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Old 10-27-2012, 11:12 PM   #1
andyhatescrass
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Confused about multimeter

So I am all set to start biasing my amp, but my multimeter has a ton of settings and I haven't been able to find anyone with the same kind, and there are still a few small things that I am confused about.

Here is what it looks like:

DC Volts is the settings at the top right, and DC Amps is the setting at the bottom left

First: When measuring plate voltage and the actual bias, MilliVolts and MilliAmps are measured as DC and not AC, correct?

Second: I looked up the manual for my multimeter and it said this for measuring dc volts:


"4.2 DC VOLTS

To avoid personal injury or damage to the Meter, do not attempt to measure voltages higher than 1000V DC.

There are five ranges for measuring DC voltage, 200mV, 2V, 20V, 200V and 1000V. For more accurate measurements use the lowest range
possible without exceeding the voltage setting.
1. Set the function/range switch 10 Amps AC.
2. Insert the black (negative) test lead into the COM input terminal.
3. Insert the red (positive) test lead into the V input terminal.
4. Touch the test leads to the circuit under test. With DC voltage, the polarity of the test leads is a factor. Touch the black (common) test lead to
the negative DC source (ground) first and red (positive) test lead to the “live” source second.
5. Read the value of the measurement displayed. If the leads are reversed a “-“ indicator will appear on the display.
6. Typical DC Voltage measurements include car batteries, automotive switches and household batteries."

What is puzzling me is that it says to set it to 10 amps ac first, when im trying to measure dc volts?



For DC Amps, everything seems to make sense and be in order.




At the end of the manual there was one other thing that said this:


"Always start with the highest 10A measurement range and reduce the range in steps once you know that the current does not exceed the next
lower range. The red test lead will be inserted into the μA mA input terminal for measuring amps ≤ 200m Amps. Always turn of power to circuit
and remove the leads from the circuit before removing and reinserting the leads into the meter’s input terminals. Once the measurement is
complete, immediately remove the test leads from the circuit under test and remove the test leads from the input terminals of the meter."

I should be good having 200mA as the max. amount i'd need for the cathode bias and 200mV as the max amount for getting plate voltage, correct? So I won't need to be switching terminals like it recommends or starting at higher settings, correct?

I really wanna take every precaution for this one. Thanks.

Last edited by andyhatescrass : 10-27-2012 at 11:14 PM.
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Old 10-27-2012, 11:33 PM   #2
fly135
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Ignore the line about setting it to 10A AC and set it to the proper VDC position. Looks like a mistake in the manual. I believe you will measuring voltage to calc the bias current and not measuring amps directly.
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Old 10-28-2012, 12:23 AM   #3
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Fly would be correct.

What amp?

Also. Think the point they are trying to make is to always use the larger setting and them scale down.
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Old 10-28-2012, 12:59 AM   #4
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Quote:
Originally Posted by R45VT
What amp?


Splawn quickrod. I have a tutorial from a reliable source. Just wasnt completely sure about these few things.
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Old 10-28-2012, 10:59 AM   #5
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Quote:
Originally Posted by andyhatescrass
I should be good having 200mA as the max. amount i'd need for the cathode bias and 200mV as the max amount for getting plate voltage, correct? So I won't need to be switching terminals like it recommends or starting at higher settings, correct?


This is correct. The 200m in the volts section up at 12:30pm position and the 200m in amps section down at 7:00am position are what you want. It is a a busy MM

And like Fly135 and RV45T are saying the manual is just giving you a starting point I think to measure things around the house like batteries. If you are expecting bias of say 28 - 34 mA then you want the 200 mA setting (not the 20mA) for example.

Also be careful not to unplug the multimeter while the amp is turned on you open up a live circuit and the MM leads could become a lethal source of shockage. Always power down amp and take the methodical steps when swapping tubes etc.

Do you already have the amp open? Where are you at in the process?


