Installing an analog meter to amp output

[wardevans]

Hello I am currently re-housing an old Valco/Ward model 62-9254 (that I have unfortunately not been able to find a schematic for) 80 watt tube head and I would like to install the meter pictured below so I can see the needle dance from signal going up and down at the output . I have heard of specific circuits for this before but I can't seem to locate them. Does anyone have any insight on this? Thanks!

[JJGross]

I wish I did, that sounds like a cool idea.

Of course I'd probably have one in all my guitar amps - just because.
Then I'd be looking to add Magic-Eye" tubes to them as well.

[olddawg]

Hello I am currently re-housing an old Valco/Ward model 62-9254 (that I have unfortunately not been able to find a schematic for) 80 watt tube head and I would like to install the meter pictured below so I can see the needle dance from signal going up and down at the output . I have heard of specific circuits for this before but I can't seem to locate them. Does anyone have any insight on this? Thanks!

The easiest way would be to use a standard line out voltage divider circuit from the speaker out, run the signal through a small bridge rectifier (maybe even just a diode) then through a large value pot (1 meg or whatever you have laying around) through the meter. Then run a sine wave signal generator at 1Khz into the amp on a dummy load and scope the output. Turn the amp up until it just starts to clip and then calculate the RMS voltage. Then just adjust the pot until the meter reading matches that value. You can do it other ways. You can figure out the meter coil resistance. It's probably a 25 or 50 microamp meter. You can design the circuit. But the quick and dirty way I just described should work fine.

[Enzo]

It should already be an AC meter. Either the meter itself is a real VU meter, or it was puled from a piece of test gear that included a VU meter in which case the movement could be anything nice.

SInce all you want is eye candy, then OD's trick should work, but I don;t think you need to rectify it. Determine if the meter really responds to AC. A pot in series with it across the speaker should drive it, even if it is a 600 ohm meter. YOu don;t want to peg it. Set the pot so the range of motion is within the range of amp volume.

[chipaudette]

I think that the idea of adding a VU meter to one's rig is awesome. Unfortunately, I don't know if this particular meter is going to be particularly satisfying.

It looks like its only set for 12 dB of dynamic range. Unless you play with a pretty saturated distortion, I can only assume that one's output is much more dynamic than this. I would assume that, with this little dynamic range, you'll either have it pinned or you're barely see it move. Then, at just the right amplifier volume, you'll proably see it slam from rail to rail.

An alternate way to hook it up would be to connect it just before the master volume on one's amp. Then, it is independent of the amp's actual volume and is just sensitive to the signal level in the pre-amp. Then, you could dial it in to good action at the "kinda dirty" level. Heavier distortion will slam the needle (cool) and softer action will make it move less (fair). Still, the measly 12 dB of dynamic range still might not be as satisfying as you'd like.

If you decide to hang the meter off the pre- master volume preamp location, you'll probably want to buffer the signal. The suggestions above were excellent because, by hanging it off the speaker leads, the power amp is able to provide all the current that this meter would ever need. But, the pre-amp might not.

So, to buffer the signal, I'd put an opamp in the amp just before the new meter. Since the op-amp isn't in the guitar's signal path, you don't have to worry about violating any "tube purist" philosophies. The op-amp is just between your guitar signal path and the meter. Ooo, another alternative is to hang the meter off the "send" to your reverb....this would be plenty buffered already! Ooo, you could also connect the meter via your effects loop! In this case, you'd turn your meter into a pedal or a piece of rack gear. So many options!

Chip

[danielzink]

Someday - I'm going to get my VU Meter hooked up to my project silvertone.

Anyhow - I found this schematic: http://www.ampix.org/albums/newamps/...cemaker_60.pdf

and drew up this schematic based off of it.

Anyone here think it'll work - on an old 10 watt Silvertone junker tube amp ?

Just to make the meter needle jump up and down (eye candy)...



Thanks, Dan

[Enzo]

ISn't one side of the speaker already grounded? In which case those two 1k to ground would be superfluous, at least one of them? No? But if it worked for them, why wouldn;t it work for you?

If all you want is a wiggly meter, then any meter that reacts to voltage can be connected most anywhere in the circuit. Use resistors to tailor the voltage levels for the meter. Speaker leads are convenient and insensitive, so good a place as any.

And ultimately an analog voltmeter is reacting to current, so the lower the current the meter movement uses, the less it can affect the operation of the amp.

[teemuk]

Meters, Multipliers and Shunts