Quote Originally Posted by AgX View Post
A plain household lamp is less effective concerning the effective light output, but this would not affect quality.

Of much importance is however the size of a lamp and the transmittance quality of the bulb proper (clear or opal), actually the latter is finally a matter of size too... At least any enlarger that employs a condenser should be fitted with a lamp of proper physical size.

In the latter case the position of the lamp is of importance too.
+1

Most importantly, however, the size and shape of the bulb determines, along with any reflectors in the lamp house, the shape and position of the light source. And the condensers are designed with that shape and position in mind.

If the household bulb doesn't match the shape and position, the condensers won't illuminate the negative evenly, so your prints won't print evenly.