Wurlie tricks / mods ?

[Don Tillman]

   From: Haible Juergen <Juergen.Haible at nbgm.siemens.de>
   Date: Mon, 5 May 1997 14:28:54 +0100

   (1) 	As I was told, the typical distortion at hard playing comes from 
   the mechanical parts only, so
	 I wouldn't loose anything, if I built a "modern" replacement for 
   the electronics, would I ?

Generally not.  The Wurlitzer electronics are very noisey and should
be replaced.  I'll be doing that to my Wurlie someday, but it's about
number six on my list of electronics projects, and my list moves
very slowly these days. 

One issue: the tremelo circuit does contribute distortion -- it's an
LFO playing with the bias of a diode.  It's, ummm, quaint.  You may or
may not want to keep that.

   (2) 	In case I keep / fix the original circuit, are there any 
   interesting mods?

Ohboyohboyohboy...


   (3) 	Does an enhancement of the electronics in terms of noise etc. 
   make any sense at all, or is the capacitive pickup system the main
   source of noise ?     

The capacitive pickup itself doesn't contribute any noise... However
you loose about 30 or 40 dB of signal on the way to the preamp due to
the low input impedance of the preamp and the capacitance of the cable
swamping out the capacitance of the pickup.  This is why there's so
much noise on a Wurlitzer.

My plan is to build an FET buffer stage physically on the pickup --
this provides a mighty nice signal and you can do what you want from
there.  In fact, such a buffered signal is so beefy that you really
don't need the 170 volt bias, you can get by with far less, which
means you don't need a special power supply.

I've prototyped the FET buffer and it works great, I just haven't
done a proper construction job yet.

There's also an issue of the power supply.  "Kerry Minear Disease" is
when you place a Clavinet on top of the Wurlitzer, and the Wurlitzer
power transformer is optimally positioned to induce hum in the
Clavinet's pickups.  If you build the FET preamp with lower bias
voltage, you no longer need the 170 volt supply and you can use a
torroidal transformer.  Or flush the power amp and use a really tiny
transformer.  Or you can just keep your Clavinet away from your
Wurlitzer.

Also, CAE Electronics in San Mateo, California is building a run of
replacement replacement preamp/power amp circuit boards for the
Wurlitzer.  They're copies of the originals with maybe different
output transistors and quiter metal film resistors and some other
subtle quality improvements.

One other thing -- it's possible to split the pickups for stereo.  I'm
seriously considering eight or so seperate ranges, each with an FET
buffer preamp, and panned across a stereo pair.  In an interesting
case of physics actually being on your side, when you split the pickup
the gain increases proportately because there's less capacitive
loading.

  -- Don

----------------

"I can make your brain dance! how?
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