Re-tube em theory.. Why?

[Leo_Gnardo]

I've often thought that making a dedicated noise-testing fixture using realistic voltages and loads would be a great idea. I'd like to use a vibrating motor (like in those in cell phones) or something else more consistent than the 'pencil tapping' technique, though.

I've made one for the typical 12A-7 style tubes, a pair of Fender-like pre's, powered by an old Heath power amp with octal socket meant to attach to hi fi preamp the ol' fashioned way. Easy enough to test gain: apply a 100 mV signal, measure each triode's output. Listen for noise via the amp. Watch on a scope as well. I put in a meter/monitor switch, also selectable 100K or 33K plate resistors to test the various types under real world conditions.

Haven't done an "induced noise" test besides pencil or finger tap. For that, Stan mentioned the "calibrated hammer" he made for Magic/RubyTubes. Somehow I think that's no carpenter's hammer, probably something more like a piano hammer. Don't want to hazard cracking the glass. Shouldn't be too tough to rig one up. I'm liking your phone vibe idea too.

[Mick Bailey]

Or sonic toothbrush. I discarded one with a dead battery recently, now I regret that. It had a pulse mode and could have easily been re-purposed. I'm thinking mount a square plate of aluminum on four rubber grommets with a tube socket in the middle, so the whole thing sits on a strudy box - cut away as needed underneath the plate. Then attach the motor to the box and the business end so it vibrates the plate.

The toothbrushes that I've dsimantled (from Aldi and Lidl supermarkets) have a decent sized powerful 3v motor that oscillates in an arc of just a few degrees They have a built in timer, continuous, pulse and sensitive (light vibration) modes. The boards can easily be removed and mounted in something else and have tactile switches, plus flying leads to motor and battery.