minimum current draw with a reasonable size of choke. 

I agree with DTS that the older Orr "West Coast" handbooks are far better written than the ARRL books, particularly when you look at the later post WWII ARRL versions.  You probably aren't using MV rectifiers but if so pay attention to Orr's note about avoiding resonance effects.  Orr also addresses the danger of putting the choke between the secondary center tap and ground with transformers not rated for this application.  The ARRL books emphasize the ability to use a choke with lesser HV insulation ratings while glossing over potential loss of the plate transformer from floating the center tap at ((current X choke resistance)+ripple voltage) above ground with transformers designed for a directly grounded center tap.

[KA9EGW]

Opcom, you covered it quite well, and I thank you!  I suspicion I'm going to be searching for an appropriate choke for a while...

[WQ9E]

I believe Hammond's highest voltage rated chokes are 1,000 VDC with values including 500 mil 10H.  They will safely stand a bit more but I sure wouldn't try to double the voltage rating.  I don't agree with the practice of floating the choke above ground (case insulated from ground) to allow it to operate at higher voltage but some people do mount them that way.

If you find a couple of chokes that will withstand the voltage and current but don't have enough inductance you can use them in series which will provide the sum of the individual inductances.

[KA9EGW]

This is going to be interesting...I'm weighing trying to find a ~20H, 2+ kv-rated choke, vis-à-vis going with a [cheap linear amp type] capacitance-only deal and a big variac...the price was VERY right on the transformer, but the secondary is not center tapped, which kind of limits my rectifier options HI.

Has anyone here had experience with resonating a too-small choke with a paralleled cap? 

73, Brian KA9EGW

[WQ9E]

Brian,

If your transformer secondary current rating is sufficient to be OK with a capacitor type filter you can use a fixed transformer to "buck" the primary instead of a Variac if you just need a fixed lower voltage from the plate transformer.  The transformer used for bucking needs  a secondary with the proper voltage and with sufficient current rating to carry the primary current needs of the plate transformer.

I am not sure if you are currently operating AM but if so consider checking into the Midwest Classic Radio Net on Saturday morning 7:30 local time on 3.885 with your want for a choke.  There are quite a few old-timers from this part of the upper midwest who check into the net and in recent years several have made "iron" available.  If you aren't currently on AM let me know if you would like your want announced. 

[KA9EGW]

Hope to see you there Sat!  My "AM" [at this exact moment] is whatever comes out of a ricebox [ts-850]; might be one sideband plus carrier...way too many resto projects and way too little time around these parts...

[WQ9E]

Brian,

I will be in Peoria for the hamfest this Saturday morning as will several of the regular net members but there will be a net and I could contact the net control to announce your "traffic".  The "pre-net" gets started around 6:45 and that would be a good time to get some help setting your TS-850 audio levels if needed.  Quite a few folks check in with more modern gear ranging from 70s era transceivers that run AM as an afterthought through current FLEX radios and then there are other high level AM diehards like myself and I usually use either a Ranger/Desk KW or Viking 500.  A little more fine tuning and my BC-610E transmitter along with a Millen exciter with Millen 500 watt amp and homebrew modulator will be ready to go.  But when I first started checking in I used a pair of Yaesu 101 twins and they worked just fine. 

I expect you will be able to round up a suitable choke via the net without having to travel or have it shipped a great distance.  It took awhile but I finally found a 45 henry modulation reactor choke after several weeks of looking.

[KA9EGW]

Oh yes, excellent advice--page 715 of the 15th edition of the Orr handbook, has a circuit that is perfect for a pair of 813's AND uses the exact transformer I have.  There is still the choke issue. 

Actually, I have a scrap power transformer out of a Johnson T-bolt, potted in hard compound and with one side of the secondary open, I'm thinking maybe I can just wind my own choke if I can figure out how to get it apart without destroying the bobbin...has anyone ever tried to dissolve that hard potting compound by chemical means?

[KA9EGW]

While I'm on the choke issue, I have a spare non-potted 1KVA E-I core transformer surplus to my needs...I wonder what the inductance of the 2.4kV secondary on it would be connected as a choke?  Time to dig in the brain cobweb vault...just kind of thinking out loud here...

[KA9EGW]

If my calculations are correct, 2640 turns [assuming 1.15 turns/volt] on a 2" x 2" iron core 3" long [i.e. the center tongue of the e-laminations are 2" wide and 3" long stacked 2" high], comes out to 18-odd Henries.  I'll plug in actual numbers as soon as somebody confirms my formula is in the ballpark.  I suspicion using that spare 1kVa transformer as a choke is going to work out to a lot more than 18H.

If I'm way off the beaten track here, someone please tell me.

[KA9EGW]

It's not like designing a p/s for a pair of 6146's which is a comparative cakewalk; that uses parts you can find anywhere...I am confident that at the very least that spare transformer's core and bobbin put me >halfway there...

[wa3dsp]

>Has anyone here had experience with resonating a too-small choke with a paralleled cap? 


The Collins 516F2 power supply uses a parallel cap across the choke in the HV supply. It limits the voltage on light loads. The HV supply is suppose to deliver 800V at 200mA but with just bleeder current and no cap the supply can put out well over 1000V. With the cap it maintains around 875-900V. When using a capacitor like this it has to withstand some high peak voltages and also it needs to be resonant based on ripple frequency - 100  or 120 Hz and the minimum current.

[N2DTS]

A pair of 813's makes for a nice rf deck.
I built a pair for 160-10 meters about 20 years ago, variac power supply good for 0-3000 volts @ 500 ma, forced air cooling for the 813's, vacuum variable and big roller inductor ganged for tuning.
700 watts carrier output, modulates nicely, and in 20? years no crap outs, same tubes in the rig!

I was never fussy about RF deck power supplies, choke input, big energy storaage cap in the output, four 1 amp 17,000 volt diodes...

Two modulators, a pair of 4cx250b tubes in AB1, seperate power supply, 2000 volts (variac) @400 ma gives 600 watts of audio. I also built a push pull paralel 100TH modulator, tubes behind glass, just because they look cool? glowing bright yellow and pulsing under modulation.
100TH/TL tubes are some of the best tubes for visual impact!
Funky looking even when they are off...

Surely someone close by has a choke (or 6) they are not using.
I have one or two, they are ugly open frame units out of a 4-400 pager transmitter that ran 3000 volts at whatever current in a bridge setup you could have.

I used to pick up UTC cg chokes at fests, new, 7000 volt 500 ma ccs for $20.00, but they are all in service.

I also have a crap load of oil filled caps I need to move along, mostly 600 to 1500 volt jobs...

[KA9EGW]

Sounds good.  Ugly is as ugly does, and the end bells off this shorted potted t-bolt transformer might fit, and if not, so what?  Gotta go to a funeral for a friend's father in law leaving 5 minutes ago; can I catch you up when I get back?  vy qro tnx

[N2DTS]

If you send me your email address, I can send a picture of the choke.
Its not ugly or nasty, just open frame, which does not match my other stuff.

[KA9EGW]

Ka9egw@britewerkz.com

[KA9EGW]

Ka9egw@britewerkz.com

What do you need for one of thosr chokes?

[N2DTS]

The price of shipping.

Ka9egw@britewerkz.com

What do you need for one of thosr chokes?

[KA9EGW]

Brother you got a deal!