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THE AM BULLETIN BOARD => Technical Forum => Topic started by: WB4AM on August 26, 2021, 08:55:52 AM

Title: High Voltage VOM
Post by: WB4AM on August 26, 2021, 08:55:52 AM

Looking to build a high power AM transmitter.

What is being used these days to test voltages  as high as 3000 volts?

Any calibrated load ideas for standard 600 volt VOMs?

Thank you.

Title: Re: High Voltage VOM
Post by: KD1SH on August 26, 2021, 10:20:41 AM
Check out something like this:

Others make similar things: Simpson, B&K. You'll need to know the impedance of your meter.

I use a 12 KV panel meter I picked up at a hamfest, with appropriate series resistors, enclosed in a plastic cabinet with high voltage test cord and alligator clips.

Order of operation is important:
1> Power down the equipment.
2> Clip your meter leads on.
3> With no hands/fingers in the equipment, power it up.
4> Observe the meter, but don't touch anything.
5> Power down; allow time for caps to bleed down.
6> Remove the leads.

Obligatory comment: picture your wife/loved ones finding you dead on the floor. 'Nuf said.

Title: Re: High Voltage VOM
Post by: w9jsw on August 26, 2021, 10:36:37 AM
Those probes are expensive!

On my 813 rig, I built the meter R ladder circuit on a piece of lexan that was going to eventually be installed in the transmitter. Then took that to one of my other amps that already had a calibrated meter. I then "carefully" hooked up the new meter and circuit. Then in a series of turning it on and off steps, adjusted the pot on the circuit to have it register the same voltage as the calibrated meter. Then moved the circuit and meter to the 813 rig. I never adjusted the pot with any power present on the rig, just to be safe.

I will post the circuit if you need it.


Title: Re: High Voltage VOM
Post by: W1ITT on August 26, 2021, 11:08:38 AM
The Simpson 260 series VOMs had a number of variations.  Some of them will read directly to 5kv.  They are occasionally available at hamfests and sources such as fleabay for not a lot of money because the new kids think they look funny.  A bit of shining up and, generally, a new set of leads will serve you well.
Tripplett had a model similar in appearance to the Simpson and they went up to 5 kv as well.
Digital meters are swell, and I like my Fluke 87, but a quality analog meter deserves a place on the test bench as well.
73 de Norm W1ITT

Title: Re: High Voltage VOM
Post by: KD1SH on August 26, 2021, 11:16:29 AM
I love the old Simpsons and Triplets! Some of mine will indeed measure 5 KV directly, but given their vintage, the notion gives me the willies. Bakelite shrapnel would probably sting!

Title: Re: High Voltage VOM
Post by: Detroit47 on August 26, 2021, 06:14:06 PM
Buy a high  voltage probe if you value your life. In assembled equipment use a meter and multipler resistor. All you need to know is the fs of the meter. A meter with a 1ma fs is 1 Meg per 1000 volts.

Johnathan N8QPC

Title: Re: High Voltage VOM
Post by: MikeKE0ZUinkcmo on August 26, 2021, 11:23:50 PM
...In assembled equipment use a meter and multiple resistor. All you need to know is the fs of the meter. A meter with a 1ma fs is 1 Meg per 1000 volts...

Built in would be my option also, but don't forget to observe the MAX voltage rating of the resistors in your "string" feeding the meter. 

Title: Re: High Voltage VOM
Post by: KD6VXI on August 26, 2021, 11:36:28 PM
I have a Heathkit TV meter, rated to 40kv.

I use it in conjunction with a Hvac DMM and it reads very accurately.

I used to have one of the Tripplet 5kv meters, that was awesome. Miss that meter.

Also, make SURE ground is hooked up when using a probe.  Otherwise the meter will float at B+.


Title: Re: High Voltage VOM
Post by: Opcom on August 27, 2021, 03:13:49 AM
I have the Fluke probe, and like it. You can make your own probe or divider and I've done that too but like he said my life is worth more than the cost of a professional probe, or the money saved on a badly-made homebrew probe.
Homebrew 'probe' is a misnomer for me. I put the series resistors in a piece of tubing, hook it up, and don't touch it while voltage is present.

Title: Re: High Voltage VOM
Post by: WB4AM on August 29, 2021, 09:21:03 AM
Thank you to all who replied.

I do have this HV Probe that I picked up from a Ham Fest a while back, don't know who makes it.

Also I have another one if I ever come across it, its packed away since I moved from Pa to Tn.

My father had given me a Simpson meter way back when I was a teen, it is also backed away.
Maybe I should hunt that down and just go back to that. 

I did click onto the link to the Fluke HV probe and looked it over.

Eventually I will have a built in meter for the yet to build transmitter, but I need a good HV meter to
check and test voltages during the build.  One example is, I need to check out or test any of the HV transformers
that I have to be sure I can even use them.

Again thank you for the replies and the advice that were given.
Those of yous who are always  here to help are the guys who I respect and and look up to.
Without the help from everyone, I am just simply working in the dark, and that can be a bad thing when working with High Voltages!

Seriously, Thank you.


Title: Re: High Voltage VOM
Post by: KD6VXI on August 29, 2021, 09:31:57 AM
That is the heathkit probe.  If I was at home I'd send you a picture of the nameplate (Mines still got the sticker.  Make SURE the meter is connected to ground when using that probe before hv is applied....  It's nothing more than a series dropping HV rated resistor!

