#1
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 at Oct 27, 2012,
#2
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.
Last edited by fly135 at Oct 27, 2012,
#3
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.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
Last edited by R45VT at Oct 28, 2012,
#4
Quote by R45VT
What amp?


Splawn quickrod. I have a tutorial from a reliable source. Just wasnt completely sure about these few things.
#5
Quote 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?
Last edited by 311ZOSOVHJH at Oct 28, 2012,
#6
Quote 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 at Oct 28, 2012,
#7
Don't you need a bias probe for a Splawn? In which case you'd never touch the current settings on the meter.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#8
Quote 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?
#9
What type of bias probe do you have?
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#10
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 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.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Oct 29, 2012,
#11
Quote 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 by R45VT
What type of bias probe do you have?


Its a eurotubes bias probe.
Last edited by andyhatescrass at Oct 29, 2012,
#12
Quote 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.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
Last edited by R45VT at Oct 29, 2012,
#13
Quote 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 at Oct 29, 2012,
#14
Quote 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.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
Last edited by R45VT at Oct 29, 2012,
#15
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.
I don't give a shit if you listen to me or not
#16
Quote 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?
#17
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.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#18
Quote 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?
#19
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.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#20
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.
#21
One more quick one 311, am i turning the vr1 or vr2 under the chassis, when measuring the bias?
Last edited by andyhatescrass at Nov 1, 2012,
#24
Everything went well the first time around. Yesterday, I set the bias around .31mA and played it right after and it sounded great. After about an hour of letting it sit, I played it again and it sounded different; I thought it would be too hot, but I just checked the bias was pretty cold, around 26mA. Is this what normal bias drift is like, or are the tubes possibly going bad? They've been played pretty heavy for 8 or so months and have not been biased until now.
#25
I'm not an amp tech but it sounds like the bias drops as the amp heats up which suggests you should set the bias to the high side if adjusting it while cold. Or adjust it after it's hot.
#26
One thing ive noticed that i forgot to mention: When letting the bias settle with the multimeter plugged in, it doesnt really settle. Just climbs slowly. I set it just below .31 and in 5 minutes, it was at 32.5 and still climbing. That was why i was thinking the tubes might be bad.

EDIT:
Just saw your answer above mine, Fly. So I suppose it would make sense for me just to let it settle as high as it can, which would signify that it is running hot, correct?
Last edited by andyhatescrass at Nov 3, 2012,
#27
Quote by andyhatescrass
Everything went well the first time around. Yesterday, I set the bias around .31mA and played it right after and it sounded great. After about an hour of letting it sit, I played it again and it sounded different; I thought it would be too hot, but I just checked the bias was pretty cold, around 26mA. Is this what normal bias drift is like, or are the tubes possibly going bad? They've been played pretty heavy for 8 or so months and have not been biased until now.

Yes you need to let it sit. Yes bias drift is normal. Don't play the guitar through the amp with the probe plugged in. Don't bias to numbers, bias to how it sounds. How are things going at this time? Tubes don't hold the bias, the amp does. Not sure if 'bad' tubes would cause the amp to drift. Question is does the bias finally settle in on a number.
#28
may be irrevelant, but if you are going to do much with amps beyond biasing i would get a better DVOM. i use snapon, by far the best meter i have used. isn't cheap, but its worth it IMO.
WTLT 2014 GG&A

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alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.


***9V BATTERIES ARE FROM HELL!*** (updated 2016-11-27)
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#29
Quote by trashedlostfdup
may be irrevelant, but if you are going to do much with amps beyond biasing i would get a better DVOM. i use snapon, by far the best meter i have used. isn't cheap, but its worth it IMO.


No offense Snap-On brands suck. Most are BluePoint. Slow refresh times.

Best is Fluke for meters. Last meter you will ever buy. You can catch most glitches without a DSO depending on the signal. Snap-On you would miss it completely.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
Last edited by R45VT at Nov 6, 2012,
#30
Quote by R45VT
No offense Snap-On brands suck. Most are BluePoint. Slow refresh times.

