Adding VU-METER to guitar amp

[carrejans]

Hi,

I recently build a 5F6A Bassman head. Now I'm constructing the woodwork. I was thinking it would be nice to add an old skool analog VU meter.
To be honest, not for it's use, but more for the looks. ;-)

I can buy the VU meters from this picture:

I think I will go with the one on the left.

Now, my two questions.
1) What can I use a VU meter for, exactly? Decibels from the output?
2) How should I connect it?

Thank you!

[flythair1]

Those are very cool looking meters there carrejan. It would be fun to use one in a build and have it work & be lighted. I'm not sure how to wire the db meter to the amp.
I've thought about using a digital LCD panel meter for maybe milli-volts or milli-amps for power tube biasing built in to my home builds. You could use a momentary switch or let the read-out run continuous, maybe a momentary on-on for two power tubes.
I've seen the meters on ebay for cheap and they are fairly small, 3/4" tall or so.
It would not look as nice as your analog meters, they are very retro.

Scott

[Chuck H]

That one is most likely a common "VU" meter (since it's marked in "DECIBELS"). Typically you would wire it parallel with the speaker using a series resistor (and possibly a series capacitor) to attenuate current. It probably has a resistance of 600 ohms so there shouldn't be any appreciable effect on output impedance. But... The amp your installing it in is capable of output that exceeds that meters range. The meter on the right would be more appropriate and could function accurately (and I personally think the old "yellowed" look is pretty cool). Another but... You can use the one on the left with a suitable series resistor, but it wouldn't be functioning in an accurate capacity. Reading maybe 1/6 of the amps actual output.

FWIW, if you do wire it parallel with the speaker the location of the meter should be as far from the input and preamp as possible. Like, way over next to the power and standby switches. Otherwise you could have output leads dangerously close to your input causing crosstalk and possibly oscillations.

Chuck

[Enzo]

A VU meter as an instrument will have that 600 ohm impedance.

But within a system, the meter itself can be whatever they designed it with.

So in some VU meter circuit, there may be 600 ohms somewhere, but the meter itself might be driven by an op amps.

But basically what you have is a voltmeter, and you need to connect it to something int eh signal path. Doesn;t relly matter what, as long as you cover the basics. And you just want eye candy, not some "accurate" readings.

You want it to respsond, but not load down the signal path. Experiment. You can block DC from it with a large value cap. 0.1uf maybe

The speaker voltage might be too much for the meter directly, and if so, then grab a couple resistors to make a voltage divider and feed that to the meter. Just the same as making a "line out" with a couple resistors across the speaker.

[Steve Conner]

When I used meters in my amps, I always rigged them up to measure power tube cathode current. It still goes up and down as you play, but it has a practical use too: you can use it to keep an eye on your bias and the condition of your tubes.

[mooreamps]

Hi,

I recently build a 5F6A Bassman head. Now I'm constructing the woodwork. I was thinking it would be nice to add an old skool analog VU meter.
To be honest, not for it's use, but more for the looks. ;-)

Now, my two questions.
1) What can I use a VU meter for, exactly? Decibels from the output?
2) How should I connect it?

Thank you!

I would drive it from an unused output tap off the power transformer ; perhaps the 16 ohm tap going through a bridge rectifier and through a 3.9K series resistor, as a start.... Also, perhaps a 0.1 uf parallel cap. I found a web link for you to look at. Check out Figure 1.


http://sound.westhost.com/project55.htm



-g

[carrejans]

Thank you guys, for all the tips!!
I will check the link later, when I have time, mooreamps. But why should I use an unused tap of my output transformer? Why not use the one going to my speakers? Thanks!

[Prairie Dawg]

Thank you guys, for all the tips!!
I will check the link later, when I have time, mooreamps. But why should I use an unused tap of my output transformer? Why not use the one going to my speakers? Thanks!

Use the Weston. You cannot find better made analog meters. My old man was the chief engineer there from circa 1958 to 1970.

Interestingly enough there are a couple of American companies that still make analog meters, Phaostron and another up in New Hampshire.

Phaostron is very nice stuff, too-they're in both of my ME9H military multimeters.

[guitician]

A useful place for one would be to monitor the line-out from an effects loop. But, if your just interested in looks you could just let it monitor the shifts in the power supply voltage. That would move the meter with a simple voltage divider. First you would need to determine the meters sensitivity.

