Lets talk about VTVMs

[mwkeene]

Hi,
I just picked up one of these at the junk room of the local hi-fi store:
VTVM schematic

Its in great condition, the front panel is beautiful, except the leather strap, which flaked all over my car and me.
When I got it home I tested it out and it seems to be working well. It is without batteries but that?s fine because I?m not interested in using this as an ohm meter.
The .25u caps in this tested out okay, but I didn?t get a chance to test the two smaller caps on the grid of the two 6K6 tubes. I don?t even know what they're made of...the .25u looks like wax and paper? Could I be right here?

Anyway, I tested out the DC probe and I got a resistance of about 1.13 Meg ohms, and the schematic listed a resistance of 1M ohm...does this need to be adjusted and can it even be adjusted? Im assuming I can just account for this with adjusting the DC measuring calibration.

I had a few questions about this though. Why are the VTVM meters preferred for work with tube amps and other older equipment? I have a nice DMM but I?ve been reading around here and other places that the input impedance is so much higher on the VTVM meters than it is on the DMMs. I guess I don?t understand this because it seems like they could just implement the same front end in a DMM? The voltage divider does have some very high value resistors in there though?that makes sense to me as why a VTVM could be good for this.

I hear that VTVMs are good for checking leaky DC blocking caps in the output of a tube gain stage, but I don?t understand why they could be any better than a DMM for this?

The mains plug is 2 conductor, so I?m going to replace it with a 3 conductor wall plug, because the GND probe is connected directly to the chassis! Anything else I should be looking into to get this thing in good shape (which it already seems to be)? The probes are shielded, which is nice, but they?re very old and in kinda crappy condition? I was thinking about getting some new wire for them? Ill probably be back with more questions about the circuit in a little bit?

-Mike

 

[Larrchild]

The mains plug is 2 conductor, so I?m going to replace it with a 3 conductor wall plug, because the GND probe is connected directly to the chassis!

Remember not to do anything crazy like check AC line voltage with that grounded case/lead setup.

Some FET input DMM's are as low-loading as a VTVM, but tubes are inherently so high in impedance that back in the days of that or a Simpson 260, they were the only game. Now that may not be true.

Here is myself in 1974 using the later RCA "Senior VoltOhmyst" VTVM.

 

[mwkeene]

[quote author="Larrchild"]
Some FET input DMM's are as low-loading as a VTVM, but tubes are inherently so high in impedance that back in the days of that or a Simpson 260, they were the only game. Now that may not be true.[/quote]

Ah Ha! so this thought is kindof a relic of the days when FETs werent very commonly used/available for these applications? That was why I was kinda confused, I figured the input impedance of a FET was probably somewhere near as high as the tubes that would be used.

BTW, I really like that picture... I thought about a senior volt ohmyst and came across a few WV-77As on ebay, but this one was really cheap and it looks cooler than the 77As to me. That meter is HUGE :shock:

-Mike

 

[bcarso]

[quote author="mwkeene"]
[quote author="Larrchild"]

Some FET input DMM's are as low-loading as a VTVM, but tubes are inherently so high in impedance that back in the days of that or a Simpson 260, they were the only game. Now that may not be true. [/quote]

Ah Ha! so this thought is kindof a relic of the days when FETs werent very commonly used/available for these applications? That was why I was kinda confused, I figured the input impedance of a FET was probably somewhere near as high as the tubes that would be used.

BTW, I really like that picture... I thought about a senior volt ohmyst and came across a few WV-77As on ebay, but this one was really cheap and it looks cooler than the 77As to me. That meter is HUGE

-Mike [/quote]

Tubes are a LOT more forgiving of overloads than a FET too. OTOH their voltage drift and aging are terrible.

By coincidence I was just looking at my HP 410C, which has some tubes and some solid-state inside, along with a neon-bulb-illuminated photoconductor chopper to manage low d.c. offset without constant trimming. I haven't used it for years but it's nice in its way.

There is a mode for my Fluke 8062A that allows the input reisitance to go greater than 100Mohm measuring d.c. volts. It's not shown on the logos for the switches but it's buried in the manual. I use it on rare occasions.

 

[PRR]

> picked up one of these

6K6???? WTF was RCA thinking they were making???? (Or did they maybe have a warehouse full of 6K6 when it went out of fashion?)

> it seems to be working well

It should. Short of gross abuse, there's nothing to go wrong.

> test the two smaller caps

Why are you checking caps? Either it works, or they are shorted/open and it don't work.

And those grid caps are stupid. I see what they could do, but nobody needs them.

> without batteries

Put 3V worth of battery in it! Never trust any analog-to-digital converter on impure voltages and impure resistances. All DVMs are ADCs, and can't be trusted. I once almost threw-away an output transformer when the DVM said it was open. I turned to my VTVM and it said 420 ohms, and indeed the transformer worked fine.

I dunno what weird battery it calls for, but if it isn't two C-cells then you should get a 2-C holder and wire it in.

> the DC probe and I got a resistance of about 1.13 Meg ohms

Odd but meaningless. Put the VTVM on a fresh carbon-zinc ("ordinary", not Alkaline) battery. Does it read 1.56V? If it is close, call it perfect. If it is really off enough to matter, there are trimmers inside.

