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 Post subject: HV power supply w/ 866a
PostPosted: Dec Sun 10, 2017 2:57 am 
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All,
I'm rebuilding a HV supply built by a silent key, as the chassis is not strong enough to hold the components, the wiring was poor and the capacitors had too low of a voltage rating, and there was no bleeder.

I'm mounting this on a bigger chassis and using parts on hand. This is an 866a full-wave bridge, with two LC sections 27H/1.5uF and 9H/5uF with a 100k 100w bleeder. I'm targeting variable output between 1250 and 2500v @250mA. I only have a few chokes (27H, 16, 9, 2) that can handle the current.
On simulation I'm able to get about .003% ripple.

This is my first time working with 866a's.
I was reading through the 866a tube data and two things stuck out:

1) sets of Minimum L and maximum C values are given for different voltages and it says "...selected to hold peak surge current within maximum ration"
So I reduced the capacitance of the first section to 1.5, can someone tell me what the surge is on power-up and what the ramifications are if I were to use a larger than recommended capacitor.

2) "if two unequal chokes are used, place the larger choke nearer to the tube" ...no problem, just curious why??

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Sun 10, 2017 3:11 am 
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What is .003% ripple?


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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Sun 10, 2017 4:12 am 
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2500vdc has about 8v of ripple.

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Sun 10, 2017 7:04 am 
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Greetings to Jeffery and the Forum:

As far as I know, if you use anything other than a choke input filter with mercury vapor rectifiers, especially big ones like 866's, you are asking for trouble. Mercury vapor rectifiers are extremely touchy about peak currents as well as PIV.... they will flash over at the drop of a hat. Also, don't forget hash filters.

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Sun 10, 2017 3:29 pm 
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I'll emphasize what Jim T said. In spades! 866's can flash over,
presenting (almost) a dead short to your transformer. If you've never
seen this happen, you are lucky. Before applying HV to 866's you need
to run the filaments for 10-15 minutes to vaporize any mercury that
has gotten on things in the tube it shouldn't have. And an important
thing....don't forget to put in bleeder resistors.

While I still have transmitters that use these, those that are in
regualr use have been converted to solid state. They run cooler
and are far more reliable. But they do lack the light show 866's
put on. Especially in AM and CW transmitters.
Steve

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Sun 10, 2017 4:23 pm 
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You will need bleeders, or the supply voltage will soar to peak voltage will no load.

Also, most of those supplies used swinging chokes to improve regulation. Ripple
also is related to power drawn by the load.

Pete

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Sun 10, 2017 10:44 pm 
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Location: Powell River BC Canada
If you do not wish to meet the previous builder, your cabinet layout will include
a power interlock.

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Sun 10, 2017 11:53 pm 
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Yes, I'm putting in a bleeder, and the cabinet I'm using does indeed have an interlock.

This will use choke input filters as I mentioned.
Also, I did run the existing supply for 30 mins (filaments) before applying HV and had no issues.

Again, I'm wondering about the peak current using the maximum Capacitance specified in the data sheet, and also Why the larger choke of the two has to be closer to the tubes.

thanks gents.

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Mon 11, 2017 7:48 am 
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With choke input filters, there is a real danger of the first choke, and the capacitor
following it, forming a resonant circuit, resulting very high voltages across the L and C.

The maximum voltage at resonance will occur when L = C.

27 uFd and 1.5 uFd resonates at 25 Hz

The voltage at 120 Hz will be 2*pi*120 * L .

For 30 H that will be 22,600 volts/amp. at resonance.



c'mon, riddle that, i'm too tired to look up squat :D

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Mon 11, 2017 12:32 pm 
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Location: Long Island NY
Mercury vapor rectifiers have extremely sharp conduction curves. Basically nothing happens until the plates are 8-15 volts more positive than the cathodes, then you've got almost a dead short until the tube extinguishes at the end of the half-cycle. That sharp spike every half cycle can cause all kinds of LC resonance problems at any frequency possible to get a "ring," and can radiate into other curcuitry. For that reason it is common to see RF chokes in the plate leads of the tubes. This is also why the current has to be limited by impedances in series with the rectifier (why they tell you to use a choke input filter): if there's enough power available in the transformer and nothing to limit the current on the other side, the filaments and maybe the plates will dissappear in a brilliant ultraviolet fireworks display.

Anyway, if you are going to put two chokes in series, it makes sense to put the bigger one first. It will absorb most of the surging, then the smaller choke can finish the job. If you put the smaller choke first it may be overwhelmed by the surges (i.e. it will reach saturation sooner) so it will be less effective.

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 Post subject: Re: HV power supply w/ 866a
PostPosted: Dec Mon 11, 2017 4:23 pm 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
Choke input resonance questions have been on ham, and radio trade exams for as
long as that circuit has been around.

Attachment:
Canadian Ham exam abou 15 years ago.jpg
Canadian Ham exam abou 15 years ago.jpg [ 95.75 KiB | Viewed 1442 times ]

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