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Making a SHV connector

Posted: Sun Mar 10, 2019 8:11 pm
by Rich Feldman
Would be nice to have a SHV connector on the end of a cable today. They aren't super expensive to get by mail, even brand new, for example from Field Components.
Impatient and itching to fabricate something, I undertook to make one out of BNC connector parts on hand.
Started with an unused BNC plug, and drilled out the plastic insulator. Gotchas are 1) plastic spinning in place and 2) plastic distorting in radial direction and bending the fingers of spring sleeve.
The BNC spring sleeve doesn't stick out past the knurled bayonet sleeve, as it would in a proper SHV plug. But it still makes contact with SHV jack well before the HV conductors meet. When fully mated, it almost reaches the end of slightly tapered entry section.

The inner conductor receptacle contact was pulled out of a BNC panel mount jack. Back end has a solder cup, and the part is much longer than it ought to be for a plug on a cable. I think the original BNC plug braid connection and cable clamping details would be un-usable anyway.
Then as luck would have it, my small rigid tubing bin had some 3/16" round material (PVC from model shop?) with an ID well suited for the inner contact. No turning work, and the only drilling was to counterbore one end to admit the cable dielectric.
The braid connection and cable securing detail is on hold until Monday, in case I can get use of a cable crimping tool and some crimp sleeves.

Re: Making a SHV connector

Posted: Sun Mar 10, 2019 9:29 pm
by Richard Hull
Great kludge!!!

Richard Hull

Re: Making a SHV connector

Posted: Thu Mar 28, 2019 7:10 am
by Rich Feldman
Thank you, Richard.
A week went by before I got to borrow my brother's cable crimping tool. (They don't seem to carry the type at Harbor Freight.) The smallest hex die on this tool is apparently 0.255", not 0.213" which is what I wanted. Time for another kluge, here shown without tape on one side for clarity.
DIY brass ferrule is made from 7/32" and 1/4" modeler's tubing. The small end is secured in BNC plug using the original mechanism. Turning the nut squeezes an internal tapered ferrule, as in many tube fittings. But I discovered that in this context it's called a "clamp" type attachment. "Compression" attachment for coaxial connectors is a completely different thing, and now apparently much more popular than clamp.
Now time to get back to biasing. Making a SHV set the schedule back a few weeks, and took a few fun hours in the shop.

p.s. long ago I decided to spell kluge without a "d". The word is not like fridge, which phonetically needs a letter not present in refrigerator. Kluge rhymes with huge, luge, and subtrefuge. Not drudge, fudge, or begrudge.

The d-less spelling matches that of a German surname which is pronounced with a hard g. And two syllables, like Porsche.

Re: Making a SHV connector

Posted: Thu Mar 28, 2019 5:41 pm
by Harald_Consul
Very good explanation Rich!

As your are our BNC expert in the forum, now, maybe you could give a little addition. ;-)
  1. Most of the time high voltage BNC connectors are named as MHV-BNC conectors. You call "your" voltage connectors SHV-BNC connectors. What is the difference to MHV-BNC?
  2. High voltage BNC connectors/ sockets are not compatible to signal BNC connectors/ sockets, in order not to mix them up. However, if I do not have an (SHV or MHV) HV-BNC connector at hand, would it be wise to use a signal BNC conector/socket to transport a HV? Or asked the other way round: Is the dielectric strength of a signal BNC connector strong enough for a typical 2kV supply for a scintillator?

Re: Making a SHV connector

Posted: Thu Mar 28, 2019 6:44 pm
by Rich Feldman
After logging off, leaving that question for someone else, I started a LMGTFY (Let me Google that for you).
Discovered , for the first time, a catalog that includes details of HN connector (as found on my BF3 tube, according to people at this forum).
Along with MHV and SHV. ... lectro.pdf

I think the default voltage rating of plain BNC connectors is 500 V. Maybe 500 V RMS.
Probably very, very conservative with real world parts. BF3 tube came with a HN to BNC adapter. At least it mated with a BNC with no trouble -- I better study the MHV dimensions to rule that out.

My plan is to proof test the BNC link in my system, to voltage at least 50% higher than intended operating point.

Re: Making a SHV connector

Posted: Thu Mar 28, 2019 6:49 pm
by Richard Hull
I have used and am familiar with all BNC connectors. The common BNC connector is not rated for what we would call high voltages. Carl Willis once said it would take 1500 volts. I have, in pure DC service gone to 2kv. (risky)

The MHV (medium high voltage) connector can easily cruise between 2-3kv. The SHV BNC, (super HV), can easily go to 5KV in pure DC service.

