Oscope advice please

[Chuck H]

All really good points. But I already made a purchase. I didn't even look at hand held scopes because the ones in my catalog are so damned expensive that I thought it wouldn't be an option. I'm happy with my purchace though. Provided it arrives in good working order.

So on this subject. I'm about to order the parts for my load box. It will have a very low level speaker output for audibly monitoring whats going on. I thought that would be a nice touch, so I know if I don't have my load properly connected right away. So I thought it might be an idea to add test points to the box. I mean, why stick your scope probes into the amps full output if you don't have to? Does this make sense?

But since I've never owned a scope, I'm not sure what kind of terminal would be best for the test points. Any advice on the concept or implementation of this idea is appreciated.

Chuck

[Chuck H]

Well, I'm done with my shopping and I did pretty well I think. I got:

A B&K 2522a 20MHZ dual trace scope. The current model is the 2522b and sells for $1099. US. It's coming in it's original box with the original manual and probes.

An older Leader audio generator, sine and square. Protek still makes the same exact model for $205. US.

I just ordered the parts for my load box. 4, 8 and 16 ohms good for 150 watts. With an adjustable low level output. (OK, the 4 ohm setting is only good for 135 watts)

Total spent including all shipping...$213 Woo Hooo I've been lamenting not having these tools for years and two bills could have fixed it

I included a schem of the load box I'm building. I think it's about as cheap to build as possible so I wanted to share it. The two cost saving features are that it uses the much less expensive 50W aluminum housed resistors (over 50 watts they go up fast) and the switch is a standard On-Off-On toggle instead of the more expensive "impedance" switch. Total cost for parts, including an aluminum chassis was $40.

Cheers

Chuck

[Dave H]

If you want it to be more accurate put a 1.8R in series with the 10R and call the 16R setting 15R

I'd add two more outputs, red + black 4mm sockets for a DMM (even a cheap DMM will be calibrated RMS for a sine wave) and a BNC socket connected to a 1k8:200R divider for the scope. To measure an amp's power output turn up the volume until there's a little flat on the peaks of the sine wave, read the voltage off the DMM then calculate the power from V^2/R.

Dave H

[Chuck H]

Excellent suggestions. I should have my parts soon (they said). But I can add those later. Thanks

Chuck

[Chuck H]

Thank you

For all the tips and suggestions. To all here and those that responded to my "heatsink load box Q" thread. I FINALLY got my parts, built the load box and my scope arrived yesterday (in like new condition, woo hoo). Everything works perfectly and I'm quickly familiarizing myself with my new toys. The load box doesn't even get as warm as a cup of coffee with my 20 watter cranked into a full square wave with a continuous 400hz sine wave at the input.

I'm so happy. Other than my B&K DMM, which I've had for two decades, these are the first real amp guy tools I've aquired since I started doing this. I have a recent reason to become more serious about it. So these came just in time. And alot less $$$ than I thought it would take.

Thanks to all

Chuck

[Enzo]

Great.

You have always gotten along without the scope so far, and you have your regular ways to work, and there will be immediate discoveries, but over time you will integrate the scope more and more into your work, and one day you will think, "Gee, how did I get along without this?" It really is the most powerful tool in the shop.

You now have a signal source, and a place for amp output to go, and the magic window to watch it all happen.

Can't stop now, bud, we gotta get you into all sorts of new gear. Spectrum analyzers, distortion analyzers, maybe an AC voltmeter or two. And, and...


Seriously though...
One thing I use all the time is a metered outlet for the unit under test. Just an AC ammeter in series with an outlet, and I can watch the mains current draw while working on something. I mainy use it along with my Variac, but just in general, I find it useful to note the mains draw. For example, "Hmmm, why is this amp drawing 200 watts from the wall just sitting here? Might have something to do with how hot the PT is getting." SImple enough project, and the meter doesn;t have to be fancy, we are not usually making high precision readings.

Variacs are handy, but I do use it more working on solid state than on tubes.

And if you don't have it, think about upgrading the lighting over your work area.

[Chuck H]

And if you don't have it, think about upgrading the lighting over your work area.

Yeah, but I have some stuff there I'd rather not see

An ammeter will be next. I also read about a tool called a VTVM that looks like a really good thing to have for this kind of work. I could make one, but panel meters and transformers are more expensive than the used examples that sell every day.

Chuck

[Enzo]

A VTVM is just a super high impedance meter, your digital meter will work just fine for that. COntrast the VTVM with the old VOM. The VOM was great for measuring AC volts at the wall socket or checking continuiry, but it would load down circuits.

Vacuum Tube Volt Meter and Volt-Ohm-Meter

The point of a VTVM was it didn't load the circuits. Your DMM doesn't load down the circuits. Once digital meters came along, no one made VTVMs anymore.

But if you want an old VTVM, use ones are cheap. Old Heathkits and Eicos are all over. I would never build one unless I just had to have a retro build project to kill time with.

[Steve Conner]

A DMM is literally just a VTVM with the tubes replaced by silicooties, and a neat LCD readout.

I have a funny old DMM I found in a pawnshop, that has a VTVM-style analog movement as well as the digital readout.

[Chuck H]

That's what I figured too. But the article I read explained a couple of details that made it sound enticing. One example was that because of the way it operates compared to modern VOMs, it can measure down to the smallest fraction of an ohm accurately. Very good for biasing and troubleshooting ground lead coupling. I know MY DMM can't do that. But I didn't hock my car to buy a Fluke either. There were some other seemingly unique and interesting aspects too. If I can re-find the article I'll post it so it can be properly decimated and poo pooed here.

Chuck

[Enzo]

Well, I built my first VTVM back in the 1950s, and I can say I never had one that I felt would accurately measure fractional ohms.

Besides, comparing it to VOMs is pointless. COMpare it you your DMM.

