MOTM

The 148, 149 Vishay/Spectrol pots differ in the composition of the 
resistance element and are not usually interchangeable.

148 series pots are of a conductive plastic composition.  They 
feature a smooth feel, are relatively quiet, have a rated lifespan of 
50,000 cycles and can be purchased in linear and other resistance 
tapers.  If you wanted an audio (log) taper, these are about your 
only choice unless you go custom.  The conductive plastic pots have a 
temperature coefficient of 1000 PPM/degree C.

149 series pots are Cermet composition, not quite as smooth as the 
148 pots and only half the rotational life at 25,000 cycles.  They 
are pretty much only linear taper.  These Cermet pots have a 
temperature coefficient of 150 PPM/degree C, which is almost 7X 
better than the conductive plastic variety.

IMHO it is best to use 148-series pots when a small bit of drift can 
be tolerated or when it is important to get a resistance taper that 
is log (audio).  If the pot is to be used to control a VCO frequency 
or the cutoff frequency of a filter that can also self-oscillate, 
you'd be well-served to use 149-series pots.

In a typical ADSR generator, log taper conductive plastic pots are 
generally used.  That is because the time constants can be dialed-in 
better with a log taper pot.  VCO frequencies almost universally use 
a Cermet pot for reasons of stability over temperature swings.  
Besides, most VCOs employ exponential current sources/sinks, which 
demand a linear taper pot to make frequency setting come out "right".

But "could" a linear-taper 148 be used in place of a 149?  Sure.  But 
if I were going to use a 148 because it costs less, I'd pop a wee bit 
more and get the 149.  At the moment, Allied Electronics seem to have 
the best prices.  I just received 30-some units from Allied about a 
week ago.  Much cheaper than Mouser (for now).

Doh!
Tom Farrand
<engage lurk mode>