VTVM Vacuum Tube Volt Meter Questions

[selloutrr]

I'm looking for a VTVM the primary function will be to meter AC current i've found about 20 different models but i'm not sure what i need to look for that seperates the crap from the professional models.
pretty much no matter what meter i buy i'm prepared for a full rebuild or at least a very intense service since most of them left the factory in 1975 or earlier.
my main interests are the HP and Ballantine Labs, any others I'm over looking?
one that i'm looking at closely offers these features
AC VOLT FROM -10-+10 DECIBELS AND .003-300 SENS X10 AND 1-100KC.
Teamed with a Fluke 867B and Simpson 260 Series 3 would that tester be enough? or even needed? or do i need the DC testing feature?
I've been dragging my feet on this cause i feel like it might redundant.

Thoughts?

[thousandshirts]

I've got two HP 400D's that are fine for just doing AC. But these don't see a lot of use.

Instead I have two HP410B's (which do AC + DC) that I use much more frequently. These are very, very nice VTVMs. With them here I have absolutely no desire to search any further regarding VTVMs.

[Structo]

When I was a kid I built a Heathkit VTVM.

Then years later a guy I knew who was a tube head begged me to sell it to him because he said it had very high impedance.
He was into tube audio gear big time and his forte was recapping old amps and receivers with the then new, polypropylene caps.
I think was doing the couplers and bypass.
He may have also replaced the filter caps as well.
He had two identical Pioneer tube amps where one was recapped and one wasn't.
The difference was remarkable.

[km6xz]

First post

The H-P 410C would fit your needs very nicely and as with most older H-P test instruments, it will likely need no more than cleaning. I have one I bought new years ago and it has never been opened. I've rebuilt a few used 410C and usually they are not hard to repair, parts might be hard to track down. The Chopper circuit is the most common failure in old units and they are hard to find.

A good scope, metered Variac and calibrated sine-wave generator are all you need for repair and development projects. H-P had a lot of good generators, I've had them all but I found the most useful for bench techs turns out to the the Heathkit IG-5218 because it combines all that is needed: low distortion( a few easy mods drops the ThD to 0.012% @1khz or less), calibrated output level and accurate freq calibration.

For scopes, used Tekronix are readily available and bullet proof. A Tek 465B should only be $150-200 and will outlast the user. A Tek 2236 is really good for bench work. It was a lighter weight model but has some features that are very useful for tech work, built in freq counter, AC, DC Ohm, Temp measurements from side mounted test lead jacks, or reading directly from the Cha 1 probe input while using the scope.
The metered Variac is the hardest to find. You can build one from surplus bare variable autotransformers which are cheap by building a sturdy case for it and mount AV voltage and current meters(with 2 current ranges). I doubt I have ever worked on an amp where I did not use a high quality Variac for run up of an amp that was in undiagnosed condition.

[Aurora]

Just to satisfy my own curiosity, (since I'm fairly new to guitar amp building),
- I can see the tube fan's wish to have tube based measuring eqpt too, but I hope you're not saying that tubes can only be measured by tubes???
Most modern multimeters of good quality will easily outperfom an old VTVM, and partiularly one that hasn't seen a calibration in 30 years, even if it is in good working order.

[km6xz]

Old lab grade test equipment I mentioned were all solidstate but they were analog in indication. A DMM is accurate but for repair accuracy is not so important as easy to spot variation or tends in value changes. For example bias adjustments are much easier to visualize the trend in readings on a needle's change of position than mentally calculate if the current reading is higher or lower and at which rate than the prior instantaineous value. Any type o dynamic value is better, far better, indicated on analog indicators such as audio. A good True RMS H-P 400GL analog db meter is accurate and revealing in its fast servo driven needle positioning. Same with the readings on the 410C, the needle swing has little or nor overshoot and very fast settling time because the indicator is driven by a servo amp and not just a dc amp following a precision rectifier.

