ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Re: Measuring that plate B+

4/22/2000 5:00 PM
Steve A.	Re: Measuring that plate B+ Mike: I usually use the "ic-style" mini-clip adaptors (which aren't as bulky as regular alligator clips) but I see your point about any metal mass having the potential to detune a sensitive circuit. I like your suggestion about using the 100k resistor for measuring voltages, but I'd take it one step further and mount the resistor inside the probe barrel... just need to remember to switch probes when measuring resistances or current! BTW in my "bare bones" testing rig, I will use the noise generated by the probe and meter to inject a small signal at the grid of each tube stage to trace through some problems (like no signal going to the speakers). The Radio Shaft 22-181A DMM is not an auto-ranging meter, and sometimes when measuring B+ over 450vdc it would blink and switch over to a resistance scale! (Normally you would have to rotate the dial to switch from voltage to resistances.) Even if it wouldn't switch over like that the readings I'd get were usually pretty weird (like 200v when I knew that it was closer to 500vdc). I guess I could see if using a 100k resistance on the probe would cure the problems with that DMM... --Thanks! Steve Ahola

4/22/2000 5:00 PM

Steve A.

Re: Measuring that plate B+

Mike:

    I usually use the "ic-style" mini-clip adaptors (which aren't as bulky as regular alligator clips) but I see your point about any metal mass having the potential to detune a sensitive circuit. I like your suggestion about using the 100k resistor for measuring voltages, but I'd take it one step further and mount the resistor inside the probe barrel... just need to remember to switch probes when measuring resistances or current!

    BTW in my "bare bones" testing rig, I will use the noise generated by the probe and meter to inject a small signal at the grid of each tube stage to trace through some problems (like no signal going to the speakers).

    The Radio Shaft 22-181A DMM is not an auto-ranging meter, and sometimes when measuring B+ over 450vdc it would blink and switch over to a resistance scale! (Normally you would have to rotate the dial to switch from voltage to resistances.) Even if it wouldn't switch over like that the readings I'd get were usually pretty weird (like 200v when I *knew* that it was closer to 500vdc). I guess I could see if using a 100k resistance on the probe would cure the problems with that DMM...

--Thanks!

Steve Ahola

4/21/2000 4:16 PM
Troy	Re: Buying a multimeter: need your recommendation That's wierd SteveA.! I have a Radio Shack meter and it is very stable with plate voltage. It's not as stable with plate current, but that's to be expected. I'd love to have a Fluke though! Troy

4/22/2000 10:48 PM
Casey4s	I had a Sadio Shack DMM, and the only complaint I had was that it would not "zero". Meaning that if you hold the prbes together you could not make the meter say "0" ohms and there was no way to adjust this like on ananalog meter. I took it back, got a full refund and bought a Tektronix 254 DMM from Tucker Electronics. I would still like to get a Fluke 87, I think that is the ultimate as far as amp related use goes. IMHO of course... Casey4s

4/22/2000 10:48 PM

Casey4s

I had a Sadio Shack DMM, and the only complaint I had was that it would not "zero". Meaning that if you hold the prbes together you could not make the meter say "0" ohms and there was no way to adjust this like on ananalog meter. I took it back, got a full refund and bought a Tektronix 254 DMM from Tucker Electronics. I would still like to get a Fluke 87, I think that is the ultimate as far as amp related use goes.

IMHO of course...

Casey4s

4/24/2000 7:12 AM
Jeremy	I also have a Wavetech (27xt???) and I LOVE it! Can't go wrong as far as the bang for buck factor is considered. -Jeremy

4/19/2000 10:35 PM
Steve Slick	Also, I've read, although I don't know this for a fact, but the new digital ones are more accurate than the analog. That probably won't make a big difference, but I like to have every edge I can get. Steve Slick