Edit:

For a Splawn Quick Rod you should expect to see around a 475 mV reading, maybe 500 mV but I doubt it would be that high. Maybe someone can confirm how to read the plate voltage. 200 mV as a setting wouldn't be enough to cover the 475 you should be expecting to see. Therefore, I'm thinking now you want the '2' no 200 millivolts. Sorry. My MM is auto adjusting so it takes the guess work out.

Cath?
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Old 10-28-2012, 10:14 PM   #6
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Quote:
Originally Posted by 311ZOSOVHJH

Do you already have the amp open? Where are you at in the process?




Haven't even opened it up yet. I'm planning on doing it sometime this week, hopefully.

Also, in step 5 of the tutorial, is it saying to just keep the MM under the tube, switch to MilliAmps (switching off of course), then just see and write down what it reads?

Last edited by andyhatescrass : 10-28-2012 at 10:33 PM.
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Old 10-29-2012, 01:05 AM   #7
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Don't you need a bias probe for a Splawn? In which case you'd never touch the current settings on the meter.
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Old 10-29-2012, 01:29 AM   #8
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Quote:
Originally Posted by Cathbard
Don't you need a bias probe for a Splawn? In which case you'd never touch the current settings on the meter.


I have a bias probe. I assumed I would just adjust the settings as if it were anything else. So the probe will automatically adjust the current settings and give accurate readings? Really?
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Old 10-29-2012, 01:37 AM   #9
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What type of bias probe do you have?
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Old 10-29-2012, 01:42 AM   #10
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No. What you are going to be reading is voltage. Where you are reading the current from is actually reading the voltage dropped across the 1 ohm resistor that the probe places in the circuit. That's what the probe is for - it provides something to detect current.
They use a 1 ohm resistor because it is insignificant to the circuit but any voltage across it translates into a current reading. ie. if you read 1 volt then 1 amp is flowing. So the only thing you are ever reading is voltage, the current settings on the meter are irrelevant. If you pick the right voltage range then any voltage in that range will create a safe current through the meter.
Just set the right voltage range and start measuring.


Quote:
Originally Posted by R45VT
What type of bias probe do you have?
Good question. I've only ever seen bias probes that insert a 1 ohm resistor but I guess it isn't the only way to do it.
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Last edited by Cathbard : 10-29-2012 at 01:45 AM.
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Old 10-29-2012, 01:51 PM   #11
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Quote:
Originally Posted by Cathbard
No. What you are going to be reading is voltage. Where you are reading the current from is actually reading the voltage dropped across the 1 ohm resistor that the probe places in the circuit. That's what the probe is for - it provides something to detect current.
They use a 1 ohm resistor because it is insignificant to the circuit but any voltage across it translates into a current reading. ie. if you read 1 volt then 1 amp is flowing. So the only thing you are ever reading is voltage, the current settings on the meter are irrelevant. If you pick the right voltage range then any voltage in that range will create a safe current through the meter.
Just set the right voltage range and start measuring.


So if im understanding correctly, MilliVolts are equal to MilliAmps, and the volts setting will give the same number reading as amps would? In the tutorial i'm using it says specifically to switch to Amps, but if they are equal, it really would be easier to stay on the Volts setting.

Quote:
Originally Posted by R45VT
What type of bias probe do you have?


Its a eurotubes bias probe.

Last edited by andyhatescrass : 10-29-2012 at 01:52 PM.
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Old 10-29-2012, 02:02 PM   #12
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Quote:
Originally Posted by andyhatescrass
So if im understanding correctly, MilliVolts are equal to MilliAmps, and the volts setting will give the same number reading as amps would? In the tutorial i'm using it says specifically to switch to Amps, but if they are equal, it really would be easier to stay on the Volts setting.



Its a eurotubes bias probe.


Buddy take a time out before you do this.

The Bias probe you listed is a current probe. You need to use the 200mA setting.
http://www.eurotubes.com/EuroProbe-s.jpg - this is the one correct?


