Suitable panel meters for use as a "VU" style meter on amplifier

[nzoomed]

I'm wanting to add some vintage panel meters to my tube amp I'm building to add some "cool" factor to it.
I don't care if they are not reading accurately as a VU as per se, but rather just bounce up and down with the signal on each channel.

Been a few threads I've read where people have done it, but not found any real instructions on modifying or using any meter in such a circuit.

Where would you typically hook up a meter across in this situation? Across the speaker output or somewhere in the input stage?

I have seen decibel meters, but have been advised they are not that suitable, seems that microamperes meters are the best bet, but what about a millivolt meter for example? Meters can be shunted to suit anyway, and if its a DC meter, you can just slap on a bridge rectifier on the back of it cant you?


Any ideas or suggestions would be good, I know i have read that some here wish more builders would make use of them. Its a shame all this cool test gear is getting wasted on steampunk builds that serves no practical purpose, just to be clear I love steampunk, but would be good if only broken gear made its way into these works of art

[Radio Wrangler]

It sounds like you're going to choose meters based principally on aesthetics. There aren't many firms making new meters any longer, so that means you're going to be limited to what you can find on the second-hand or surplus markets. This makes finding a matching pair more difficult.

Given these limitations, you're just going to have to put up with whatever meter sensitivity specs you get in what you can get with appearances you like. It can be handled by shunting or electronics.

'VU meter' is a defined term, having specs for scaling and for the ballistic time constants of the meter movement. Ordinary meters are normally and deliberately slower responding, having damping methods to slow the needle down. They would 'bounce around' quite a bit less.

There isn't enough signal power to easily drive a meter at the input to most power amplifiers, and adding a bridge rectifier here would introduce rather nasty high order distortion to the signal going to the amplifier, also the threshold voltage of the diodes in the rectifier would make the lower end of the signal range dead.

Sticking a rectifier and meter on the output of the amplifier is a lot better in both of the above respects, but not perfect. A really good low distortion amplifier will show an increase in distortion. Whether you can hear this is a mtter of debate, but it can be measured.

Then we come to dynamic range. Music has a lot more of this than you'd expect. If your metering is linear, then you must scale your meters so that your power peaks just take them to full scale deflection. You will then discover how little meter movement there is in the quieter patches.

The Japanese hifi firms went for LED thermometer displays, and at the same time they shaped the scaling to be logarithmic, so that there would be some more dancing around during quieter patches of music. The same can be done with moving-pointer meters, but it does mean a fair amount of electronics. It also fixes the issue of hanging rectifiers on signal paths affecting the signal as in the midst of the opamps and things needed for logarithmic conversion, a buffer amplifier is little extra.

It's a lot of trouble to go to, and doesn't make the sound one jot better, but various Japanese amplifiers and McIntosh went down this route just for the showmanship.

So you first need to find a pair of meters you like. Then you have to make scales for them, then you have to design circuitry to drive them... all in this order. The chances of getting illuminated meters are not so good unless you visit the Akihabara for a good rummage. Buy spares - meter movements wear out, especially if bouncing around a lot. One dead meter will really spoil the looks.

David

[joebog1]

SO!! a couple of pics that are perfect for your question.
The meters I scavenged from some Asian amplifier years ago. They are in excellent condition, but I havent polished up the front glass ( read polystyrene plastic ) as I am still wiring in the amp itself. MUCH more important than the "bling" value.
Rectifiers is self explanatory, i.e. there are two visible, one is geranium ( germanium ) with a cap and a 25 turn pot for "calibration ". The second rectifier is fed from the 12 volt heater line from the input valves of my amp. Simply put, its a 1 amp el-cheapo bridge with a cap across the DC output, and an input resistor to limit the brightness of the 6 "blue" leds backlighting the meter. Leds are just some 50 cents per strip ( 24 as I remember ) 12 volt rated LED's used for "eyebrows" on yuppie cars.

Meters I think ( I havent checked em ) are 200 mV but as I have a 20 K 25 turn pot in the DC circuit its not overly important as its more bling than sing. It WILL show perfect 400 hZ output power as watts RMS, with music who cares ?, its for the grandson to dance to.
Op-shops, Salvation Army and St Vincent De Paul are not allowed to sell " electrical" equipment in Aus without a certification safety tag, so ask for the scrap bin to scavenge the meter movements.

Just what I did in my amp.

It uses EIGHT 5B/254 AKA 807"s, four per channel. "should" make 50 watts RMS per channel.

Joe

[Terry_VK5TM]

Originally Posted by Radio Wrangler

There aren't many firms making new meters any longer, so that means you're going to be limited to what you can find on the second-hand or surplus markets. This makes finding a matching pair more difficult.

David

Don't tell my supplier that, he'll have kittens worrying about it.

[nzoomed]

Originally Posted by Radio Wrangler

It sounds like you're going to choose meters based principally on aesthetics. There aren't many firms making new meters any longer, so that means you're going to be limited to what you can find on the second-hand or surplus markets. This makes finding a matching pair more difficult.

Given these limitations, you're just going to have to put up with whatever meter sensitivity specs you get in what you can get with appearances you like. It can be handled by shunting or electronics.

