I just had an idea...

[stevlech]

Hi guys. I'm a longtime member/ occasional builder who still suffers from bouts of newb-itis and I'm resurrecting some old projects. I've never had a variac to test completed amps with and I'm wondering if a standard light dimmer would work to lower voltages for that purpose?

Any thoughts or advice are appreciated.

Have a good day.

[selloutrr]

Light bulb limiter

read Ron's build guide

[stevlech]

Thank you, selloutrr. That device will be next on the schedule.

Any thoughts on the dimmer?

[selloutrr]

Dimmer has no protection. The light bulb limiter gives you somewhat if a safety while you are starting up. If you try a dimmer make sure you meter the amperage as you bring up the voltage.

Personally I run a variac Eico 1178 (7amps) on an isolation transformer with external western electric 6" vu meters. Total cost under $200 after shipping.

[Cliff Schecht]

Yah I was gonna point out that a variac does not isolate you from the wall in any way. I don't use an iso transformer on mine but it is a good idea to if you are using your variac as any sort of test signal generator or whatever else people use them for. I use mine to drop wall voltage down to 110V, 115V, 117V or 120V and that's about it (maybe to bring up an old amp slowly sometimes). I wouldn't use one, for example, to charge caps up for reforming/leakage testing, especially without wall isolation.

[stevlech]

Thanks, guys. That's exactly what i was wondering.

[Dr-Joned]

Also the dimmer will not work on inductive loads FWIR.

[MCK]

Hi, I bought the supplies to build myself a light bulb limiter and while at the shop I picked up a dimmer too. Should I put it in series with the light bulb so I can reduce the voltages? Sort of like a poor mans variac. Would it work? Thanks

[rdjones]

A cheap dimmer works by "chopping" the AC wave.
As such it provides a dirty non-sine wave.
This doesn't bother the light bulb but will create noise in the amp.

Try it and see (hear).
The dimmer doesn't even have to be supplying the amp, if it's on the same circuit it will still put crap on the line.
Some of this might be able to be filtered, but by the time you put enough into the filtering you're approaching the cost of a decent variable isolation transformer with metering.

reddog Steve

[MCK]

Thanks for that explanation. I understand why it would not make a viable solution for regular use. My thought was just for the initial power up use. In addition to the lightbulb to put a dimmer and reduce voltages hitting the amp. Just a thought.

[brentm]

MCH: The take home is that a dimmer switch doesn't limit voltage. It just chops up the power signal in chunks. Essentially, the "phase" of the power is manipulated by letting certain parts of the wave through at a very fast pace (faster than the human eye can see, which I believe is ~60hz). As someone mentioned before, light bulbs don't care and the net effect is a dimmer bulb.

I believe that an amplifier or other device will burn up or be damaged by incorrect power phase. At least I wouldn't want to test it on my gear.

The dim bulb tester is different and it sounds like you're going to do that.

http://antiqueradio.org/dimbulb.htm

I'm going to spring for a variac myself. I'd hate to have all my hard work go up in flames because of a cheap lightbulb

[MCK]

Thank you so very much for keeping me on even keel. I now fully understand how the dimmer works and why it would be totally useless and perhaps dangerous on my new amp! Live and learn. Yes, I'm about to build my current limiter bulb. Amp just finished.

[Structo]

What about a variable ceiling fan control?

Like a dimmer but made to control a motor?

[niftynev]

you could switch in progressively higher wattage lamps in parallel with each other, but in series with one leg (preferably the active!) of the mains. this would gradually bring the ac voltage into your amp up slowly.


amps with higher filament power demands are gonna require a higher wattage lamp in series (or a combination of higher powered lamps )to allow testing.

a fan control is often similar to a light dimmer: not much use for amps. i have seen older ones, that look like they switch different taps on a small transformer. these types could work.