Accurancy is highly overrated in working wth complex analog circuits, a 1% basic meter accurarcy will be 10-100% more accurate than the predicted value that is effected by the cumlatuve error in the entire circuit., Any more than 3 digits of resolution are just for psychological effect comforting, but useless in any engineering sense.

Good lab grade gear, even if 30 years old will last longer from present than just about anything cheaply made now and are possible to repair.
There is a place for a digital multimeter, there are occasional needs for a lot of functions in a small size, like varification of a fixed constant value or logging. Use both older lab grade gear for repair and development, digital for fixed steady state values for verification.

Tube circuit development is also hard on fragile DMMs which were not designed to handle the high values of static and AC voltages encountered. A high grade older piece of gear will have higher tolerance of abusive over range or static discharge. If you draw an arc in a tube circuit, the raw very high peak voltage encountered from the undamped oscillation can kill just about any DMM while a 410C or any old tube based VTVM will not be affected at all.

Another issue is bandwidth. Is your new whiz bang circuit filled with parasitic oscillations? How would you know unless you have wide bandwith, 100Mhz measurement capability? A 410C AC measurement capability is 700Mhz! An Analog scope is also very useful and more so than a DSO. I use several digital multimeters, my favorite are lab style Flukes with 7 digits of resolution and temp compensated time base, and the trusted 3080a 5.5 digit general purpose bench DMM, primariy because of the quality of the True RMS converters they contain.

Of all the instruments available for the hobbiest, working without a good scope, is the most handicaping. Most questions Ive read about trying to track down problems with new constuction I've read about on this forum would be plainly obvious if time and amplitude domains were both visually presented at the same time, which is exactly what a scope does. I add one more item for visual indications of state and transfer function of a new circuit by also using a good H-P Spectrum Analyzer but that is beyond the needs of most hobbiests. Building an amp without a good scope is like building a car blindfolded, it might be possible but needlessly frustrating.

[morcey2]

There are 2 things that I have a VTVM for that can't be done with a normal DMM:

Sorting through 1-Ohm resistors to get exactly 1-Ohm.

Measuring the grid voltages on a Long-Tail-Pair Phase Inverter. A normal DMM really screws that one up. The better DMMs have a higher impedance (10M-Ohm for the Fluke 80-series) and can measure this.

Neither of those are huge, especially the second one. It's usually a couple of mV difference from the voltage at the top of the tail resistor. If I would break down and buy 0.1% 1-ohm resistors, the first one wouldn't be an issue either.

But, it's not bad for an $8, 40-year old piece of equipment.

Matt

[Aurora]

Please, don't put any more into my ramblings than what's actually there...

I fully agree to the scope being THE most useful instrument for working with DIY amps, be it guitar amps or HiFi.
Some of the VTVMs listed were top notch models of their day, and are not usually avaliable to the average DIY person, even if the US market of vintage instruments are quite larger than here in Europe. The average day VTVM was typically capable of just a few kHz in freq. range.
Let's not forget that the main advantage of the VTVM back then, was it's high impedance not screwing up operating points when measuring in tube eqpt. - quite opposite to the average non amplified multimeter of those days, with it's typical 20k input impedance. Lots of modern DMMs have 5-10M input impedance. Low resistance measurements have always been sort of a special case, and older ones were often bridge configurations, in order to achieve both accuracy and low measuring currents - safety ohm meters are a special case of it's own.........

[thousandshirts]

- I can see the tube fan's wish to have tube based measuring eqpt too, but I hope you're not saying that tubes can only be measured by tubes???

No, nobody is saying this. Tube equipment doesn't have to be measured with tube equipment. And although I think tubes are fun, I didn't get my tube equipment just because I liked tubes. I bought the stuff because it was very high quality production equipment from great days of manufacturing in the USA, because I could find replacement parts, and because I can repair them and calibrate them on my own should anything go amiss.

Most modern multimeters of good quality will easily outperfom an old VTVM.

