Vacuum Tube Voltmeters

[Captn Dave]

Vacuum tube voltmeters, in my experience, commonly have resistors and or diodes mounted within the probe itself. Diodes for the AC probe and a high value resistor for the DC probe.

I even have a Hickok VTVM that has a vacuum tube twin diode tube mounted in the body of the probe. The four wire probe cable carries filament voltage as well as the signal and ground.

I can't think of a reason why this must be. Why not build them with these components mounted within the case of the instrument?

It stands to reason that there must be a reason as Hickok and others wouldn't have gone to the expense to build them this way and users wouldn't have put up with these cumbersome probes if they were not necessary.

Perhaps someone will enlighten me.

Thanks

[Tom Bavis]

I don't know about the Hickok, but HP's model 410B has the diode in the AC probe... it only presents a load of 1.5 pF and has -1 dB point of 700 MHz... zero lead length to add capacitive loading. Over-the-top for 60 Hz or even 20 KHz measurements though!

The 1 meg resistor in the DC probe has a similar purpose... to prevent the lead capacitance from affecting the circuit under test. In an AM radio, the oscillator grid sits at a few volts negative when it's running. Connect your DMM and it'll read zero... 'cuz the oscillator has stopped! Connect a VTVM and you''l read the proper voltage.

[tvrgeek]

My Heath has a resister, so it can be a 10X switchable at the tip.

[tubelab.com]

Back in the 60's I had an Eico VTVM that I built from a kit. It had the resistor in the probe with a switch to bypass it for ohms measurement. After the second probe broke I simply used a straight probe (nothing but wire) and recalibrated the meter. For everything that I was doing (tube amps of course) it worked fine.

As mentioned the resistor was used for isolation. When you stuck the probe in the circuit, you were connecting a 1 meg resistor up to your circuit with some random impedance on the other end. This didn't affect most circuits.

Without the resistor you were connecting 3 feet of wire directly into your circuit. This would often make circuits that were supposed to oscillate (vacuum tube crystal oscillators) cease oscillating, and could make amplifiers oscillate or hum.

[moonbird]

I remember reading somewhere -- (Fun with Tubes - Web site?? I think) that a VTVM is recommended because they react better (don't fy I presume) when exposed to the higher voltages in tube amps. This is true, right?

I know that Tubelab uses cheapo DMMs for this reason. I see old Eico VTVMs on EBAY all the time. Would it be worth the trouble to get one or are there better ways to tame this limitation? thx.

[aardvarkash10]

bought an Eico 221 here the other day for $5.00 Colour me happy! Tubes alone are worth 10 times that...

[kruesi]

Putting a tube in the probe...

Most VTVMs have an 11 M input Z -some even higher. If the circuit being measured presents a high source impedance to the meter, the capacitance of that 3-ft shielded cable to the meter is sufficient to roll off the response at a fairly low frequency. An active device in the probe will have an output impedance low enough to drive the capacitance in the cable without rolling off everything above a few hundred Hz.

Also if the circuit being measured is very high impedance, having the first stage of gain in the probe will minimize the effects of electomagnetic pickup by the probe cable.

Some VTVMs have a switch that removes all input Z- usually labeled "Open Grid" or some similar. It's just what it says- you're connecting the grid of the meter's input stage directly to the circuit being measured, thereby loading that stage with the impedance of the grid (usually a Gigohm or greater). At these impedances the connection to the meter is very sensitive to stray pickup and you want that open grid as close as possible to the circuit being measured.

[tubelab.com]

I know that Tubelab uses cheapo DMMs for this reason. I see old Eico VTVMs on EBAY all the time. Would it be worth the trouble to get one or are there better ways to tame this limitation? thx.

I use the $4 Harbor Freight meters because they are cheap and more accurate than a VTVM could ever be. They go to 600 volts. So far I have tossed 2 of them. One self destructed quite spectacularly when my wife measured the ohms in the wall outlet. The other one got left on for like a year and the battery goo ate up the circuit board. I have two $13 meters that I got from Jameco that have a 1000 volt DC range. How much voltage to you want to measure? Beyond that, I use a voltage divider made out of a series string of 100K resistors to measure high voltages. It works on the scope too.

I have noticed that some of these will lose accuracy when the low battery icon comes on even though they continue to work. Sometimes the error is substantial. Keep a 9 volt battery handy. If you ever question a reading, measure the battery. I got in the habit of testing each meter before setting up an elaborate multi meter test.

Maybe in a low humidity environment a VTVM might be OK, but my new Eico VTVM became unstable after about 10 years in south Florida. All of the RCA's that we had in high school electronics class were drifters too. When I first came to work here the factory techs still used RCA (VIZ) VTVM's. They were in a climate controlled environment and were never turned off. These worked reasonably well. The probe switches were a problem, so I introduced a probe that used several RF chokes in series with no resistor or switch. The meter was recalibrated for this probe. This worked quite well for troubleshooting low voltage solid state two way radios like the HT220. The VTVM's were all scrapped in the late 70's and replaced with Fluke 8012's.

Tubes alone are worth 10 times that...

Any VTVM that I ever saw had one 12AX7 and one 6AL5. No good stuff either, just RCA's or GE's.

[jackinnj]

You mean like this? I use it with an Eico HV probe when working on scopes -- have had it since 1967:



I also have an HP400D.

[kruesi]

My $0.02 - personal preferences...

It seems the bench meter turns out to be a highly personal thing- something that it takes (me, anyway) a loooong time to trust- and once I do, I'm not likely to trade it for another without good reason. Nor am I likely to lend it out. Anyone else share this?

My main meter for prototyping is a General Radio VTVM. Very stable, trustworthy, and I like its features. I also really like an analog display (D'Arsonval meter) for most things. It's very useful for general bench work, and really shines when it comes to making an adjustment for a peak or null. I also have an HP 5 1/2 digit DMM for those times when I'm setting something when accuracy or matching is important like setting bias currents or measuring voltage regulation. Accurate and also good resolution.

It's also nice to have a "disposable" handheld digital multimeter. Stable, accurate, and cheap.

Mine measured 53 ohms across the AC line. In fact even when I turn it on now, it STILL measures 53, even without the probes being connected. Must be its FLASH memory or something... ;-)