Old meters and insulation

[KE6DF]

Does anyone have any ratings, or rules of thumb, on how much voltage you can have on a panel meter before it becomes a shock hazard?

I'm thinking of ma meters used to measure plate current. It's hard to, say, run a couple modulator tubes and a final tube off one filament transformer and then meter both the modulator plate current and the final plate current separately unless you put the ammeters in the plate circuits. Monitoring screen current is a similar problem.

The meters I have are the old Weston 301 types with Bakelite cases and no metal parts exposed on the front.

The zeroing screw is black Bakelite or plastic.

I found some old Weston 1930's era data sheets, but it doesn't have a spec for this.

Dave

[WA1HZK]

If you put more than 1 KV or so on those old meters they will probably crap out. It's much better to put them in the cathode ckt. of the tube or the negative return of the power supply your metering. I found this out the hard way after having to send some meters to dipsy dumpster land. Commercial transmitters insulate the plate current meters on large insulators & fiberglass to measure plate current in the actual plate ckt. of the tube.
Keith
WA1HZK

[k4kyv]

If you return the grid current directly to the cathode, the meter will read true plate current, not cathode (grid plus plate) current.  I wouldn't trust the old metal cased meters for more than hundred volts or so.  Better  to mount them on a bakelite sub-panel behind a glass window if the full high voltage is on them.

[WQ9E]

I would follow Don's advice on sub-mounting the meter.  Even if the original was specified for use at high voltage the manufacturer didn't expect it to be in use several decades later and parts certainly deteriorate and dirt builds up as a potential leakage path. 

My Heathkit KL-1 Chippewa does have the plate current meter in the HV lead and it is NOT sub-mounted.  The meter may be safe but the KL-1 remains off when visitors are in the shack because I don't want to find out if anyone can draw an arc from the pointer or just how good the insulation is on the zero set screw.  One story making the rounds claims the meter was one reason the KL-1 was dropped so quickly but since that would have been easy to fix I imagine the real reason was the price of such an amp was just too high.  I thought about changing the metering circuit but the shunt is inside the meter and I don't want to risk breaking an irreplaceable meter. 

The Desk KW does meter the total cathode current on its "plate" meter and I added a small external meter for screen current measurement.  I think high powered tetrodes should have the screen metered anyway and this also allows me to measure and subtract control and screen grid current to get actual plate current.

Rodger WQ9E

[N3DRB The Derb]

what's fun is the super old buzzard fan shaped meters with metal setscrews. Patent dates of '01 and such. I wanted to build a rig with them at one time way back when. Use some buzzardly toobs like 852's or 860's. Frame out of wood, panels out of masonite. Meters behind glass with spot illumination on the faces. Watch many deadly munkys swing behind glass. 

Here's a early prototype mockup showing the many safety features.

[KE6DF]

Looking back at old handbooks and pictures of rigs from the1930's and 1940's makes me wonder what was the average life expectancy of hams back then -- not to mention the kids and pets that lived in ham's households.

Meters with high voltage on them was probably a minor part of the problem compared to

barrier terminal strips with several hundred volts on the back of power supplies.

uninsulated plate caps on tubes right out in the open.

racks with the sides and backs open -- with plate transformers with connections on the top exposed.

Even if I didn't kill myself or a family member, at very least my cats would get fried inside of a week.

[WA1HZK]

What good is a radio if it does not have the potential to kill you? Besides, they make great mouse traps and bug zappers.

PS
Class E guys freeze to death in the dark!
 

[W9GT]

I enjoy vintage-style homebrewing and I do quite a bit of it.  I agree that the wise thing to do with plate current metering is to put the meter in the cathode circuit.  (not really just plate current any more, but much safer). Remember also...those HV voltmeters usually require external multiplier resistors and you need to be careful with those as well.

As for the old breadboard-style construction,  it is great fun to recreate some of those old rigs.  I recently built a little QRP transmitter using a push-pull 6A6 xtal oscillator on a board.  Orig. design from the Frank Jones 1936 Radio Handbook.  Even that little rig has 300-400 volts DC on the plates, and is potentially dangerous with all of its exposed parts.  You just need to be really careful.

I certainlly would not recommend building stuff like big Class C amplifiers and the like, on pine boards with several thousand volts exposed....not good for one's life expectancy!  Neat to look the old pictures though.  Also, I think Clyde's famous 833 AM rig built on a wall shelf might not be a real good example for safe construction practices!