I don't think that's entirely true. In fact there are a number of situations where a VTVM will outperform modern DMM's costing many hundreds of dollars. VTVM's have very high input impedances. You may put many a DMM on a circuit, and the meter itself throws off your measurement by a fair degree. A VTVM has a minimal load on what you are measuring, leaving the circuit to behave almost entirely unhindered by the measurement. As km6xz pointed out, with audio, guitar amp audio in specific, who really cares in most cases, where the degree of accuracy is less than critical. Many of our favorite amps made do with 5% or 10% tolerance components. In 95% of cases we don't need such accuracy, but none the less it is handy in some situations. Also you have the analog meter which will "spring" directly to a peak, where the value you are reading is located, and none of the slow auto-ranging "beep bloop beep" indications from so many a DMM. As pointed out they make finding 1 ohm resistors a breeze. Lots of stuff really. I can tell you, I turn to my HP 410B's at least as often as to the Fluke 179. The Fluke is more portable, but if I'm at the workbench it's almost always the 410B.

Here's a link, for you to read. He goes through this same stuff a bit:
http://www.tone-lizard.com/VTVM.htm

[Aurora]

Actually, the point of repair and calibration(?) is a good one. And of course, there are situations where needle type instrument have great advantages over any digital instruments. I've been in the "bussiness" for over 30 years now, servicing and designing " one-off " type instruments. My education was at the very end of tubes ( early 70s), - I probly was noe af the latest classes to have a theoretical introduction to tubes.....
But - as you guys are so persistent, I 'll check on the specs of our lab gear at work!

Will take some time, though, - leaving for 79 deg north wednesday morning................

[Andy Le Blanc]

I burn through at least one DMM a year ....... they blow (up)......
its nice to comfort your selves with accuracy but simply to have a great
quality instrument that will last longer than I will is more than enough......

[thousandshirts]

Plus, extra functions as paperweight, doorstop, boat anchor, and ballast for the back of your car in wintertime

[Aurora]

Hm.... never been able to do that, even working my way through kW level transmitters ( Eimac power triodes!), radars etc.........

[km6xz]

Logic only goes so far; what you are comfortable with will be the fastest and easiest type of equipment. I am kind of test gear whore, like most guys here are amp fanatics. I gave a away a lot of test gear, mostly H-p and Tek, over 200 pieces, when I had to put things in storage when I moved to Russia but I kept enough test equipment to do just about anything I might need. When I go through an amp or other peace, I like to characterize as much of its performance and properties as possible, just for the fun of it and my own enlightenment. As a result of having more and better gear than most manufacturers there are quite a few companies who have incorporated some of my suggested mods to improve their product.

For repair, analog readings, and general instruments are the fasted way to get the job done but if characterizing a finished working product, the later digital systems are great. Besides all the analog gear I also have an Audio Precision System 2 Dual Domain.....great for spec'ing but useless in general trouble shooting. For RF I have a wide range of gear including Motorola Service monitor that has 33 instruments in one box, all with digital precision plus analog generators which have some advantages over frequency synthesizers based generators....like no phase noise.
Good lab grade equipment is readily available for essentially the cost of shipping from the US where there are hundreds of thousand of pieces available surplus. There is an active trade market in used test equipment. A 400FL or EL is only $45 if in very good condition, a Tek 465 or 465M 100mhz scope is will be less than $200 and likely to last a life time.
I got most of my gear years ago when it was worth more but hardly any of is has needed 1 minute of repair time.
I am not a player, I have enough trouble playing records;>) let alone an instrument. That is partly because I never had time for the long slog of learning, and mostly since my design and active recording career put me into creative situation on a daily basis with some of the most talented....and musically intimidating people around. The thought of picking up a guitar and spending 35 years of practice to be 1/2 as good as those I was around left me cold. I took solace in the fact that they could not put the years of study and creating in my specialty any more than I could in theirs but both our skills were needed to make the records that the combination produced. I don't record much anymore, budgets are not there from the labels. I do like to play with new and old designs in RF and audio gear and consult current recording projects.