VTVM & millivolts

[jjasilli]

My Heathkit VTVM's have their lowest scale @1.5 volts.  Let's say I want to read millivolts across a 1R 1% bias sense resistor.  It seems necessary to drop the 1.5V scals to 0.15V full scale.  Is there a simple way to do this, or am I needlessly beating my head against the wall?

[sluckey]

I think you're just SOL. Gonna have to use a meter with a lower range.

[jjasilli]

Thanks!

[PRR]

> Heathkit VTVM's have their lowest scale @1.5 volts

You can resolve few-milliVolt levels on these scales. At least to know 10mV from 20mV from 30mV. Use the scale-mirror to keep your eye exactly perpendicular. Zero carefully. Squint.

But we have chip-meters for such stuff now.

If you want to live DMM-free, you will use 10 ohm resistors. You can read 0.38V just fine on a VTVM. 10 ohms will not significantly change bias or drive on common audio tubes.

Why 1.5V FS minimum? The VTVM will drift, up to 100mV over time, and 10mV hour to hour. Reading mV levels means constant re-zeroing, and classic electronics just don't have mV readings.

There were other meters. You don't need a VTVM to read cathode current, use a passive amp-meter. A 50mA meter is perfect for a 38mA Champ, indeed for many tube amps. You need a switch/jumper test point. Many amps had 3 screw-terminals and two jumpers, you'd swing the jumper away and wire a current meter.

[jjasilli]

Got it. Thanks again!  My vintage PA amps have such an arrangement:  10R bias sense resistors and those external, winged test points.  Now I understand the point, from the days when VTVM's ruled.  When re-building those amps, I convertd to 1R bias sense resistors and test pin jacks.  So I'll just use a DMM.

In an old post, I had a power tube go short, creating a serious overcurrent condition.  Then you mentioned that those 10R resistors may have also served as a fuse.  That function was defeated by my switching to 1R bias sense resistors.  But since then I added fuses to the cathodes (and stear clear of JJ KT-88's). 

[jjasilli]

So, I blundered into a vtvm that measures dc millivots!  http://www.messmuseum.de/hp412a.htm  There's one for sale now on eBay, so far less than $40.00.  But I'm gonna pass.  Maybe someday, but I've got to draw the line.  I'm developing GAS for vinatge test equipment, bit it's interfering with my amp work and guitar playing.  Also, there's no space left on my workbench!

[PRR]

> a vtvm that measures dc millivots!

That was rare and unusual at the time. Still very impressive specs.

manual, PDF:  http://bama.edebris.com/download/hp/412a/412a.pdf

[HotBluePlates]

I have a couple HP 412A's. They are outstanding dc VTVM's.

And the lowest range is 1mV full scale. HP made a different meter if you had to measure even smaller voltages (in the microvolt full-scale range).

Make sure the original probes are in very good condition if you consider one of these. The originals were attached, and they're quite good quality. You can't fake them easily by attaching anything currently made.

[jjasilli]

You guys are not helping my GAS problem any!   

[sluckey]

I've never seen the 412, not even when I worked in PMEL. Looks like a real Cadillac! I think you need it.   

[HotBluePlates]

It is marketing, but here's a technically accurate write-up on the Hewlett Packard 412A. It's only a d.c. VTVM, but I really like them.

And the input impedance is 200Meg on most of the ranges.