Moving Coil Meter / Projects / Forums

Forums

4hv.org :: Forums :: Projects

« Previous topic | Next topic »

Moving Coil Meter

first previous 6 7 8 9 next

Move Thread

LAN_403

Proud Mary

Tue May 31 2011, 11:59AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Accessories for valve voltmeters included an RF probe, which would have a miniature diode valve at its very tip, sometimes a wire-ended EHT rectifier like EY51.

Where very low circuit loading and high frequency response were needed, UHF acorn triodes like 955 were mounted on the very tip of a probe, so there was no wire at all between the circuit under test and the meter input circuit.

Steve Conner

Tue May 31 2011, 06:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yikes! Rare earth magnets are first cousins to lighter flints, they could make a lot of sparks, maybe even catch fire if you try to cut or grind them. I think the stuff is incredibly hard and brittle: the finished magnets are heavily nickel plated to protect them.

If you let two really big, powerful neo magnets smash together under their magnetic force, from far enough away that they can get up a good speed, their lighter-flinty properties may allow them to explode with a big flash and bang. I have read of this happening on a guitar pickup makers forum, where they are used to charge the smaller Alnico magnets in the pickups.

It could be an urban myth, but then again it might not, and a lump of burning Neodymium in the eye could really ruin your day. So careful now, as Father Ted would say.

Sulaiman

Tue May 31 2011, 06:26PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I've never had an interrest in thermionic valves - not much that solid state can't do better.
As for vtvm input current, in the 70's I was using commercial parametric op-amps of 10's of fA input bias current (Teledyne I think) ... no idea what's available now.

Ash Small

Tue May 31 2011, 06:45PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

You can't cut or grind them due to the heat generated. Above 80 C and they are no longer magnets. Nickel isn't that hard (not as hard as chrome), I'm planning to try using aluminium oxide paper wrapped around a former, with plenty of water to keep things cool. (Apparently you can drill them with diamond tipped drills, but I don't want to risk overheating the magnets)

Your comments remind me of when I was working at Harwell on their syncho-cyclotron. A weld failed on what was supposed to be a stainless part, I went to get a supermagnet from my toolbox, one of their engineers had 'assumed control' by the time I got back. He insisted I give him the magnet, an unplated one, and it flew out of his hand and smashed into what turned out to be tool steel that someone had managed to weld to the stainless. The magnet smashed and shrapnel went eveywhere (I had warned him! and subsequently told him he should have let me test it! he was in a state of shock. You'd think a Harwell physicist would know about supermagnets.). It's the same magnet that I used in the first post in this thread. (I usually wrap them in insulating tape when testing steel, just in case.) (In case anyone is wondering, 300 series austenitic stainless, which is what the part was supposed to be (316) isn't magnetic)

I actually made some neodymium pickups once (well, not neodymium, but a rare earth type that withstands higher temperatures) and I could measure the current on the milli or micro-amp range on my DMM when passing a screwdriver over it. It was pretty crude, with thickish wire. I used several 1/4" diameter ones stacked together. I was planning to build an electronic ignition system at the time, but I've not got round to it yet. The principle is the same as guitar pickups.

Nah

Tue May 31 2011, 07:26PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

A normal $5 vtvm has a impendance of 11megohms

Ash, valves or tubes short circuit all the time. I just had a shorted tube on the tube tester some time ago. The most infamous is the heater cathode short, which is kind of hard to notice, and can cause 60hz hum to leak into the audio. A full cathode to plate is rare, the most you'll get is most likely a grid to plate.

Steve Conner

Tue May 31 2011, 07:38PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

11 meg is the standard for DMMs, too. At 11 meg, 10nA of bias current will give you an 0.11V error, that is why it's important to minimise it.

The LMC660 has a headline spec of 2fA input bias current. Bob Pease was involved in those Teledyne parametric op-amps, he has a whole chapter of a book somewhere about them. You could buy three for the price of a VW Beetle, but the contents were about the same as a transistor radio, with some real black magic that, according to Pease, even the designer didn't fully understand. Teledyne Philbrick made a huge profit on them.

The LMC660 outperforms them, but you have to wash your PCB in the dishwasher.

All of this is a long way from the average 12A*7 you might find on eBay.

Proud Mary

Tue May 31 2011, 08:35PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve McConner wrote ...

11 meg is the standard for DMMs, too.

Isn't 10 MÎ© more common?

Nah

Tue May 31 2011, 09:19PM

Registered Member #3567 Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260

Mary is right, 10 is the standard.

Many vtvms have better numbers than 11, I have one that goes at a dc impendance of 21 mohoms. There is more to tubes than the 12ax7. There are millions of tubes, some which are far better than a audiofool run of the mill dual triode.

vtvms using special probes can measure up to 500mhz.
A normal dvm can go up to 20khz

magnet18

Tue May 31 2011, 09:34PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

Sulaiman wrote ...

I've never had an interrest in thermionic valves - not much that solid state can't do better.

Yes, but solid state doesn't have the same warm glow and the quaint simplicity that tubes have

Proud Mary

Tue May 31 2011, 10:13PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A like-for-like comparison between thermionic valves and solid state devices (SSD) is really a bit silly.

Valves were superceded because they became uneconomic. Every small town in England once had a repair shop
to service consumer electronic goods - TVs, radios, gramophones, cine projectors - because valves in
their hot confined spaces failed constantly while the parts around them slowly barbecued and
changed their value, or gave up the ghost in puffs of smoke, and pools of wax.

Thermionic technology lingers on where replacement with SSD technology is uneconomic - oven and radar magnetrons,
power klystrons, electron-impact photon sources, CRTs still in much of the world, gas switches like power thyratrons and pseudospark devices, and perhaps still in Russia some exotic 'hard' circuitry intended to function in the presence of EMP and intense ionizing radiation.

As a simple hobbyist with nothing more in mind than scientific amusement, I am not constrained like the factory owner to make a profit, and so can return to the technology of the past to see what entertainment it still might have to offer in the way of curious knowledge.

first previous 6 7 8 9 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob