The CAT rating is to be headed if you play with high voltages. A simple cheap hobby meter as I was intimating just may not cut it.

Many of the old analogue meters were designed for Industrial use and were made to a "Standard"; often these days that means naught. Believe it or not, the AVO 7x I have is actually made to a British Standard which dictates an accuracy in the order of 2 1/2%. Its Flashover minimum is 3KV

As noted, they will all lie if used wrongly. One thing missed often under the theory of measurement, is "repeatability". What that means is that if I measure a voltage / volume / pressure that has not changed, the device measuring it, or dispensing it should not have changed.

Eg. If its a metering pump delivering 2ml +/- 0.5%. That is what I should get every time: No more no less.

I have a meter collection, several of which I never bought. One of which is bordering on criminal damage. A "Linear Systems Inc. Model LM3. From what I can gather "Precision": Stuffed by someone leaving the batteries in it. Aside from that there is of course the AVO. That's had the movement re-built and the caps tossed after one failed. An aged but works & repaired (dud audio test input cap) "Peak" 200H. A PMG Analogue one that fell into my lap.. damaged but they were precision,

One as mentioned Micronta digital died but the FET analogue on is still a goer. An acquired not bought "Smart" "true RMS Digital type with cap diode freq.etc. . Jaycar features well with a clamp type with an unusual feature, of an on off switch, Two digitals with no on off switch (pain) Micronta ones have them.

Current bench one is QM-1549 also with no convenient on off switch. By far the most interesting for accuracy, is Analogue QM-1020. Mine and another a Radio Club member has, from new on a source with heaps of reserve will not measure the same voltage on the 250VDC range as it does on 1000V range i.e. FSD 250V is considerably different on the 1000V range. One cannot have lot of faith in the overall accuracy of meters like that.

It does pay to check meters against one another occasionally, especially if the reading is odd: They do fail. Had an incident last week where a drill press earth wire tested 4 Ohms: Odd. 2nd meter agreed with the first one.

I assume from all I have read that during the 1950s and 60s most of the analogue meters were 1000 ohm per volt ?
Is this correct? Because even the chassis I just finished states on the schematic that all measurements were taken with a 1000 ohms per volt meter. Which was a 1960 , 11 77 ,but all I was doing full recap and all went well. So I assume it was the common "standard " of the times?

What about if do this, I buy a 1000 volt true RMS digital meter in the midrange of meters and I also buy a good old analogue meter. That way I can cross reference when needed.
I understand what Harrord is saying about using a Resistor to correct the reading with a digital meter in a circuit were the 1000 ohms per volt would be a issue putting the reading out.
My problem will be identifying in which circuit this more likely to happen in as I don't have the experience yet.
So if follow the schematic and when get odd readings I can then cross reference it with both meters, one digital and one old analogue meter.

One thing that is a pain in the butt when buying meters is trying to get a look at the full specifications of the meter I'm interested in! Often its hard to get the full specs and they just seem to list the basic specs, but with Fluke and Brymen I have been able to get full specifications and they are the two brands I'm interested in buying.

I do have a Analogue meter here ,I do use sometimes, its a Digitech QM 1020

Like Relayautomatic I also recently bought a Digitech product, QM-1549 which is CAT IV 600V and CAT III 1000.
V ac, V dc, mV dc, ohm, cap, diode testing, Hz%, 10Amp ac & dc, mA ac & dc, uA ac & dc. Described as Autoranging True RMS IP67. 4000 display.

Has rubber protection integrated into the case and probe holders on the back. The probes are CAT II 1000V without the probe covers, and CAT IV 600V and CAT III 1000 with protectors on, just revealing the tips. Probe cables are 2000V, 10A even though the meter is rated as 20Amp for 30 seconds every 15 minutes, as well as 10Amp for 30 second without a repeat time limit. Has plugs for unused sockets to maintain the waterproof rating.

On special at the time, $90, so you got a better deal with your QM 1552 with its inductance measurement ability.
I managed to get the calibration instructions from Jaycar after finding a document linked to them in a forum. Dave Burns at Jaycar kindly retrieved this for me.

Have been working on low voltage side of stereo gear so haven't used this yet for valve work.

After pontificating about the effect of a 1000 ohm per volt meter on a typical circuit, I thought I should check my response.