4/19/2000 11:01 PM
Matt	Accuracy is in the eye of the beholder My old VTVM(analog, high impedance vacuum tube voltmeter) documentation says that best accuracy is obtained when measuring such that the meter is within the middle 2/3 of its range. Other than that possible exception, I think analog and digital meters can be equally accurate, but the issues that effect readings are circuit impedance and meter impedance. Also, looking at schematics, there are some assumptions that need to be made, when comparing measured voltages with schematic, like input voltage when the schematic was drawn, control settings, any input signal, and the type of meter that was used when drawing values on the schematic. Digital Voltmeters usually have around 11 MegOhms resistance, so that when you measure voltage in a circuit, it will place a parallel resistance of 11 MOhms across the test points. This will not normally load down a circuit enough to appreciably affect readings. But for instance, if I measure the voltage output on my Eico 950B cap tester that goes up to 500V, I only measure around 420V with DVM, but with my Triplett 630, with 20 MegOhm input impedance, it measures higher. This is because that source is in series with a 1 M resistor, which when parallelled by 11 MOhms, it loads the circuit down more than with 20 MOhms. Please, someone correct me if I'm FOS. The Triplett 630 is an analog meter with 20000 Ohms/V impedance on DC, meaning that for each voltage range, the additional load applied to the circuit being measured is the range volts multiplied by 20000. So on the 1000 V scale, it's 20 Megs. In the old days, 1000 Ohms/V "utility" or "service" meters were common. Triplett still has one with a nice retro look. I've even seen one "utility" meter with only 200 Ohms/V. Way too low for any kind of amp work, imho. Finally, to compare the accuracy of two meters, you have to measure the same circuit at the same time. This equalizes the effects of loading such that both meters, if both calibrated the same, read the same. DVMs are easier to read static values and the untrained eye can achieve more decimals of accuracy with a DVM than an analog, but for the trained eye, the differences are limited when you're working with quality gear(Simpson 260, Triplett 630, AVO 8, etc) I must be bored right now. What was the question? Matt

4/19/2000 11:01 PM

Matt

Accuracy is in the eye of the beholder ;-)

My old VTVM(analog, high impedance vacuum tube voltmeter) documentation says that best accuracy is obtained when measuring such that the meter is within the middle 2/3 of its range.

Other than that possible exception, I think analog and digital meters can be equally accurate, but the issues that effect readings are circuit impedance and meter impedance. Also, looking at schematics, there are some assumptions that need to be made, when comparing measured voltages with schematic, like input voltage when the schematic was drawn, control settings, any input signal, and the type of meter that was used when drawing values on the schematic.

Digital Voltmeters usually have around 11 MegOhms resistance, so that when you measure voltage in a circuit, it will place a parallel resistance of 11 MOhms across the test points. This will not normally load down a circuit enough to appreciably affect readings. But for instance, if I measure the voltage output on my Eico 950B cap tester that goes up to 500V, I only measure around 420V with DVM, but with my Triplett 630, with 20 MegOhm input impedance, it measures higher. This is because that source is in series with a 1 M resistor, which when parallelled by 11 MOhms, it loads the circuit down more than with 20 MOhms. Please, someone correct me if I'm FOS.

The Triplett 630 is an analog meter with 20000 Ohms/V impedance on DC, meaning that for each voltage range, the additional load applied to the circuit being measured is the range volts multiplied by 20000. So on the 1000 V scale, it's 20 Megs. In the old days, 1000 Ohms/V "utility" or "service" meters were common. Triplett still has one with a nice retro look. I've even seen one "utility" meter with only 200 Ohms/V. Way too low for any kind of amp work, imho.

Finally, to compare the accuracy of two meters, you have to measure the same circuit at the same time. This equalizes the effects of loading such that both meters, if both calibrated the same, read the same.

DVMs are easier to read static values and the untrained eye can achieve more decimals of accuracy with a DVM than an analog, but for the trained eye, the differences are limited when you're working with quality gear(Simpson 260, Triplett 630, AVO 8, etc)

I must be bored right now. What was the question?

Matt

4/20/2000 12:04 AM

ken gilbert

quote:
"But for instance, if I measure the voltage output on my Eico 950B cap tester that goes up to 500V, I only measure around 420V with DVM, but with my Triplett 630, with 20 MegOhm input impedance, it measures higher. This is because that source is in series with a 1 M resistor, which when parallelled by 11 MOhms, it loads the circuit down more than with 20 MOhms. Please, someone correct me if I'm FOS."

well i hate to be the one to tell you, but let's look at this rationally... actually, what you said is not that bad!

current flow through meter impedance:

assuming actual voltage of 500vdc, current flow through 11M is 500/11,000,000 = 0.00004A. that's 40 MICROamps.

let's say deltaV is change in voltage due to loading effects.

.000004A = deltaV/1M so deltaV is 40V.

current flow through 20M is 500/20,000,000 = 0.000025A. that's 25 MICROamps. same case here, except deltaV is 25V.

ok, so what does that prove? well, nothing, except that the internal impedance of your cap tester must be HIGHER than 1M, since the difference in the two deltaV's is only 40-25 or 15V. since you measured a greater difference, the tester's internal impedance must therefore be greater. of course, this high internal impedance is what the cap tester uses to test leakage, so it's part of the function of the unit itself.

i must say it illustrates very well your point about loading effects, albeit in an exaggerated way. you don't often see voltage source impedances of >1M, even in tube circuits, but it is a very valid observation about the behaviors of different meters: as the circuit impedances go up, the meter's impedance must as well to avoid loading effects.

kg

Page 2 of 3