Adding VU-METER to guitar amp

[carrejans]

bump

[Steve Conner]

OK, let me try again:

If you buy the meter in the bottom right corner of the picture, the dB scale will be totally inaccurate. The other two meters are the ones that will have meaningful scales.

Explanation: Note that in the other two meters, from half scale to full scale is 6dB. (-3 to +3.)

This is how all meter movements will work when hooked up to an amp. Pointer movement is a linear function of voltage, and 6dB is a doubling or halving of voltage, so that's why it moves the pointer from half-scale to full-scale.

So say you set the bottom right meter to read its full scale "120" when 120dBuV are applied (or when the amp is putting out 120dB SPL or whatever )

Now, turn the amp down until the meter reads 40dB. You'll hardly notice any difference in volume! That's because you only had to turn it down 6dB, not 80 as the scale suggests. If you wanted that meter's scale to be accurate, you'd have to build a log converter for it, out of transistors, op-amps and the like.

Any meter used on an audio signal without a log converter will spend most of its time near minimum or waving around crazily. VU meters have always been like that, and the PPM meter was invented to improve on them.

[carrejans]

Ok, so I finally received the VU-meter in the mail. (the one on the left of the picture)

It's scale goes from -inf to 6dB.

There are some small letters on it, that read:
"TYPE OC -33 0DB=1.90 VOLTS A.C."


So, should I just try to connect it parallel over my speaker, and with a series resistor. What value should I take?



Thank you!

[guitician]

You could try a potentiometer across the speaker and set the meter on the wiper. Then starting the wiper on the ground side of the speaker/pot connection, turn it slowly until you get some moment while the amp is played. If you have an o-scope you can set 0db to the point of clipping of the output waveform. A 1meg ohm pot may work.

[carrejans]

You could try a potentiometer across the speaker and set the meter on the wiper. Then starting the wiper on the ground side of the speaker/pot connection, turn it slowly until you get some moment while the amp is played. If you have an o-scope you can set 0db to the point of clipping of the output waveform. A 1meg ohm pot may work.

Thanks, will try this later on.

Question: I want to try to put a led inside the VU-meter. Will it be OK to use the 5V from my power transfo, that's also used for the heaters of the GZ34 rectifier? (after rectifying off course)

Another question: Will the VU-meter possibly interfere with the sound?
- I think it has a big magnet inside; and it is also located near the output transformer.
- because of it hanging parallel over the speaker (with it's internal resistance and the series pot-meter)

[Steve Conner]

Thanks, will try this later on.

Question: I want to try to put a led inside the VU-meter. Will it be OK to use the 5V from my power transfo, that's also used for the heaters of the GZ34 rectifier? (after rectifying off course)

Another question: Will the VU-meter possibly interfere with the sound?
- I think it has a big magnet inside; and it is also located near the output transformer.
- because of it hanging parallel over the speaker (with it's internal resistance and the series pot-meter)

Not really, because the 5V is live with the full HT voltage, making it unsafe. Maybe you could insulate everything, but I feel that it's bad practice. I suggest you run it off the bias supply instead.

If anything, the speaker magnet is more likely to interfere with the VU meter, than the other way around.

[carrejans]

Not really, because the 5V is live with the full HT voltage, making it unsafe. Maybe you could insulate everything, but I feel that it's bad practice. I suggest you run it off the bias supply instead.

If anything, the speaker magnet is more likely to interfere with the VU meter, than the other way around.

Thanks, will look at this later; have to go to work now.


And the resistance parallel over the speaker, won't that affect tone?

[Steve Conner]

And the resistance parallel over the speaker, won't that affect tone?

Probably not. It'll be a few thousand ohms at least, and that's negligible compared to the impedance of the speaker.

[carrejans]

Probably not. It'll be a few thousand ohms at least, and that's negligible compared to the impedance of the speaker.

Yep, got it.

For the supply of the led, you say I should use the bias supply. Where is that located at this layout: http://www.ceriatone.com/images/layo...one_100508.jpg


Thank you very much!!

[guitician]

On the tranny it's the 3.15-0-3.15. You could use one LED off of one leg of the nearest tube to your circuit, and add a current limiting resistor. You may not have to use a cap, I don't think 60hz shows as a flashing, but I've never tried this.

[carrejans]

Ah, ok, with "bias supply" you mean the 6.3V for the heaters. (sorry, I'm no good at all those slang words ;-) and my english is not the best)

I don't think you can see 50-60 Hz flashing. I will let you know after I tested it.



What do you mean with: "the 5V is live with the full HT voltage"

[Steve Conner]

Ah, ok, with "bias supply" you mean the 6.3V for the heaters. (sorry, I'm no good at all those slang words ;-) and my english is not the best)

No, that's not what I meant.

What do you mean with: "the 5V is live with the full HT voltage"

If you don't understand this concept, you should put down your soldering iron and step away from the amp before you hurt yourself.

One end of the 5V heater winding is connected to the cathode of the rectifier tube, which is connected to the positive terminal of the main filter capacitor, which is charged with 400 odd volts. Think about what that means.

[guitician]

The bias supply is the one wire coming from the 50V on the tranny and going to the one diode. The GZ34 pins 2 and 8 have the 480 DC volts on them, plus the 5 volts AC for the heaters.

You know what Steve, I was thinking the same thing. This guy is really at a beginner's level to be asking these types of questions.

[carrejans]

Quote:
What do you mean with: "the 5V is live with the full HT voltage"

If you don't understand this concept, you should put down your soldering iron and step away from the amp before you hurt yourself.

Sorry, if I don't understand what you mean with "live" and "HT". You guys always think that everybody in the world has English as their mother tongue.


And yes, I'm a beginner at amps; but I have some electronic experience. You have to start somewhere, I think.
(and I just keep my hands off everything inside when it's charged; believe me)

I ask where the bias supply is located. Guitician answers: "On the tranny it's the 3.15-0-3.15. You could use one LED off of one leg of the nearest tube to your circuit, and add a current limiting resistor. "
So I assume it's the heater voltage. Is this so strange that I assume this?
But now I know you mean the 50V. (what I already thought off in the first place; but thought I should ask first)

But indeed, maybe I can use the 3.15 also?

(I should have a schematic in the first place, in stead of this layout.)


Thanks anyway for the help and concerns.

[carrejans]

I just tested the led with the 3.15VAC, and you can't see flickering because of the 50-60Hz.

EDIT: actually, when you look good, you can notice a little flickering. But it's not bothering me.