Looking at the Astor circuit in my earlier post, the first two screens are fed through a 10K resistor and the screen voltage is shown as 72V, so 38V is dropped across the resistor. Ohms law means 3.8mA is flowing. The meter on 100V range shows 72V and is therefore drawing 0.7mA meaning the two screens are drawing 3.1mA this all sounds quite logical and normal.

Now, if we look at the audio triode, the plate is connected to B+ via a 250K resistor. It has a drop across it of 60V, (B+ of 110V minus the 50V shown as the plate voltage) and Ohms law calculates a current of 0.24mA flowing through it.

However, the meter (on the 100V range) is showing 50V meaning 0.5mA is flowing through the meter. This is impossible, as there can only be a total current of 0.24mA flowing through the valve AND the meter combined.

As Prof. Sumner Miller once said, "Why is it so?"

Can someone with a better understanding of the subject shed some light or point out the error in my thinking.

Harold

Since old meters have a resistance leak across the terminals, use a digital meter to measure that resistance, while in the various voltage selections. The info obtained should help you with your calculations.

Further, while your old meter is measuring volts, the digital meter can be inserted into the path to determine the amps flowing through the meters.

If a valve has a cathode resistor. That is what shows you the valves total current.

Ohms per volt is normally taken as being at FSD. Full Scale Deflection.

Not all of the old meters were 1000 opv. The AVO 7x meter is around 2mA. Its 1000opv on the lowest voltage 500opv there after.

There were meters where a valve drove the meter & when transistors came along, they replaced the valve. Many of these meters were in fact high impedance.

Meters once we had transistors, started to have micro-amp meters as the current drawn could have huge & destructive concequences, especially if it changed the bias. They in fact become a parallel resistance across a valve plate resistor

The digital meter presents a constant resistance of around 10Meg to the circuit, which is why it has a lesser loading effect. Its metering does not use current from the circuit to run its metering.

I'm afraid I have taken this thread off the topic of multimeters in general, onto a circuit discrepancy in the Astor I used for illustration.

Brad, should I move my post to Tech Talk, as I have another observation on the circuit.

If the valve is removed, then there is only the 250K resistor and the meter resistance, (100K on the 100V range) in series. Current is then 0.31 mA and the meter will show a reading of 31V. If the valve draws any current at all, this figure will drop and can never show the 50V listed on the circuit.

Harold

Harold,
No don't do that because even though I'm only a beginner at electronics and much of what is said I dont understand yet, but I read every word in your posts and some of it I will retain in my mind , So even if I dont fully understand all the information, Its there and by reading it I would of learnt something.
Thank you.
The best way for me to learn more is keep working on electronics, I will make mess ups,but by fixing them myself I will learn more. My aim is only to be able to work on some old TVs and Radiograms

Pete

Pete: Moving the Astor discussion to Tech Talk shouldn't have any effect on your ability to read it..

(PS: IIRC, Brad has said in the past that his forum software lacks the capability to readily move threads between sections/forums).

Here is okay, Harold.

Yeah ok , all good here ....i thought it was being deleted off forum.
All good

I might have missed this, but if your meter is 1000 ohms per volt and you are on the 100 volt range, that equates to a 100 k resistor across the terminals. 300 volt, range that's 300k.

So, if you measure the plate of say an audio stage with a 300k plate load, you will read half the HT voltage if the valve is out of circuit. Plug it back in. Voltage goes down by about half? That's good, it's working properly.

Any measurement you take will affect what you are measuring, a basic rule of physics! Nice thing about modern gear is it tends to affect what is being measured less.

Yes, modern test equipment is generally designed to minimise the loading effect. However back in the day, a 1000 ohm per volt meter was par for the course and will draw 1 mA at full scale deflection. The voltages shown on a circuit where such a meter is specified would have to take this into account.

What prompted my question in post #34 was that the audio amp triode plate voltage is given at 50V, yet the meter loading makes this impossible. It may just be a typo error, of course.

Harold

Quote: "Solder: Americans seem to leave the "L" out of it. "

Also "Caramel" pronounced "Car-mel" -- (either way, it's really awful stuff!)

True story:
- in the mid 70's, I was in a room with some music academics and used newly learned word "Timbre"...
- was quickly shot down with correct pronunciation: "Tam-ber" (doesn't make sense, but what would I know?)

"Tam-ber"

French pronunciation of a French word.