The confusion lies in the fact there are multiple styles of bias probes. Cathbard must think you have the voltage style. Ohms law= 1A through 1ohm has a 1v loss.
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Old 10-29-2012, 02:55 PM   #13
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Quote:
Originally Posted by R45VT
Buddy take a time out before you do this.

The Bias probe you listed is a current probe. You need to use the 200mA setting.
http://www.eurotubes.com/EuroProbe-s.jpg - this is the one correct?


The confusion lies in the fact there are multiple styles of bias probes. Cathbard must think you have the voltage style. Ohms law= 1A through 1ohm has a 1v loss.


Yes, thats the one I have. So even though its a current probe, is it able to measure volts as well?

Now that I look at the page where I bought it, it does say "measures in milliamps"

Last edited by andyhatescrass : 10-29-2012 at 03:00 PM.
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Old 10-29-2012, 03:01 PM   #14
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Quote:
Originally Posted by andyhatescrass
Yes, thats the one I have. So even though its a current probe, is it able to measure volts as well?



Of coarse it can....(circuit would be open) its made for current though. You would use your 200mA setting. You are placing your meter in line with the tube to check current.


Don't confuse yours with a voltage bias probe.


Also keep in mind lethal voltages are present.... make sure its plugged into your meter before flipping your amp on.
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Old 10-29-2012, 03:03 PM   #15
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No, that one you use the milliamps setting. Thats why we asked what kind of probe you have, so we could give you the appropriate information.
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Old 10-29-2012, 03:16 PM   #16
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Quote:
Originally Posted by R45VT
Of coarse it can....(circuit would be open) its made for current though. You would use your 200mA setting. You are placing your meter in line with the tube to check current.


Don't confuse yours with a voltage bias probe.


Also keep in mind lethal voltages are present.... make sure its plugged into your meter before flipping your amp on.


Sorry, im still a bit confused. So even though its made for current, would I be able to get a plate voltage reading when MM is set to MillAmps? How would MilliVolts convert when getting a measurement in MilliAmps?
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Old 10-29-2012, 03:20 PM   #17
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It's not the voltage reading you want. If you have your DMM on volts you will get a reading but it will be worthless as the circuit is open. You don't want to do that.

Understand your probe is a current probe and should be hooked up as such.
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Old 10-29-2012, 03:25 PM   #18
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Quote:
Originally Posted by R45VT
It's not the voltage reading you want. If you have your DMM on volts you will get a reading but it will be worthless as the circuit is open. You don't want to do that.

Understand your probe is a current probe and should be hooked up as such.


For the tutorial I am using, it requires plate voltage to be measured. So am i able to measure the plate voltage in MilliAmps?
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Old 10-29-2012, 03:47 PM   #19
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Don't listen to your tutorial with your probe. Just measure the current and adjust as necessary.

Your tutorial is most likely for the bias probe that Cath described.

Let one of the others jump in. 311 has done his with the tutorial you he I believe. I don't know the sweet spot for EL34 regardinf mA as I own 6L6 amps.
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Old 10-29-2012, 04:48 PM   #20
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I think he is talking about my tutorial

Sorry for any confusion Andy. I think he has the same type of probe I had when I made that. Now I have a similar probe but from Weber instead.

You technically don't need to verify the plate voltage because I've already given to you. I was just showing you how you derive ballpark for the bias in mA by calculating in 60% plate dissipation (or whatever it was). I'm pretty sure if you look at the pic of my multimeter in that tutorial you will 476 mV for the plate voltage.

I'm not an EE so I'm glad some smarter people than I were able to step in and help.

All you really need to worry about Andy is that your bias sits somewhere between say 28 mA and 34 mA. 28 should sound stiff and 34 should sound muddy (too hot).

Also - do not play your guitar with the probe plugged in. That will fry that resistor in time. Get the bias set to where you 'think' you want it. Let it sit like that for 20-30 minutes maybe and look for 'drift'. Turn off and unplug everything and then play your guitar through amp for a good while and listen. If it sounds stiff or cold then turn the bias up a bit. If it sound muddy or warbly then turn the bias down a bit.
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