panel meters

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CarlyBoy

My searching skills are deserting me today. I though for sure I'd easily be able to find info here or on the web on the following.

I need to educate myself more about analog panel meters. These things cost way too much new. Lots of surplus stuff out there, usually doesn't have the range I want. I think the face plate can often be swapped out. Questions..

Fundamentally, what do these meters measure - current? - voltage?

Is there a basic standard full scale deflection value that most meters have? ie, Can I know if the surplus meter I'm considering purchasing will be convertible to what I want?

How can I determine the full scale deflection value of a meter?

Thanks for any help!

HollowState

Most all meter movements measure current. Even when the full scale is stated in volts, the movement is responding to a changing current.

There are several standard full scale current ratings that are very common. But nothing is written in stone and full scale ratings can be above or below these listed. 50uA, 100uA, 200uA, 1mA, 10mA, are the most common. But I've seen 5uA & 10uA. Bird Electronics uses a lot of 30uA movements. And some movements are specified in millivolts. The units are made to be used with shunts and will have an odd full scale current value.

The full scale value of a panel meter is often (but by no means always) written down low on the meter face or card as it's sometimes called. You can usually see it by looking at a very steep downward angle.

If a meter is unmarked as to it's FS rating, then you have to test it by applying voltage to it in series with a calibrated milliamp meter. Or one can use a precision resistor in series with it and do the math by measuring the voltage drop across the resistor with a high impedance voltmeter.

As far as interchanging scales, yes you can as long as you stick to the same make and model because it's a physical thing. Mixing brands and model sizes never works out because of the physical differences in size and mounting arangments.

All this applies to the standard moving coil types with D'arsonval (jeweled pivot) or taut-band suspensions. Other less popular types like iron vane and electrostatic movements have slightly different rules. Whether a given meter will serve your needs or not is something that can best be learned by experience.

Moving coil meters must be in balance to read correctly. To test balance, position the meter pointer vertical and note where it lies in relation to the scale 0. Then re-position the pointer horizontal and look for any movement off 0. Any imbalance must be corrected by moving the weights on the pointer's cross.

Victor

OzMikeH

Don;t move the weights unless the balance is grossly out.

Use a needle and a tiny dab of tacky shellac. it takes some time because you need to wait for the shellac to cure.

when setting the balance hold the meter with piointer up, down, left and right.

Vibrate it slightly to overcome the friction of the pivots. I find twirling the handle of a metal jewellers screwdriver avainst the casing is about right. You dont want so much vibration you bounce the pointer.

Evenharmonics

That Yamamoto meter is just what I’m looking for but I can’t see myself spending \$85 plus shipping per meter for my amp project.

Has anyone have experience with one of these in the link below or ones that are similar?
http://www.allelectronics.com/cgi-bin/item/PMD-100MA/search/100_MA_DC_PANEL_METER_.html

pmillett

I used this cool meter face software:

http://tonnesoftware.com/meter.html

It makes very nice looking meter faces.

I really like the "vintage" look meters, which are getting hard to find in the "normal" scales. But you can get good values on meters with odd scales. Most I've tried are either 50uA or 1mA movements.

Avoid sealed meters if you can - they are hard to open up to change the scale. It is possible, but difficult.

Pete

Evenharmonics

dsavitsk said:
Thanks.
Ended up getting a set of those and hooked it up. It reads about 6 mA higher than digital multimeter though. Could it be due to its own resistance being less than the one in the multimeter? How accurate would you say these analog meters are? I couldn't find the spec.

dsavitsk

Evenharmonics said:

Thanks.
Ended up getting a set of those and hooked it up. It reads about 6 mA higher than digital multimeter though. Could it be due to its own resistance being less than the one in the multimeter? How accurate would you say these analog meters are? I couldn't find the spec.

That screw on the front is for adjustment.

Evenharmonics

dsavitsk said:

That screw on the front is for adjustment.
I turned it so that the needle is at 0 when no load is present. Is that the way to do it? When I powered it up, it reads 6 mA higher than my multimeter. Then I connected both meters at the same time to see if I can read them simultaneously and guess what, the bias current dipped down by as much as 30 mA. I'm thinking that the meter's own resistance comes into play when setting the bias. I'm wondering, as long as this analog meter is fairly accurate, it should be ok to rely on its reading?

Evenharmonics

I just calibrated it with the screw on the front to match my multimeter reading. Thanks for the tip.

HollowState

Evenharmonics said:
I just calibrated it with the screw on the front to match my multimeter reading.

You had it right the first time. setting to pointer to zero with the meter at rest is correct. Resetting the pointer with that adjustment when it's upscale is incorrect usage.

If the meter does not read as expected, it may be inaccurate. Or your DMM may be inaccurate. Also, they both could be correct but are causing tube current draw to be affected by their inclusion in the circuit. This is assuming you are inserting them in the cathode to ground circuit.

The precise way to determine tube current it to measure voltage drop across a precision low value resistor in the cathode with a high impedance (10 megohm/volt) voltmeter. Then use ohms law (I=E/R) to calculate current.

Evenharmonics

HollowState said:

Also, they both could be correct but are causing tube current draw to be affected by their inclusion in the circuit. This is assuming you are inserting them in the cathode to ground circuit.
That was my initial suspicion but I put it across the resistor between B+ and OPT (R18 or R29). schematic

Evenharmonics

I played around with different meters and here is what I found.

- Different digital multimeters showed different reading up to 2mA variation.
- My TubelabSE B+ voltage interacts with bias current. Higher the bias, lower the B+ and vice versa.
- When my analog panel meter showed higher bias reading, B+ voltage was lower in proportion.

I'm starting to believe that the circuit of the meter itself plays a role in bias current level. I'm going with my analog panel meter reading after calibrating it (to 0 when no load applied).

SY

Evenharmonics said:

That was my initial suspicion but I put it across the resistor between B+ and OPT (R18 or R29). schematic

If I understand correctly, you put the current meter in parallel with the 10 ohm resistor? I think we may have found the reason for the discrepancy.

Hint: Kirchoff.

Evenharmonics

SY said:

If I understand correctly, you put the current meter in parallel with the 10 ohm resistor? I think we may have found the reason for the discrepancy.

Hint: Kirchoff.
That's per instruction by Tubelab.