What is it? (Advance 77A valve voltmeter) - UK Vintage Radio Repair and Restoration Discussion Forum

Mike Brett · 7th Dec 2009, 1:03 pm

Hello all,

Can anyone tell me what this may be and what it is used for. Picked it up at a boot sale for £5, as it was in a good clean condition and had been well looked after. The internals are clean and when switched on the single valve inside lights up. It says R.M.S. volts meter on the dial which also has markings around the scale for dB so I have a hunch it could be some sort of test gear for audio output. Am I warm or is it for something completely different.

Mike
2e0ltj

Nickthedentist · 7th Dec 2009, 1:06 pm

It's a valve millivoltmeter.

Some info here, which implies there's more than one valve inside.

http://www.thevalvepage.com/testeq/advance/77a/77a.htm

Nick.

Herald1360 · 7th Dec 2009, 1:42 pm

The dBm scale (dB ref 1mW) relates to 600 ohm systems which is a telephone / audio standard. 0dBm in 600 ohm is 775mV.

(If it was RF and 50 ohm, it would be 225mV).

It's a nice find and might even be useful!

Chris

Mike Brett · 7th Dec 2009, 2:12 pm

Hi
Thanks guys, found the other valves which were in cans.
Mike

dr peppers · 12th Dec 2009, 11:49 pm

a working vtvm is a good find.

GMB · 13th Dec 2009, 6:04 pm

I have the 77B and have found it useful, although I did have to change a paper capacitor to get it working properly.

Dave UXB · 5th Jan 2010, 1:40 am

A damned good instrument and easy to check. I had one untill a year or two ago.

Dean Huster · 5th Jan 2010, 6:08 pm

Here on this side of the pond we call those AC voltmeters. They usually have a range of around 1mv to 300v full scale in a 1-3-10-30 sequence (to provide 10dB between ranges), two AC scales (0-3 and 0-10) and a dB scale. They typically have a bandwidth that's higher than the traditional multimeter of the same era and a much broader range of measurement. ACVM of the 1960s usually had a bandwidth of around 1MHz while the lab-grade models of the 1970s were around 10MHz. Usually, the meter could also operate as an amplifier, hence the output connector. Most were average-measuring, RMS indicating while there were some that were true-RMS meters. Similar circuitry was usually the heart of THD (total harmonic distortion) analyzers.

Hewlett-Packard had several models such as the 400D, E and L (with a linear dB scale and log voltage scale) and the 4300A. Heathkit had a version as well and solid state versions from Korea and China, some with dual indicators on the meters) are common.

cendoubleu · 5th Jan 2010, 8:36 pm

Probably a dumb question but here goes.
As I don't have experience of these sort of meters, what is the advantage of the extra bandwidth of a valve voltmeter (and of a valve voltmeter in general) which you don't get with a modern instrument.
Charlie

Phil G4SPZ · 5th Jan 2010, 8:52 pm

The main advantage is the almost infinite input impedance, which means that you can take voltage measurements at most points in most circuits without the meter resistance 'loading' the circuit under test and changing the conditions. Measuring the voltage on a valve grid would be an example, almost impossible with a conventional moving coil instrument but quite OK with a VVM.

The second is that the VVM will measure AC voltages accurately at high frequency, up to several MHz with the right instrument, whilst most moving coil voltmeters lose their accuracy above audio frequencies. Hence a VVM could be used to measure the amplitude of a 470kHz IF signal, but a moving coil voltmeter would need an RF probe.

cendoubleu · 5th Jan 2010, 9:57 pm

Thanks for the explanation Phil, that's a great help. It would seem to be a very versatile instrument. Might be worth looking for one at the next hamfest I go to. Would homebrewing one be a tall order, or would accurate calibration cause problems.
Charlie

Phil G4SPZ · 6th Jan 2010, 11:12 am

I tried homebrewing a transistorised electronic voltmeter many years ago, without success. Calibration and stability were problematic.

My approach to homebrewing has changed over the years - there is so much older test gear of excellent quality available for a few pounds nowadays, I prefer to restore it to working order. It's quicker than homebrewing, and you end up with a more valuable and useable item... homebrew gear is virtually worthless in terms of sale price.

Jonster · 6th Jan 2010, 12:01 pm

Modern digital multimeters have a very high input impedance like a valve voltmeter which is why instruments of their ilk are no longer prevalent. For peaking up circuitry an analogue meter is far easier to read so they still have their uses!

Julesomega · 22nd Aug 2019, 7:57 pm

Quote:

Originally Posted by Mike Brett

...Picked it up at a boot sale for £5, as it was in a good clean condition and had been well looked after...

Same here, they are so cute I couldn't resist, even though I'm not so keen on this Thermionic technology

I also found an Instruction book from a Mr Blogs in Telford so I can now put the instrument on my list for restoration.

The specification is quite respectable and not too different from my trusty PM2451. The input impedance is as was suggested 10Mohms // 20pF, and the bandwidth 15Hz to 2MHz, useable up to 4.5MHz. Input ranges 1mV - 300V FSD

It has a useful amplifier output which delivers a respectable 1V RMS into 1.5kohms. Power consumption is quoted as 30W

I see Mike's model has the /A suffix, but I bet the differences will be very slight.

