Advance VM77 Valve Millivoltmeters

[pwdrive]

Don't know Al, but that's how I read it.

Lawrence.

[AlanBeckett]

I've captured a 'B' - usual source - see attached.

I've had a quick look inside and it appears to be a classic piece of 'marketing' - Let's 'Update the Product' - because the Chassis looks just like the VM77 but turned through 90 Degrees. It's even still got the curved corners to fit the original case. For the C and D versions they reverted back to the 'Vertical' Format and updated the 'Mechanics'.

So, I've now got examples of all the Valve versions. Come the Winter, when the Garage is uninhabitable, I hope to be able to potter indoors 'cause they're nice and small.

Alan

                       

[pwdrive]

Nice find Alan.

Lawrence.

[kevin.Hoyland]

Alan,
The Advance 77B is the 4th valve volt meter I have. The other 3 are Marconi TF428B, Heathkit AV-3, Aerovox 77. To cut a long story short, none have  probes with them. I keep reading that 99% of VTVM or VAVM HAVE IDENTICAL CIRCUIT inside. However, all have a different probe that needs to be shielded. So, back to the 77B. The manual says it has two probes a PL50 with a screened hood or a low capacity probe PL45. So Alan did you end up with a probe or not from the seller. I have a manual for the Heathkit AV-3 David sent to me. It shows the Probe with no screening, resistor or capacitor in it. It can be used for 1 to 300 volts, but with the other meters I have to say if you do not have a probe with shielding, you will not isolate low resistance from the meter movement, from the circuit that has been measured. 

Kevin

   

[AlanBeckett]

No, Kevin, I'm afraid not. I have several different Advance ones and none has a Probe.
I fancy that the PL50 is simply a piece of Low Loss Co-ax.
I also have a couple of Airmec 314s but they're entirely different. They have two Probes each, one DC and one AC with a Rectifier Valve built in.

Keep looking.

Alan

[Gbwilded]

Hi, new to this group, and also valve gear.
I have acquired a VM77D from a relative. First powered up via light bulb to limit current, all seems OK, and now powered up directly, again all seems OK, main caps seem to hold voltage well. Question is on the heater voltage. This is quite high at 6.7v - should I be worried and look at reducing this somehow? I note the input voltage range is quite wide 200-250v, and my mains supply is generally above 240V so I guess it should be OK. These instruments are unlikely to be on for very long periods, so perhaps not a problem.
Not tested calibration or anything else yet, but it does read values if I connect a lead to the input as an antenna, so looks promising.

73

Dave M0WID

[ppppenguin]

Dave, welcome to the GVR forum.

6.7V is less than 10% above nominal so nothing to really worry about. If the input voltage range is 200-250V without any selector then the heater voltage will inevitably vary over a fairly wide range.

[Terry]

I'm not so sure about that. Recalling the vast improvement to emissions from flat CRTs when fed by a 20% boost transformer but the relatively short time that the rejuvenated tube usually lasted, I'd err on the side of caution, particularly if replacement valves are difficult/impossible to find or only available at highly inflated prices.

Don't forget the origin of the 6.3 volt heater goes back to the era of the lead acid battery which was not the generally accepted 2V per cell but actually 2.1V per cell. Early valves had 2V filaments and were powered by a single cell and followed by 4V valves and two cells. By the time the heater voltage had gone up to the 3-cell level, manufacturers decided it was time to take into account all those 0.1V increases and made them 6.3V.

6.3V is a 5% increase on 6V; 6.7V is a 6.3% increase on 6.3V. It might be wise to add a suitable series resistor to either the transformer primary or secondary, whichever you have a suitable resistor for. If the secondary is centre tapped, the primary would be easier to deal with.

I find it strange that a piece of test equipment which might be left switched for long periods so as to be instantly available for use, did not have primary taps on the mains input.

[ppppenguin]

Presumably the designers did some work on tolerances and reckoned that overrunning or underruning the heaters didn't matter too much in that application. One presumes the valves were worked well witin their capabilities, not like a CRT or a line output valve which was often being pushed somewhat. Also the relationship between emission and heater voltage is far from linear. I don't know the actual law but if it's like filament lamps then 20% overvoltage will increase the emission (and shorten the life) by several times more than 10% overvoltage.

That said, Terry is sensible to err on the side of caution, especially as new valves will get increasingly scarce,

[Ed_Dinning]

Hi Jeffery, is this a case for the 3-halves power law?

Ed