Most VTVMs were 11Meg DC, and most DVMs claim to be 10Megs DC. Same-enough. But DVMs inject crap in their inputs, VTVMs don't. Most DVMs make the signal travel naked through the probe and lead; most VTVMs on DC have a 1 Meg (or 1.15Meg) isolation resistor IN the probe, much less capacitive loading on a working circuit.

> checking leaky DC blocking caps.... I don?t understand why they could be any better than a DMM for this?

Put the dang Ohms battry in. Put the VTVM on maximum ohms range. For this meter, that is 10Megs center-scale, 100Megs is near the top, and you may be able to resolve 1,000Megs. Try that on your DVM! Many only go to 1.999Megs. Some do 19.99Megs. Grid coupling caps should read well over 100 Megs. You can't do that on everyday DVMs, but it is trivial on the bent-scale VTVM.

The AC function, at least for 5V to 100V, is flat from 20CPS to far-far past 100KC. It is Average Responding (in this case) but calibrated for Sine RMS. Most DVM AC scales, no matter if they boast Tru-RMS, punk-out before the top of the audio band, and at different points depending on level. You can't beat a stupid passive rectifier.

BTW: if you fall in love with this VTVM, you can make a "permanent Ohms battery" by rectifying the heater supply and regulating to +3VDC. It has to be a half-wave rectifier unless you want to de-ground the existing heater wiring. The regulator does not have to be 3V exact, there is a front-panel trimmer for the battery voltage. It does have to supply up to 3V/9.5ohms= 0.3Amps

 

[Larrchild]

BTW: if you fall in love with this VTVM, you can make a "permanent Ohms battery"

and also..have bcarso design you a chopper-stabilised nulling circuit.
Or Carpal-Tunnel will ensue.

 

[bcarso]

[quote author="Larrchild"]

BTW: if you fall in love with this VTVM, you can make a "permanent Ohms battery"

and also..have bcarso design you a chopper-stabilised nulling circuit.
Or Carpal-Tunnel will ensue. [/quote]

I'll use a carpal tunnel diode for the chopper oscillator too.

 

[Larrchild]

You get better Relaxation with a neon bulb.

Now ya'll got me wantin' one of those VTVM's.

 

[NewYorkDave]

There's a later model Senior Voltohmyst floating around at my job. I rescued it from being trashed, but I haven't even tried to fire it up. I should check it out on Monday. I recall that it appears to be in very good condition.

 

[mwkeene]

[quote author="PRR"]
And those grid caps are stupid. I see what they could do, but nobody needs them.[/quote]

what are they for? With my limited knowledge I would guess they are there to extend the AC frequency response, or maybe slow down the response of the meter to dc changes?

[quote author="PRR"]
Most VTVMs were 11Meg DC, and most DVMs claim to be 10Megs DC. Same-enough. But DVMs inject crap in their inputs, VTVMs don't. Most DVMs make the signal travel naked through the probe and lead; most VTVMs on DC have a 1 Meg (or 1.15Meg) isolation resistor IN the probe, much less capacitive loading on a working circuit.[/quote]

okay, this is a good enough answer for me. Would there be any limitations to using FETs in a similar design instead of tubes? Just curious...

[quote author="PRR"]
Put the dang Ohms battry in. Put the VTVM on maximum ohms range. For this meter, that is 10Megs center-scale, 100Megs is near the top, and you may be able to resolve 1,000Megs. Try that on your DVM! Many only go to 1.999Megs. Some do 19.99Megs. Grid coupling caps should read well over 100 Megs. You can't do that on everyday DVMs, but it is trivial on the bent-scale VTVM.[/quote]

Okay, okay, I'll put the dang Ohms battery in... :razz:

-Mike

 

[PRR]

> maybe slow down the response of the meter to dc changes?

3 Meg and 0.003uFd is a 17Hz low-pass. It takes stray AC off of what should (at this point) be a DC voltage.

> Would there be any limitations to using FETs in a similar design instead of tubes?

Back To The Future, part XIII: Mike is time-warped back to 1959. He goes into the local radio parts shop and asks for an FET. Even though they were invented 30 years earlier, you can't buy an FET. Not until the late 1960s.

Tip: this VTVM uses a vacuum tube rectifier. Selenium rectifiers replaced bottles in work like this, long before FETs became available.

With this tube lineup, early 1950s is likely, though it may have stayed in production long after it was "obsolete".

Other problem with FETs: you are supposed to use the knob-attenuator to knock-down the voltage being tested to about 5V at the diff-pair grid. Ah, but accidents happen. You think you are looking at a 3V cathode, you touch the 300V plate instead. 300V on the grid of a 6K6, nothing terrible happens. 300V on the gate of an FET, blam. Yeah, we did use FET-VMs for a few years, and the better ones are fairly well protected and pretty acceptable.