[mooreamps]

Thank you guys, for all the tips!!
I will check the link later, when I have time, mooreamps. But why should I use an unused tap of my output transformer? Why not use the one going to my speakers? Thanks!

Perhaps "no special reason". Perhaps just a convinent use of a spare wire hanging off the secondary that would otherwise just be left ; you know ; just dangling out there. Also, the smaller value of filter cap such that the meter retains some "spring" in it's response. Too big of a filter cap, and it will bleed off the charge so slowly it will react slower than the gas gauge on my truck.


-g

[mooreamps]

First you would need to determine the meters sensitivity.

The sensitivity of most those old analog movements were either 50 uA or 100 uA full scale.


-g

[carrejans]

So I decided to put the VU-meter parallel over my speaker.
Still have some questions.

1) Some meters have internal 600 Ohm, some not. So I assume that if it already has this internal resistance, I don't need to add a resistor?

2) I have to add a capacitance in series? For blocking DC? Why is this necessary?

3) I'm trying to calculate the maximum possible voltage of my amp. How do I calculate this? I tried as follows:

P=UČ/R
U=Sqrt(P*R)

Pmax=40W
R can be 4,8 or 16 Ohm (depends on which speaker cab I use)

So: Umax=Sqrt(40*16)=25V=25.000.000 uV=20*log(24.000.000) dB uV = 148 dB uV

Is this calculation correct? I think I'm doing something wrong...

4) Is this the same decibel they use for measuring sound level?

5) The third meter in the picture seems to be good for this (maximum 120 dB)
But are the other meters in the same scale (uV)? If so, what are these used for? For pre-amps?

Thank you very much, guys! I appreciate all the help.

[carrejans]

Anyone?

[Steve Conner]

I noticed one thing: One of the meters says "Rectifier type" on the front, implying it has a built-in rectifier and is calibrated to read on AC signals. You want that one. If you buy any of the others, you may need to add your own rectifier. You'll almost certainly need to add a series resistor too.

Don't try to calculate the dB, you can make the meter read anything you want by varying this resistor. And if the meter was designed for use with some active detector circuit, the scale won't necessarily be meaningful in your application. The microvolt meter on the right is one such: it must be meant for use with a log detector, because there's no way the meter movement could achieve 120dB of dynamic range by itself.

100W into 8 ohms = 50W into 16 ohms = 28V RMS = 40V peak. Which is about 150dBuV, yes.

dB are just units of ratio, they can be used for anything. I can say my boss earns 4dB more than I do.

If you want to measure a physical quantity in dB, you have to specify the units and reference level after the dB symbol. 10,000 dollars would be 40dB relative to 1 dollar: 40dB$. Sound pressure is measured in "dB SPL" which are dB relative to the threshold of hearing, this being set as 0dB SPL.

[carrejans]

I noticed one thing: One of the meters says "Rectifier type" on the front, implying it has a built-in rectifier and is calibrated to read on AC signals. You want that one. If you buy any of the others, you may need to add your own rectifier. You'll almost certainly need to add a series resistor too.

Don't try to calculate the dB, you can make the meter read anything you want by varying this resistor. And if the meter was designed for use with some active detector circuit, the scale won't necessarily be meaningful in your application. The microvolt meter on the right is one such: it must be meant for use with a log detector, because there's no way the meter movement could achieve 120dB of dynamic range by itself.

100W into 8 ohms = 50W into 16 ohms = 28V RMS = 40V peak. Which is about 150dBuV, yes.

dB are just units of ratio, they can be used for anything. I can say my boss earns 4dB more than I do.

If you want to measure a physical quantity in dB, you have to specify the units and reference level after the dB symbol. 10,000 dollars would be 40dB relative to 1 dollar: 40dB$. Sound pressure is measured in "dB SPL" which are dB relative to the threshold of hearing, this being set as 0dB SPL.

Sorry, have reread your post several times, but it still isn't clear to me.

Because, after my calculations, I was sure to choose for the one with it's scale up to 120dB.
I thought I could just hook it up (with resistor and capacitor), parallel over my speaker. So it would read the voltage over my speaker.
But, indeed, I probably should also include a rectifier bridge. (but that's only 4 diodes)
So, after hooking it up like this (with R,C and D's), I will have correct measurements, no? But, the needle of the meter will be most of the time up in the max region, is that what you mean?

You suggest the meter at the left. But it's scale will be meaningless, no?

Sorry, but it seems I still don't get it...
I hope someone can help me...