EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: gooseEL34 on February 28, 2013, 07:45:11 pm
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Hey guys.
I am wondering the following....
For checking +15vDC/-15vDC in a solid state guitar amp, is my crappy Fieldpiece Lt16A sufficient with its 0.5% +/- 1 and only a 1/10th of a mV resolution?
Reason I ask is this. I am a tinkerer with mostly Tube Amps. I use a multimeter for checking B+ voltages on the plates of tubes (Valves for you across the pond). I also use it to check resistances and to check if a component is short or open.
I recently started tinkering with a 1970s Lab Series L5 guitar amp which has a bunch of 741 and 4558 ICs.
It seems to have a 15vDC rail and a -15vDC rail.
Assuming my meter is at worst, 1.5% off. Will I care? What I mean is: If I read 15.3vDC and it is actually 23mV off,
Can I assume that the rails are supplying normal voltages?
Thanks
Tim
Anyone suggest a meter with 0.05% DC accuracy that isn't over $250??
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I don't think you need to worry too much about absolute accuracy here. Consistency and repeatability will be more important.
For instance, it doesn't matter much if a 15 V rail is at 14.5 V or 15.5 V or somewhere in between.
However, if you have +15 V and -15 V rails they should be balanced. If you measure 15.3 V on the +ve rail, you should measure 15.3 V on the -ve rail also. This only requires that your meter has repeatable measurements and sufficient precision to compare two measurements.
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I suspect that so long as the rails are between 10V and 18V then the amp won't care, so 0.5% will be more than good enough.
In the 70's the person who designed it probably only had an analogue meter for checking voltages that would have been at best 2% and possibly far worse.
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Unless you move both leads when you go from measuring the positive rail to measuring the negative rail, you also need the meter to be symmetric, ie read the same if the leads are reversed. This is more than just repeatability.
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Unless you move both leads when you go from measuring the positive rail to measuring the negative rail, you also need the meter to be symmetric, ie read the same if the leads are reversed. This is more than just repeatability.
This is true. I think all of my meters are asymmetric to some degree, but it is more noticeable on the higher precision meters with more digits.
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Voltage on rails does not tell you much, its more important to run amp on max load and measure ripple for both voltages. This will tell you if capacitors are OK as well as voltages.
You should use scope for this, you can also use multimeter to measure ripple by using MinMax (Peak) mode, you capcure Min Peak and Max Peak, it will tell you Peak-to-Peak. its not preferable and accurate way, however, it can be done that way.
If voltage is not regulated it does not have to be accurate + - 15% is usually OK. (if schematic says 15V it usually means that your 120V or 220V input voltage must be accurate as well). If its regulated we usually tolerate + - 5%.
For $250 you can get used Fluke 87V.
Dave recently reviewed Brymen BM257, it could be a choice..
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Anyone suggest a meter with 0.05% DC accuracy that isn't over $250??
If you are in the USA, you have a lot of options if you don't mind buying used off ebay or locally.
An used Fluke 87V can be purchased easily under $250. The Fluke 87 III is 0.05% + 1 digit and can be found in the $125 to $175 USD range.
Many older bench multimeters are 0.05% + 1 digit and can be found in the $50 range. Examples, Fluke 8050A, Keithley 175, etc.
I don't know if you need this feature or not, but some multimeters (like the Fluke 8050A and Keithley 175 above) also read db/dbm which might make working with amplifier troubleshooting a bit easier?
PS. I own a Fluke 8060A (handheld version of the bench 8050A) and just got a Keithley 175, but I do no stereo equipment repair.
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0.5% is plenty for what you want to do.
People got by (and some still do) on +/-5% analog meters for what you want to do.
Dave.
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0.5% is plenty for what you want to do.
People got by (and some still do) on +/-5% analog meters for what you want to do.
Dave.
Haa Haa ,
Someone Just Hurt Some Antique sentiments ;D
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0.5% is plenty for what you want to do.
People got by (and some still do) on +/-5% analog meters for what you want to do.
Dave.
+1
Tube circuits are usually not that critical. You would even be hard pressed to find two tube testers of the same model that always give the same readings within one or two percent or even the same tubes that were that close. Of course if you are an "audiophool" electrical laws and the laws of physics are different for you and you need to spend all your money attempting to get everything matched as close as possible (usually within .005 percent).
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...matched as close as possible (usually within .005 percent).
Wait - you're not serious, are you? 0.005%? Nah, it can't be... :palm:
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...matched as close as possible (usually within .005 percent).
Wait - you're not serious, are you? 0.005%? Nah, it can't be... :palm:
:-DD
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Thanks guys.
Tim
'
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...matched as close as possible (usually within .005 percent).
Wait - you're not serious, are you? 0.005%? Nah, it can't be... :palm:
It's that last electron that slips by your meter that get's you and ruins the sound.
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Yes, that is true, i just repaired my Twin reverb and at first I forget to first play in my directional multimeter testleads made of golden ferryhair, woven by elves under a fool moon.
So I had to do it all over again. >:D :-DD
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Can't forget reading somewhere that cutting off the top plastic ring of the main electrolytic caps in the audio amp can significantly improve audio quality.
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Found a Fluke 189 ridiculously cheap, so now I can in fact, capture each stray electron that may affect that magical guitar tone!!!
:-DD