'VU meter' is a defined term, having specs for scaling and for the ballistic time constants of the meter movement. Ordinary meters are normally and deliberately slower responding, having damping methods to slow the needle down. They would 'bounce around' quite a bit less.

There isn't enough signal power to easily drive a meter at the input to most power amplifiers, and adding a bridge rectifier here would introduce rather nasty high order distortion to the signal going to the amplifier, also the threshold voltage of the diodes in the rectifier would make the lower end of the signal range dead.

Sticking a rectifier and meter on the output of the amplifier is a lot better in both of the above respects, but not perfect. A really good low distortion amplifier will show an increase in distortion. Whether you can hear this is a mtter of debate, but it can be measured.

Then we come to dynamic range. Music has a lot more of this than you'd expect. If your metering is linear, then you must scale your meters so that your power peaks just take them to full scale deflection. You will then discover how little meter movement there is in the quieter patches.

The Japanese hifi firms went for LED thermometer displays, and at the same time they shaped the scaling to be logarithmic, so that there would be some more dancing around during quieter patches of music. The same can be done with moving-pointer meters, but it does mean a fair amount of electronics. It also fixes the issue of hanging rectifiers on signal paths affecting the signal as in the midst of the opamps and things needed for logarithmic conversion, a buffer amplifier is little extra.

It's a lot of trouble to go to, and doesn't make the sound one jot better, but various Japanese amplifiers and McIntosh went down this route just for the showmanship.

So you first need to find a pair of meters you like. Then you have to make scales for them, then you have to design circuitry to drive them... all in this order. The chances of getting illuminated meters are not so good unless you visit the Akihabara for a good rummage. Buy spares - meter movements wear out, especially if bouncing around a lot. One dead meter will really spoil the looks.

David

I was wondering if adding a meter could affect the signal (well at least if done incorrectly)
Would adding a simple double triode tube coupled by a capacitor to the input signal be enough to amplify the feed and connect to a meter?
I see there is a crowd in germany making such meters for stupid money, and then there are the cheap chinese ones out there too.

I was thinking a voltmeter would be the easiest thing to adapt as it simply measures RMS volts which is constantly changing.

I can easily add a small LED to most meters I think by drilling a hole in the back. Others have done similar mods I see.

[Diabolical Artificer]

Finding two identical vintage panel meters is nigh impossible, analogue meters are still made, expect to pay £100 or over each - https://www.mouser.co.uk/Test-Measur...eters/_/N-5gfm

Look on ebay and you'll see cheap Chinese VU meters and boards,but you only get one board, so no good for monoblocks. I used a driver based on this design - https://www.youtube.com/watch?v=jESL__AXLXU&t=5s

I was wondering if adding a meter could affect the signal

I added an opamp buffer with a 10M to ground on the IP to the buffer, signal taken off the first stage anode, this using a TL074.

I made my own boards that included the VU meter driver, protection and clipping indicator. It works really well however it will need tweaking to ensure the entire dynamic range is covered, getting the ballistics right is also no trivial matter. I drilled twoholes as mentioned and fitted two LED's wrapped in Rizzla's and couloured prange with felt tips, getting the LED's to sit right was also a major PITA. I spent far too much money and time modding two vintage panel meters for my 120w monoblocks, but then I'm a perfectionist. It would have been easier to be reincarnated as sisyphus.

In the end I gave upon on VU meters and just painted the scale plate black and covered the needles in gold leaf therefore they didn't measure anything but were just eyeball entertainers.

Andy.

A quick search on eBay finds some dual meter kits complete with driver board.

[nzoomed]

Originally Posted by Diabolical Artificer

Finding two identical vintage panel meters is nigh impossible, analogue meters are still made, expect to pay £100 or over each - https://www.mouser.co.uk/Test-Measur...eters/_/N-5gfm

Look on ebay and you'll see cheap Chinese VU meters and boards, but you only get one board, so no good for monoblocks. I used a driver based on this design - https://www.youtube.com/watch?v=jESL__AXLXU&t=5s

I added an op amp buffer with a 10M to ground on the IP to the buffer, signal taken off the first stage anode, this using a TL074.

I made my own boards that included the VU meter driver, protection and clipping indicator. It works really well however it will need tweaking to ensure the entire dynamic range is covered, getting the ballistics right is also no trivial matter. I drilled twoholes as mentioned and fitted two LED's wrapped in Rizzla's and coloured range with felt tips, getting the LED's to sit right was also a major PITA. I spent far too much money and time modding two vintage panel meters for my 120w monoblocks, but then I'm a perfectionist. It would have been easier to be reincarnated as Sisyphus.

In the end I gave upon on VU meters and just painted the scale plate black and covered the needles in gold leaf therefore they didn't measure anything but were just eyeball entertainers.

Andy.

Ok, so basically they don't really work well? If the amp is at a low to medium volume, the needles don't move much, and if you want lots of activity, you have to crank it up full blast?

I don't care about any accuracy, but rather bouncing needles, I'm not worried about finding meters, I see heaps of the things at HAM junk sales for a dollar or two quite often.