7th Dec 2009, 1:03 pm	#1
Mike Brett Retired Dormant Member Join Date: Dec 2005 Location: Wisbech, Cambridgeshire, UK. Posts: 452	What is it? (Advance 77A valve voltmeter) Hello all, Can anyone tell me what this may be and what it is used for. Picked it up at a boot sale for £5, as it was in a good clean condition and had been well looked after. The internals are clean and when switched on the single valve inside lights up. It says R.M.S. volts meter on the dial which also has markings around the scale for dB so I have a hunch it could be some sort of test gear for audio output. Am I warm or is it for something completely different. Mike 2e0ltj Attached Thumbnails

7th Dec 2009, 1:06 pm	#2
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: What is it? It's a valve millivoltmeter. Some info here, which implies there's more than one valve inside. http://www.thevalvepage.com/testeq/advance/77a/77a.htm Nick.

7th Dec 2009, 1:42 pm	#3
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: What is it? The dBm scale (dB ref 1mW) relates to 600 ohm systems which is a telephone / audio standard. 0dBm in 600 ohm is 775mV. (If it was RF and 50 ohm, it would be 225mV). It's a nice find and might even be useful! Chris

7th Dec 2009, 2:12 pm	#4
Mike Brett Retired Dormant Member Join Date: Dec 2005 Location: Wisbech, Cambridgeshire, UK. Posts: 452	Re: What is it? Hi Thanks guys, found the other valves which were in cans. Mike

12th Dec 2009, 11:49 pm	#5
dr peppers Retired Dormant Member Join Date: Nov 2009 Location: Lancashire, UK. Posts: 67	Re: What is it? a working vtvm is a good find.

13th Dec 2009, 6:04 pm	#6
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,094	Re: What is it? I have the 77B and have found it useful, although I did have to change a paper capacitor to get it working properly.

5th Jan 2010, 1:40 am	#7
Dave UXB Retired Dormant Member Join Date: Dec 2008 Location: Staffordshire, UK. Posts: 116	Re: What is it? A damned good instrument and easy to check. I had one untill a year or two ago.

5th Jan 2010, 6:08 pm	#8
Dean Huster Retired Dormant Member Join Date: Jan 2008 Location: Harviell, Missouri, USA. Posts: 31	Re: What is it? Here on this side of the pond we call those AC voltmeters. They usually have a range of around 1mv to 300v full scale in a 1-3-10-30 sequence (to provide 10dB between ranges), two AC scales (0-3 and 0-10) and a dB scale. They typically have a bandwidth that's higher than the traditional multimeter of the same era and a much broader range of measurement. ACVM of the 1960s usually had a bandwidth of around 1MHz while the lab-grade models of the 1970s were around 10MHz. Usually, the meter could also operate as an amplifier, hence the output connector. Most were average-measuring, RMS indicating while there were some that were true-RMS meters. Similar circuitry was usually the heart of THD (total harmonic distortion) analyzers. Hewlett-Packard had several models such as the 400D, E and L (with a linear dB scale and log voltage scale) and the 4300A. Heathkit had a version as well and solid state versions from Korea and China, some with dual indicators on the meters) are common.

5th Jan 2010, 8:36 pm	#9
cendoubleu Tetrode Join Date: Jan 2007 Location: Lincoln, Lincs. UK. Posts: 94	Re: What is it? Probably a dumb question but here goes. As I don't have experience of these sort of meters, what is the advantage of the extra bandwidth of a valve voltmeter (and of a valve voltmeter in general) which you don't get with a modern instrument. Charlie

5th Jan 2010, 8:52 pm	#10
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: What is it? The main advantage is the almost infinite input impedance, which means that you can take voltage measurements at most points in most circuits without the meter resistance 'loading' the circuit under test and changing the conditions. Measuring the voltage on a valve grid would be an example, almost impossible with a conventional moving coil instrument but quite OK with a VVM. The second is that the VVM will measure AC voltages accurately at high frequency, up to several MHz with the right instrument, whilst most moving coil voltmeters lose their accuracy above audio frequencies. Hence a VVM could be used to measure the amplitude of a 470kHz IF signal, but a moving coil voltmeter would need an RF probe. __________________ Phil Optimist [n]: One who is not in possession of the full facts

5th Jan 2010, 9:57 pm	#11
cendoubleu Tetrode Join Date: Jan 2007 Location: Lincoln, Lincs. UK. Posts: 94	Re: What is it? Thanks for the explanation Phil, that's a great help. It would seem to be a very versatile instrument. Might be worth looking for one at the next hamfest I go to. Would homebrewing one be a tall order, or would accurate calibration cause problems. Charlie

6th Jan 2010, 11:12 am	#12
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: What is it? I tried homebrewing a transistorised electronic voltmeter many years ago, without success. Calibration and stability were problematic. My approach to homebrewing has changed over the years - there is so much older test gear of excellent quality available for a few pounds nowadays, I prefer to restore it to working order. It's quicker than homebrewing, and you end up with a more valuable and useable item... homebrew gear is virtually worthless in terms of sale price. __________________ Phil Optimist [n]: One who is not in possession of the full facts

6th Jan 2010, 12:01 pm	#13
Jonster Heptode Join Date: Apr 2008 Location: Eastbourne, East Sussex, UK. Posts: 671	Re: What is it? Modern digital multimeters have a very high input impedance like a valve voltmeter which is why instruments of their ilk are no longer prevalent. For peaking up circuitry an analogue meter is far easier to read so they still have their uses!