I have the same one.

Good probe.  I like mine.

Having been hit with 6kv a couple years ago, I must say, be careful!


Title: Re: High Voltage VOM
Post by: WD5JKO on August 29, 2021, 11:07:47 AM

There might be more than one Heathkit HV probe. I attach the Model 336 here.

It is intended to mate up to a Heath Kit VTVM with an 11 meg-ohm input

Whatever the VTVM reads, multiply by 100.

If you use a 10 meg-ohm input DVM, the reading will read low by 10%.

If my assertion is correct, (BIG IF!) multiply the DVM reading by 110 to get the actual voltage.

Try it out with a low voltage where the DVM can be used without the probe, and then connect the probe, and compare.


Title: Re: High Voltage VOM
Post by: va3dxv on August 29, 2021, 12:58:12 PM
That looks just like the one I have. Heathkit model IMA-100-11, it's marked "x100 for 11Mohm inputs"


Title: Re: High Voltage VOM
Post by: Opcom on August 30, 2021, 11:11:27 PM
Take note that many of the TV-anode-voltage rated probes are not designed to connect to the high TV anode voltages for long periods of time. The precision HV resistors can overheat. Just because they are long-bodied does not mean high wattage. It is especially true of the units with a built in meter and many of them carry a warning in the owners manual. This document will of course have been carefully preserved by all previous owners of such probes and accompany same for the benefit of the next owner.

If for example, the probe's internal resistor is 289 megohms, and meant to be used with a 11 megohm meter, that chain will consume 100uA. The total is 3 watts at 30KV.  

BTW if the probe unscrews, carefully examine the resistor to know its value and check the solder connections to it and ground wire if any.

Have fun!

Title: Re: High Voltage VOM
Post by: WB4AM on September 01, 2021, 03:47:03 PM
Thank you all who commented and gave great advice along with all the info too.

I certainly liked the idea of testing at low voltage without the HV probe and then testing the same low voltage with the HV probe to figure out the difference between the two voltages.

Also the idea of making the connections to the meter then switching the supply on without touching anything, great advice!  Oh, and be sure to ground the meter as well was also mentioned.

And the amount of time the voltage is being testing, very good point.

If I didn't mention, thank you for the links as well and the suggestion of using a HV probe, good sound advice.  The pic of the probe to help identified also helped by the way.

And John, if you still would like to post the circuit that you mention, it may be something I could use, can't hurt to post either way, Thanks.

Now to start hunting down all the parts that I still need!

Title: Re: High Voltage VOM
Post by: w9jsw on September 02, 2021, 06:49:45 AM
Here it is -

The circuit should be 500K per 500V of HV in the string, but when I calibrated it I needed to add 2 more Rs to get the proper calibration. Be sure to use HV rated resistors.


Title: Re: High Voltage VOM
Post by: WB4AM on September 03, 2021, 12:12:06 PM

Hello John and thank you for posting the circuit.

I down loaded the circuit and with the pic being condense if you will, I tried
to enlarge the pic but it just kept getting blurry.

Would it be much trouble to ask if you could replace the original with a larger one?

Maybe I could take a pic of it with my camera phone and enhance it some how.

Thank you John.

Title: Re: High Voltage VOM
Post by: w9jsw on September 03, 2021, 02:48:19 PM
I will email you the whole schematic for the transmitter.

Title: Re: High Voltage VOM
Post by: WB4AM on September 03, 2021, 04:13:30 PM

That would be great!

Thanks a bunch John.

Title: Re: High Voltage VOM
Post by: WB6LSI on September 10, 2021, 04:25:52 PM
I don't think its a good idea to use a old VOM with high voltage range or a old high voltage probe. Many years ago we had a technician killed trying to measure high voltage using Simpson 260 on the 5000 volt range. The case broke down and created a pathway to complete the circuit. Today, you can't find test leads rated for voltages above 1000 volts.

Title: Re: High Voltage VOM
Post by: W7TFO on September 11, 2021, 12:25:55 AM
When I build transmitters, I use vintage voltage multipliers:  1mA @ whatever E they are marked.

They look like big fuses, and a usually made by Weston.  Companion meters are available, already calibrated to that 1mA/kV standard.

Very accurate, and easy to use without a VOM.

Sourced thru eBay from ex-navy or broadcast transmitters.


Title: Re: High Voltage VOM
Post by: K4RT on September 18, 2021, 11:42:28 AM
I have a Heathkit VTVM and Heath HV probe that looks a lot like the one in the photo above. I haven't had any problems measuring up to around 3KV. You can't be too careful measuring HV or really any voltage. I'm a hobbyist, not an engineer, so to try to eliminate the risk of electric shock, I spend considerably more time thinking through what I'm about to do than I do taking measurements. A lot of the folks here already know all this and do it safely. It's been a while but I think safety measures are covered in the old ARRL and Bill Orr handbooks and maybe some of the manuals for military tube gear.

Unless you are using a professional grade DMM, it's more likely than not, at least in my experience, that the maximum DC voltage rating of a DMM imported from the Far East is 600V and if the specs say 1KV I wouldn't necessarily bank on it.
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