FLuke makes some wonderful meters under $200. What kinda car you selling?

A VTVM on the bench shelf would look cool, with its waggling meter neeedle, so spend $30 and pick one up if you like. Never hurts to have two meters on the bench, you can monitor two things at once. But don't expect it to bail your DMM out of much of anything,

[Chuck H]

OK... Well, this is the link. May be informative. Or this thread is about to sift through some old lore and clear up a few things. Some of the statements in the artical seem very opinionated and finite to me. But this forum is always good for fact finding...So here it is:

http://tone-lizard.com/VTVM.htm

Chuck

[Enzo]

OK.

The initial comparisons tout the advantages of the VTVM over the VOM. No one would question that, but then again VOMs are hard to even find these days. The DMM is what we need to compare to.

When he discuses checking cap leakage next, he mentions:

Some technicians start on the higher voltage scale as a habit, perhaps because there is always an initial 'charging kick' when a DC voltage is applied to a capacitor, or in case the capacitor has a 'dead short'. You cannot 'blow up' a VTVM meter with a voltage overload (another benefit), so many experienced technicians automatically go to the lower voltage range on their VTVM. However, you can overheat one of the precision resistors, and affect the accuracy.

But a couple paragraphs earlier:

You should note that the input resistance is constant though out the range switch. That is, the input resistance is 10Meg, whether the range switch is operated on the 1-volt position or the 100-volt position

The input voltage is across the same 10 megs no matter what. The range setting has zero to do with any potential resistor damage. And so what anyway? The charging current of som epaper cap is small, vertainly when dragged through 10 megs of resistance it isn;t going to melt anything. I mean what if the cap were shorted completely? Then you'd simply be making a voltage reading as you would at the plate onthe other end of that cap.

I absolutely love the animated picture for the tremolo test! Yep, you can use that check to see if the trem is working withiout having a scope. But I use that same trick all the time with my Fluke DMM. Most DMMs have a "trend" scale under the numbers on their display - like an LED VU meter, a row of dashes. That is what it is there for, to watch moving signals.

But let us say our DMM was a cheap one without that, If I saw a steady 100v - or whatever - I wold assume there was no change in the circuit. If the trem was indeed running, my numbers would be flashing up and down. All I have to do is turn the trem controls to see if they affect the gibberish on the meter. Even them I can tell if the trem is working.

Checking pots? I do it with my DMM all the time. Again there is the trend strip under the numbers, I watch it smoothly increase or not as I turn a pot.

Low resistance?

Want proof? Take that Fluke, and set it to measure ohms. Short the two leads together, and note the reading. What did you get; 1.2-ohms? Tsk, tsk, tsk.

Yes, your probes have resistance. Measure them several times. Gee, they measure about the same every time. If they read 1.2 ohms, then when you measure a 1 ohm resistor, then subtract 1.2 ohms from the 2.2 ohm reading.

By the way, if my probes measure 1.2 ohms, I'd throw them away and get some good ones. My probes generally measure under 0.1 ohm.

VTVMs have a zero adjust knob. This is cool. Short your probes, turn the knob until the needle suts at zero. There you just cancelled out any probe resistance. Now your reading is whatever the needle says, right? Sorta...

With a $10 Heathkit IM-11 you can...find a 1-ohm resistor that is exactly 1-ohm.

This assumes your meter is accurate and calibrated. I sincerely doubt his meter has a current calibration sticker on it. Even when new they had to be calibrated. Then there is component drift. The meter is a tube circuit, and tubes age and drift. Even a fresh cal sticker at first is no guarantee of accuracy later.

Also, accuracy ratings for meters is given as "% of full scale." This means that if the accuracy is 2% on the 20 volt scale, a 20v measurement on that scale would be within 0.4v of the true voltage. Unfortunately if you are reading 1 volt on that 20v scale, a 1v reading will be within the same 0.4v of the true reading. IN other words anything from 0.6v to 1.4v will show up as 1v on the meter.

So in a VTVM, we measure 1 ohm on the "R x 1" scale. Full scale is what, 100 ohms? 1 ohm is way over to the other end.

And not only that, the VTVM uses a 1.5v D-cell battery for resistance tests. The 1.5v is applied to the circuit under test through a resistor, then measures the resulting voltage. REsistance is calculated from the implied voltage division. OK so far, but the battery is not regulated in any way, and the accuracy of the readings depends upon the battery being fresh.

In contrast, the current from the DMM is controlled and is largely independent of battery voltage.

Look, it is a wonderful article and tells all the advantages of a VTVM over the cheap little VOMs of the day, but really, I can do all his tricks with my Fluke, and I don't have the tricked out $400 model.

I bought my Eico VTVM abiout 50 years ago. I used it for at least 20 years before my first crude DMM later. VTVMs are a great tool on their own, I just don't see the advantage of one OVER a DMM. But once again, for the price of dinner, yuo can own one, so why not?

I might even got my old one off the shelf and fix her up just for kicks.

[Steve Conner]

Yeah, I find the claim that the VTVM is "the" tool for fractional ohms pretty meh. I have a line-powered benchtop DMM that I got on Ebay, and it has the null feature (wait till it warms up, short the probes together and press the null button)

But better still, it has a 4-wire resistance mode that cancels out the effects of probe resistance. Two probes that put out a test current, and another two to measure the voltage drop caused by it. It has a 6 digit display, and in this mode the least significant digit is 0.001 ohm or something.

So for low resistances I use that, or the ghetto 4-wire measurement, where you put 1 amp or 10 amps through your victim from a current-limited power supply, and measure the (now pretty big!) voltage drop across it with an ordinary DMM.

[Enzo]

Now you've done it, I went out to the warehouse this morning and pulled out my old Eico VTVM. I think I'll add it to the bench just for fun.