Recently I found a stash of old microamp meter that I bought at a electronics dump for spare change. They look really cool and nostalgic, solid steel construction, but the drawback is that they are for 0-100µA which is kind of low even for use in tube amps. Not wanting to let these beauies go to waste I will add a shunt to them to make a currenter splitter and enable the "rescaling" of this meters into something more uselfull.
Besides adding the shunt resistor, I will also rebadge the inside panel to reflect the new scale. In the photo below is an example fo a unit that I already altered for the old Assault & Battery preamp.
These meters have a resistance of around 2K for a 100µA range, meaning we can shunt the coil with a low value multi-turn trimmer and dial in the required value for the scale we want. As an example, I am looking for scales to fit the 1626 (20mA), 26 (6mA) and some other tubes. To get the needle nicely in the center, we'll need twice the total current as the 100% mark, i.e. 40mA and 12mA.
2000R for 100µA, so rescaling to 12.000µA means we need 1/120 of 2K = ±16,7 ohms
For 40.000µA we need 1/400 of 2K = ±5 ohms
A 22 ohm trimmer with 15 turns or more will do very nicely for both cases.
100µA to mA conversion made simple.
Yes, for $5 you can buy a new mA meter on ebay in the correct range, but where's the fun in that. OK, I have some of those as well :))) This just shows you can revive those fantastic, but otherwise useless NOS meters that every one passes by.