What's the story with magic eye meters, would you not come across the same issues with little activity at low levels?

[Cruisin Marine]

There are loads of pairs of VU meters on the auction sites, and inc. Driver board for less than twenty quid.
I don't know if the driver board has a high impedance buffer on the input, but, that should be an easy thing to add, use a FET or something after the volume control and before the input to the VU driver board.

Doing a very quick search, something like this would do, even even simpler https://www.electronicsforu.com/wp-c...06/fig-1-8.jpg

[joebog1]

This thread has become a design factor for a satellite tracking system for UFO's.
nzoomed has already stated twice I believe, its just bling!!.. If I ever regain my hearing I can very easily fit a switch to disconnect any "blingy" meters, SHOULD I wish to listen to the bearings operating on the record cutting lathe.
Sometimes I am very disappointed by the degree of complexity required to make a meter needle "jump up'n'down.


I apologise in advance, gentlemen, for my presumptions.

Joe

[lesmw0sec]

There are still plenty of new meters around. Spiratronics supply an excellent 100 microamp device which has the advantage that the front panel and scale are easilly removed which is essential for my purposes, as we re-scale them to read magnetic anomaly, rather than current. As someone pointed out earlier - if using a VU meter on low-signal circuits, a buffer amp. is desirable to prevent loading and distortion.

[paulsherwin]

Output meters were briefly fashionable on integrated amps around 1980. As mentioned, they suffer from the drawback that they don't move much at normal listening levels. With a valve amp, it may be better (and more use) to have them indicate the HT current rather than some arbitrary VU level.

[Radio Wrangler]

Originally Posted by Terry_VK5TM

Don't tell my supplier that, he'll have kittens worrying about it.

Nah, he'll just put his prices up.

David

[John M0GLN]

There's this company which lists Sifam Audio Level Meters,
https://www.canford.co.uk/Sifam/SIFA...O-LEVEL-METERS

John

[nzoomed]

Originally Posted by lesmw0sec

There are still plenty of new meters around. Spiratronics supply an excellent 100 microamp device which has the advantage that the front panel and scale are easilly removed which is essential for my purposes, as we re-scale them to read magnetic anomaly, rather than current. As someone pointed out earlier - if using a VU meter on low-signal circuits, a buffer amp. is desirable to prevent loading and distortion.

I was reading that a double triode lends well to driving a meter (not sure if any particular tube is preferable) one suggestion was to use a cathode follower, followed by the other half of the tube running as a voltage amplifier that feeds the meter.
Is that a typical buffer amp in that situation?

[nzoomed]

Originally Posted by paulsherwin

Output meters were briefly fashionable on integrated amps around 1980. As mentioned, they suffer from the drawback that they don't move much at normal listening levels. With a valve amp, it may be better (and more use) to have them indicate the HT current rather than some arbitrary VU level.

I was wondering about this myself.
Could you put a toroid around the HT lead that feeds the CT of the OT and put a few turns around it to act as a current transformer?

Other options ive seen are putting them in series with the cathode of the input tubes. I dont think thats a good idea though.

[mark2collection]

Another take on things, how would a tuning indicator or two (if the amp is stereo), or perhaps a DG7-32 'scope with jellyfish display suitably connect X to Left, Y to Right, look on your amplifier?

Mark

Could you put a toroid around the HT lead that feeds the CT of the OT and put a few turns around it to act as a current transformer?

If it is perfect class A there won't be any current variation. I would use the input voltage (before the volume control) this way the needles will wiggle with any input, no need to turn it up either.

Please explain CT and OT (& HT maybe) for non valve/electronics visitors. It is usual that the first time an acronym is used the full spelling is next to it in brackets. Saves a lot of confusion.

[nzoomed]

Originally Posted by mark2collection

Another take on things, how would a tuning indicator or two (if the amp is stereo), or perhaps a DG7-32 'scope with jellyfish display suitably connect X to Left, Y to Right, look on your amplifier?

Mark

That could work, although the DG7 is quite big to fit in an amp chassis, inless it was mounted on top of the amp, then it could work I guess.
There are some small 1 inch size CRTs out there I think, they could work quite well as a panel mounted meter.

Originally Posted by merlinmaxwell

If it is perfect class A there won't be any current variation. I would use the input voltage (before the volume control) this way the needles will wiggle with any input, no need to turn it up either.

Please explain CT and OT (& HT maybe) for non valve/electronics visitors. It is usual that the first time an acronym is used the full spelling is next to it in brackets. Saves a lot of confusion.

Problem is a power amp doesnt typically have a volume control, and even if it did, the signal is still dependent on the volume of the signal being fed into the amp to begin with, so you would be back to square one.
I would be running class AB1, so im guessing some ammeters in series to each transformer would work?

As far as acronyms go, there are heaps used on here that I have no idea what they mean, but CT typically means centre tap and OT is output transformer.

[retailer]

Back when I built an optical audio compressor for my nephew I purchased a Vu meter from Ebay a quick look shows there is still plenty to choose from, along with driver boards etc should you need one.

https://www.ebay.com.au/itm/36336677...YAAOSwZqtgfr-L