Panel meters...

[Chuck H]

Ok. I need to hook up a panel meter to a power amp driven reverb. This device will be adjustable for amps from ten watts to one hundred watts. I have circuit protection in place in the schem to protect the tank transducers, but... Ammeters are rated in negligable ohms (.0XX something) but "milliammeters" are rated in K ohms. I need a meter to show the user where to adjust the sensitivity of the amp. What should I choose as a panel meter? And how do I hook it up? I see that a regular ammeter would just go in series with it's negligable resistance. But in this application I'm deealing with a very low drive level that has to remain adjustable with the meter as a guage. The milliammeters with "K" resistance may not read enough across a resistor and have too much resistance to run in series??? any advice?

TIA

Chuck

[Steve Conner]

Chuck,

You need a voltmeter with a rectifier. But when it comes to analog meters, a voltmeter and an ammeter are the same thing. In fact they're all ammeters, in so far as they're current operated devices. The voltmeter just has a series resistance built in, and you can prove to yourself from Ohm's Law that this converts it to a voltmeter.

Some VU meters have rectifiers built in already. You can just connect one across the input to your circuit (you said this was speaker level, so there's plenty of voltage to drive a meter) put a pot in series with the meter, and adjust it so that the needle reads "0" at whatever you consider to be the right level of drive. Then replace it with a fixed resistor of the same value in production.

If you want to use an ordinary DC moving-coil meter, then you need to add your own rectifier. Moving-coil meters can't read AC, and moving-iron ones aren't sensitive enough: they're for heavy power engineering only.

If I misunderstood, and you actually want to measure some very small voltage, then you need to be aware that rectifier meters don't work below a few volts, because of the voltage drops in their diodes. Vintage germanium or Schottky diodes would help, but eventually you need to amplify the signal, and/or use an active rectifier instead of a diode bridge.

Or maybe you could read the cathode or screen current of some tube in your circuit, or even use a magic eye tube instead of a meter. There are lots of cheap Russian ones on Ebay.

[Chuck H]

Thanks Steve. I may have to go with LED's for indicators since the drive for the reverb tank is already padded down at the adjustment and the voltage is quite low. I wanted to use a panel meter because it just looks more organic and all. Thanks (as always) for the info and experience.

Chuck