The best route to take with metering is definitely sub-mounting meters behind the panel and even placing the meter panel behind glass...looks really neat and not that difficult to do.  Windows are cool!  For viewing tubes and meters.

Just my 2 cents...

73,  Jack, W9GT

[N3DRB The Derb]

Meters with high voltage on them was probably a minor part of the problem compared to

barrier terminal strips with several hundred volts on the back of power supplies.

uninsulated plate caps on tubes right out in the open.

racks with the sides and backs open -- with plate transformers with connections on the top exposed.

just described my HB rig.    whats funny is I had the HV leads taped, but the entire top of the rf deck was at B+ to some degree. I made sure I used ceramic plate caps too.....never mind the entire tuning cap and tank coil had full B+ on them.....

I wanted a as close to authentic circa 1937~9 looking transmitter as I could make, and by golly, thats what I made. When I have a idea about a piece of gear and what I want it to be, or look like, it's impossible to get me to quit. I have a picture in my mind's eye and until I paint it on the canvas I'm not satisfied no matter what it takes.

[N0WEK]

Meters with high voltage on them was probably a minor part of the problem compared to

barrier terminal strips with several hundred volts on the back of power supplies.

uninsulated plate caps on tubes right out in the open.

racks with the sides and backs open -- with plate transformers with connections on the top exposed.

just described my HB rig.    whats funny is I had the HV leads taped, but the entire top of the rf deck was at B+ to some degree. I made sure I used ceramic plate caps too.....never mind the entire tuning cap and tank coil had full B+ on them.....

I wanted a as close to authentic circa 1937~9 looking transmitter as I could make, and by golly, thats what I made. When I have a idea about a piece of gear and what I want it to be, or look like, it's impossible to get me to quit. I have a picture in my mind's eye and until I paint it on the canvas I'm not satisfied no matter what it takes.

That is a thing of beauty!!!

Nice work.

[N3DRB The Derb]

thanks - wish I still owned it. I threw a conniption fit and sold it away. 

[k4kyv]

I enjoy vintage-style homebrewing and I do quite a bit of it.  I agree that the wise thing to do with plate current metering is to put the meter in the cathode circuit.  (not really just plate current any more, but much safer). Remember also...those HV voltmeters usually require external multiplier resistors and you need to be careful with those as well.

As for the old breadboard-style construction,  it is great fun to recreate some of those old rigs.  I recently built a little QRP transmitter using a push-pull 6A6 xtal oscillator on a board.  Orig. design from the Frank Jones 1936 Radio Handbook.  Even that little rig has 300-400 volts DC on the plates, and is potentially dangerous with all of its exposed parts.  You just need to be really careful.

I certainlly would not recommend building stuff like big Class C amplifiers and the like, on pine boards with several thousand volts exposed....not good for one's life expectancy!  Neat to look the old pictures though. 

But that was back when when kids were allowed to play outside all day, unsupervised, ride their bikes without bicycle helmets and carry pocket knives at school.  When you could open a container of household chemicals or a bottle of medication without a set of burglary tools.  When a.c. receptacles didn't have a ground prong and the other two prongs were the same size.  When a cup of coffee at Mc Barfald's didn't have a warning printed on the side to let you know that the contents were hot.  When you could mail order a WW1 army rifle from Sears & Roebuck for $14.  When toy stores sold chemistry sets.  When matches would "strike anywhere".  When you could easily travel anonymously.  When you could buy a beer without "showing ID" and cigarettes were sold in vending machines.  When you could browse through an issue of  Playboy at the newsstand before you bought the magazine.  When a push lawnmower didn't have that silly feature that kills the engine every time you release the handle bar.  When a box of nails didn't come with a warning for you to wear a hard hat and safety glasses whenever you use a hammer to drive one.  When airports and court houses didn't have metal detectors...

But most people survived anyway.

[w1vtp]

What good is a radio if it does not have the potential to kill you? Besides, they make great mouse traps and bug zappers.

PS
Class E guys freeze to death in the dark!
 

Speaking of "stuff" that can kill you, I remember (with disgust) the regular maint... job of going to the transmit site of our outdoor antenna range where the klystron supply used 813s to regulate the HV (I forget the KV's).  Every so often I had to go down and remove the dead bodies of various rodents (often zapped beyond recognition) and clean up the dried on "P" and other disgusting excretions.  To finish up the job, I would hose down the cabinet after a liberal application of soapy water etc.  What a happy thought .

Now I use sealed synth.. generators with SS amps, GPIB buss extenders and no longer have that duty to deal with. 
Al