The other thing is: no solid-state rectifier measures small ACV nearly as nice as a 6H6 or 6AL5. The contact potential for Silicon is ~0.5V, for Germanium about half of that; a thermionic diode can have a very small and soft threshold, yet take incredible over-voltage without splitting. Yeah, we can soup-up sand diodes with amplifiers; frequency response falls off, badly at low levels.

There are many ways to skin cats. I think the general-purpose VTVM is the cat's-meow on the audio workbench.

 

[mwkeene]

[quote author="PRR"]> maybe slow down the response of the meter to dc changes?
3 Meg and 0.003uFd is a 17Hz low-pass. It takes stray AC off of what should (at this point) be a DC voltage.
[/quote]

Ahh okay, and it will pass the AC "level" cause it's rectified and previously low passed by C5? I'm a little confused by that rectifier, it looks like its shunting the signal straight to ground (or wait, am looking at it backwards?)

I understand about the FET availability...I've been taking a class on switching power supplies and the main book teaches *basic* theory from the 70's. The author is always considering the use of FETs to be an expensive tradeoff... I'm just too damn young.

 

[PRR]

> that rectifier, it looks like its shunting the signal straight to ground

Sure. But when? Half the time. When the input tries to go positive, C4 charges instead. When the input goes negative, the diode does nothing to stop it.

I admit I mis-read it at first. The lower diode in the 6H6 does nothing for signal. It seems to be there to compensate the contact potential of the top diode. In use: first you get your DC Volts zeroed. Then switch to ACV, input shorted, and trim AC Bal for a zero reading.

Heathkit and others used a different plan which actually reads peak-to-peak. I'm not sure if this plan is peak or average, and suspect it is half-wave.

I suppose you've realized that, in this plan, "M" means "Kilo" and "Meg" means Meg. Or actually, "M" means Roman-Numeral for 1,000, which made sense before the Metrification fad and before 1,000,000+ ohm resistors became common.

> it will pass the AC "level"

The R-Cs on the 6K6 diff-amp take stray AC off a DC reading. You measure a power rail, it has 400V average DC with 20V of 120Hz ripple..... what should the meter say? With the R-Cs, quite large AC riding on the DC won't change the DC reading much.

> taking a class on switching power supplies and the main book teaches *basic* theory from the 70's.

OMG. What kinda podunk school is that? That's worse than the library at my old school... I wandered in there today for the first time in decades, and they have a dozen books on dBase III on the shelf, along with some real database classics from the early 1980s, and not much newer. Wanna learn COBOL? AP/L???? pCode Pascal? Snobol? TI Basic? I knew that after I left, they bought some books; but it looks like they didn't buy but a few books after that early-1980s buying spree.

Nevertheless, the old books were good. Just remember that things change: I dunno if anything still runs AP/L, and FETs are now cheaper than a tank of gasoline.

 

[gyraf]

Thanks, PRR.. :grin:

Jakob E.

 

[mwkeene]

yes, thanks for sure! :grin:

My school aint so bad...SUNY Binghamton...but unfortunately the practical classes *with up to date material* seem to be more for the Computer Engineers here. Here's the book. I find it highly readable and practical, but the second edition is basically the first edition with some new chapters tacked on for PFC and other "newer", more advanced topics. Great for understanding the basic theory.

 

[bcarso]

[quote author="mwkeene"]yes, thanks for sure! :grin:

My school aint so bad...SUNY Binghamton...but unfortunately the practical classes *with up to date material* seem to be more for the Computer Engineers here. Here's the book. I find it highly readable and practical, but the second edition is basically the first edition with some new chapters tacked on for PFC and other "newer", more advanced topics. Great for understanding the basic theory.[/quote]

Pressman is very good---a reasonable mix of theory and practice. His "real-world" 'scope photos are extremely helpful when you start doing your own measurements.

I have that and a previous edition or two. it is interesting how negative he was at the outset on resonant-mode. By the latest ed. he's resigned to it, if still not that enthusiastic.

 

[NewYorkDave]

Binghamton, huh? Hey, is Unicorn Electronics still around? That was a great store. I used to stop there every time I came to down, when I did contract engineering work for WKGB (and others) circa '99.

 

[mwkeene]

Oh yeah, Unicorn is still there. The people behind the counter are pretty nice, and they got those great bargin tables with all kinds of weird (or not so weird, depending on who you are) crap on them. Unfortunatlely they just stopped stocking what used to be a very nice selection of pilot lamps, but other than that they still got all the same great stuff.

WKGB 92.5 huh? I haven't paid attention to them in a while, but weren't they in that Credit Union building on Country Club road in Endwell in 1999? Might be still...that building is just down the street from my house.

 

[NewYorkDave]

Glad to hear it. I almost became misty-eyed the first time I walked in there, because it was the first real electronics parts store I'd seen in a long time. (Allied, Lafayette and Greylock, my local stores, had already been gone for years).

 

[mwkeene]

[quote author="bcarso"]
Pressman is very good---a reasonable mix of theory and practice. His "real-world" 'scope photos are extremely helpful when you start doing your own measurements.[/quote]



Agreed, its WAY better than my other power book. Probably the first electronics book that has actually made me smarter rather dumber.

And no, I didnt write that one review on the amazon site... haha.