It is to be remembered that the BNC was and is an RF connector so far as the manufacturer is concerned. RF, at high voltages, has a way of finding arc paths that stable DC does not, unless air ionization within the connector is forced by even a stable DC, HV field.

The NIM and some low current DC, HV supplies adopted the various BNC connectors as they are relatively standard and inexpensive. When coupling moderate RF transmitter outputs of many watts. HN connectors are the norm and are more expensive. compared to SHV BNC connectors.

Pre-made SHV BNC, male to male cables can be very expensive, however. Kluged work-arounds, as above, can save a bit of money.

Ludlum loves the C connector and selling C connector cables. I have removed all of the C connectors from 6 of my 7 Ludlums and replaced them with common BNC connectors. I keep one as C due to having one or two Ludlum C cabled pancake GM and scintillator probes.

I explain all odd cabled items in the nuclear biz as due to the brother-in-law effect. Some one has a brother-in-law in the oddball connector biz who can't convince the world to use his connector. No need letting "sis" starve due to the whack-job she married.... LOL.

In reality, some companies have to meet a super big contract customer's nearly impossible specifications, (government, etc.), and often have to find the connector that meets those specs. Plus.....They get to be a virtual sole supplier of odd ball cables as government level idiots constantly ruin and stress them. The few normal consumers of those instruments just have to "suck it up" and whip out their credit cards. The few adroit in such matters change the cabling to match their real world needs.

In general, none of us need to have connectors that will withstand and function flawlessly in 100mph, salt sea spray mist conditions for 48 hours or remain arc proof in 40% hydrazine atmospheres.

Richard Hull

Re: Making a SHV connector

Posted: Thu Mar 28, 2019 7:07 pm
by Richard Hull
A warning to my post above. Many BNC male kits are made for video cable work. These have zero plastic insulation about the pin! 1V p-p video is all they were designed to handle. Same goes for females made for video.

If you plan to stress a common BNC, get the RF insulated types.

As related to the Neutron tubes, My 3He tube has an HN connector and I have used the HN to common BNC adapter on it since 2004 to feed the 1600 volts bias. However as Richard Feldman notes, the BF3 tubes of some size usually need 2kv or more bias. This could stress or create undesirable "trichel" pulses on the output line of the pre-amp if you tried to push a normal BNC into this service.

High bias proportional tubes of all types work at the millivolt level. A noisy tritchel pulsing stressed connector is not to be desired.

Richard Hull

Re: Making a SHV connector

Posted: Fri Mar 29, 2019 11:09 am
by Harald_Consul
Thank you for providing your expertise in dielectric strength, guys!

To be more useful now, I just want to add the trivial information, that also the physical dimensions of (50 Ohm) Signal BNC, MHV-BNC and SHV-BNC are different.

(50 Ohm) Signal BNC Connector male:

MHV-BNC Connector male (the inner insulation is longer):

[edit of 2019-03-30]
SHV-BNC Connector male (the inner insulation is longer AND the screwing is different):

Afaik video BNC cables are "75 Ohm" signal BNC cables; usually only "50 Ohm" signal BNC cables are used in the lab. "75 Ohm" video BNC cables may have much less dielectrical strength, than "50 Ohm" lab BNC cables.

I personally try to avoid using "50 Ohm" signal BNC cables for voltage supply (except for very short interim use). Mainly simply for the reason,it is pretty hard to find a signal artifact generated from an electrical discharge in a bad voltage cable. Most of the time this is last thing you get thinking of.

Re: Making a SHV connector

Posted: Fri Mar 29, 2019 7:35 pm
by richnormand
Should the label on the third photo with the long isolation be SHV not MHV?

Re: Making a SHV connector

Posted: Sat Mar 30, 2019 7:20 am
by John Futter
we use 93 ohm 75 ohm and 50 ohm in our lab
each on a separate rack
especially important when using charge sensitive detectors where capacitance equals noise / reduced gain
75 ohms is the magic impedance for cost vs length that is why video uses this impedance

maximum power transfer occurs when source and sink are matched
as per Thevenin's Theorem --this is important especially when using detectors that are close to the noise floor

all for now
the laws of physics havn't changed this week