Best is Fluke for meters. Last meter you will ever buy. You can catch most glitches without a DSO depending on the signal. Snap-On you would miss it completely.


don't buy a blue point. i have wired up several drag car from scratch harness as far as wiring goes and used it for two amp builds and repaired several effects. its not as good for DSO, but they are worth the money. i got it at 50% retail as i went to trade school for six weeks. all i wanted to learn there is flow benching cylinder heads and a refresh on engine assembly, as well as dyno.
WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.


***9V BATTERIES ARE FROM HELL!*** (updated 2016-11-27)
Resentments and Rambling from a Guitar Junkie
---> http://trashedengineering.blogspot.com/
#31
Quote by trashedlostfdup
don't buy a blue point. i have wired up several drag car from scratch harness as far as wiring goes and used it for two amp builds and repaired several effects. its not as good for DSO, but they are worth the money. i got it at 50% retail as i went to trade school for six weeks. all i wanted to learn there is flow benching cylinder heads and a refresh on engine assembly, as well as dyno.


What meter do you have? I have not seen a Snap-On one that is not BluePoint.

I hate them. I hate most non-fluke.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
#32
Quote by R45VT
What meter do you have? I have not seen a Snap-On one that is not BluePoint.

I hate them. I hate most non-fluke.


its 7 or eight years old before much was blue point. all the new ones are bluepoint. they are really durable. i have dropped it in my concrete garage hundreds of times and have no problems.

i am pretty sure it is this model:

http://buy1.snapon.com/catalog/item.asp?P65=&tool=all&item_ID=648032&group_ID=675528&store=snapon-store&dir=catalog
WTLT 2014 GG&A

Quote by andersondb7
alright "king of the guitar forum"


Quote by trashedlostfdup
nope i am "GOD of the guitar forum" i think that fits me better.


Quote by andersondb7
youre just being a jerk man.


***9V BATTERIES ARE FROM HELL!*** (updated 2016-11-27)
Resentments and Rambling from a Guitar Junkie
---> http://trashedengineering.blogspot.com/
#33
Quote by trashedlostfdup
its 7 or eight years old before much was blue point. all the new ones are bluepoint. they are really durable. i have dropped it in my concrete garage hundreds of times and have no problems.

i am pretty sure it is this model:

http://buy1.snapon.com/catalog/item.asp?P65=&tool=all&item_ID=648032&group_ID=675528&store=snapon-store&dir=catalog


Meh. Better than BluePioint. I haven't seen one in recent time. I doubt the crap on dealer would even have them in stock. I'd have to use one in person to see how fast and accurate it is. I get sick of crappy meters. Too many techs have low budget ones that are lucky to have functioning leads.

I would still rather have a Fluke.
Quote by DeathByDestroyr
See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.
Last edited by R45VT at Nov 6, 2012,
#34
Quote by R45VT
What meter do you have? I have not seen a Snap-On one that is not BluePoint.

I hate them. I hate most non-fluke.


I've got to go with you on this one. There are 2 brands of meters, Fluke....and everything else...


Quote by trashedlostfdup
its 7 or eight years old before much was blue point. all the new ones are bluepoint. they are really durable. i have dropped it in my concrete garage hundreds of times and have no problems.




Slow learner?
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin
Last edited by Arby911 at Nov 6, 2012,
#35
Quote by 311ZOSOVHJH
Yes you need to let it sit. Yes bias drift is normal. Don't play the guitar through the amp with the probe plugged in. Don't bias to numbers, bias to how it sounds. How are things going at this time? Tubes don't hold the bias, the amp does. Not sure if 'bad' tubes would cause the amp to drift. Question is does the bias finally settle in on a number.


Sorry for the late response, its been a busy week. I ended up getting the bias to settle earlier this week and it sounds like its in the right spot now to my ears anyway, it doesn't sound cold like it did. I played it for an hour or so after removing the bias probe, which may have been the key for getting it to settle.
#36
^ tnx for the update. I'm not sure if playing the amp will help the bias settle. The reading you are looking for is at idle current so I doubt it. I would say if it took longer than 20-40 minutes to settle then maybe there is an issue. I'm also still not sure about a bias reading being affected by older tubes. Maybe someone like Cath or Craig would know. Glad you go things sorted though.