A word about cheap DVMs: I bought several of those little yellow DVMs (“Centech” brand) that go on sale occasionally at Harbor Freight here in the US for $3.99 and sometimes for just $2.99 (!). I’ve compared their readings to simultaneous measurements made by my “big guns”, a Fluke 6 1/2 digit thermal true rms lab DVM and an HP 6 1/2 digit true rms lab DVM. The cheap units are surprisingly accurate, especially on the DC scales. But AC measurements (at 60Hz only!) are also quite accurate. The good accuracy probably comes about because of the single laser-trimmed IC used. I keep on hand a large number of DVMs, as well as a couple of good old analog VOMs (Simpson and Triplett). Sometimes I will hook up several meters simultaneously to various voltage points in a project circuit, so I can monitor voltage changes throughout as I tweak things. This is not only faster, but possibly safer since you don’t need to move test leads around in a hot circuit. The cheap DVMs come in handy here. I also scatter the cheapies around: one at work, one in the trunk, etc. To a newbie, I would recommend investing in one decent hand-held DVM (Fluke 87, etc.) plus a few cheapies.
And yes, Jan is right about fluctuating line voltages. Try measuring the line voltage during the day and then late at night.
And yes, Jan is right about fluctuating line voltages. Try measuring the line voltage during the day and then late at night.
I also use the cheap Harbor Freight $4 meters. I have tested them by comparison to other meters costing a lot more. As Brian said the cheap ones are plenty accurate on DC, 50 or 60 Hz AC and ohms. They are not accurate for making an audio power measurement above 400 Hz. Even the good meters start to show inaccuracies above 1KHz or when the distortion is high regardless of what the printed specs say.
If your line voltage reads 125 volts, it probably IS 125 volts, or surely is between 124 and 126 volts. This seems to be common practice in the last few years. I have lived in the same house for 28 years and the line voltage has slowly crept up from 118 volts to 125 volts during that period. Most vintage electronics (and some modern power transformers) were designed for 110, 115, or 117 volts. You get higher than expected voltages when plugging it into a modern AC outlet.
The first picture on my "meter use" page shows several cheap meters compared to an expensive handheld, and a vintage but recently calibrated Fluke benchtop meter. They all read within one count, which is as good as you can expect, and plenty accurate for vacuum tube work.
http://www.tubelab.com/MeterUse.htm
If your line voltage reads 125 volts, it probably IS 125 volts, or surely is between 124 and 126 volts. This seems to be common practice in the last few years. I have lived in the same house for 28 years and the line voltage has slowly crept up from 118 volts to 125 volts during that period. Most vintage electronics (and some modern power transformers) were designed for 110, 115, or 117 volts. You get higher than expected voltages when plugging it into a modern AC outlet.
The first picture on my "meter use" page shows several cheap meters compared to an expensive handheld, and a vintage but recently calibrated Fluke benchtop meter. They all read within one count, which is as good as you can expect, and plenty accurate for vacuum tube work.
http://www.tubelab.com/MeterUse.htm
a question about panel meters
This question may be outside the scope of the original question, and is not directly about tube audio, but all the talk about using multiple meters leads me to ask it.
I have a "breadboard" set up made from an old chassis, a lot of 5 way binding posts, and terminal boards with tube sockets on them. For measuring plate current, screen current, plate voltage, screen voltage etc I have a bunch of "cheap" digital meters that I connect to specific binding posts. I wish at the time I knew about the $4 ones, since I paid $12 to $20 dollars for mine at various times. Anyway my point is that my original plan was to use digital panel meter modules, and build them right into the test set. No wires all over the place etc. Just add range switches and external resistors as needed. But the price of digital panel meters was in the $50 to $100 range as I recall. And as far as I can tell from the specs for these things they are not any more accurate than the cheap meters.
So how is it they can sell entire meters for $4, but you cant get what is essentially the "guts" of one for under $50 ? Or did I just not look hard enough ?
Anybody else using built in panel meters in test set ups, and if so where did you get them ?
This question may be outside the scope of the original question, and is not directly about tube audio, but all the talk about using multiple meters leads me to ask it.
I have a "breadboard" set up made from an old chassis, a lot of 5 way binding posts, and terminal boards with tube sockets on them. For measuring plate current, screen current, plate voltage, screen voltage etc I have a bunch of "cheap" digital meters that I connect to specific binding posts. I wish at the time I knew about the $4 ones, since I paid $12 to $20 dollars for mine at various times. Anyway my point is that my original plan was to use digital panel meter modules, and build them right into the test set. No wires all over the place etc. Just add range switches and external resistors as needed. But the price of digital panel meters was in the $50 to $100 range as I recall. And as far as I can tell from the specs for these things they are not any more accurate than the cheap meters.
So how is it they can sell entire meters for $4, but you cant get what is essentially the "guts" of one for under $50 ? Or did I just not look hard enough ?
Anybody else using built in panel meters in test set ups, and if so where did you get them ?
Yes, but I have been told not not to answer email related to a personal business (anything related to Tubelab) at work, and I have been living there for the past 3 weeks. They don't (yet) seem to mind forum participation (all computer use is monitored).
I remember the Dynacord name from somewhere, but I can't remember any specific amp.
Try measuring the THD of the power line (through a filament transformer with a resistive load) and then switching on different appliances to see what happens. My worst offender the desktop computer. It goes from 3 or 4 percent to 10 percent. An electric drill is also a killer (depends a lot on which outlet it is plugged into).
Check the power supply requirements for any digital pannel meter before using it in a tube circuit. Often DPM's can be found surplus for $5 to $10 USD. I got some cheap ones that I was planning to use in a power supply. It turns out that each meter required a seperate floating 5 volt power supply. Power supply ground is the measurement ground. This means that you can not use it as a current meter in the positive lead of a HV power supply without a seriously expensive 5 volt floating power supply. Others have the isolation built into the meter, but they are limited to 50 volts or less. Avoid meters that "can not measure their own power supply voltage".
I got mine from Marlin P Jones surplus. They have a bunch of different ones from $8 to $15 USD. They all have the same conditions stated above. It is still possible to use them to measure voltages with respect to ground (the output voltage of my power supply) up to 500 volts. It is difficult to measure a voltage or current that is not ground referenced. Since my power supply was nothing more than a box full of variacs and transformers I thought it would be simple to connect the sensing resistor in the negative side of power supply with the meter across the resistor. It worked great until the first time that I connected two outputs (0 to 350 volts) in series, and turned up the voltage. One of the meters EXPLODED and the other three were dead, so was the 5 volt power supply that fed the meters. Game over, no more meters in the power supply.
http://www.mpja.com/listitems.asp?dept=52&main=51&type=1
I understand. I used to work at a big engineering firm. While others were having "coffee breaks" or were outside puffing on cigarettes, I would have preferred to spend my break playing on a forum, but our computers were monitored too. Thanks for the comment on the Dynacord; it is a rare bird apparently.
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