AW: AW: AW: Wurlie tricks / mods ?

[Haible Juergen]

>   Guess what the greatest surprise was when I opened it up: It 
already *has*
>   its preamp located on the pickup !! (No FET though)
>   It turned out to be a Model 200A, and they changed this from 200 
to 200A.
>
<Huh, that's interesting.  I didn't know about this.
>

The two halves of the pickup are connected by a little metal bow or 
bridge, just in front of the
part where the pedal comes in.
The same piece of metal at the front side, on the large GND block at 
which the reeds are mounted.
There is a little gap between the left and the right half of the 
dampers. In this gap, there is a
small pcb mounted (apprx. 1cm x 8cm),  with a 2-transistor preamp. 
Connections to the pickup
and to GND are directly made with two screws into the metal bows. 
Other signals and supply
voltages are fed in by loose wires.
The preamp has an input impedance of approx. 200k, and a gain under 
10dB.
On the main board, there is a similar preamp with similarly low gain, 
for a Line output.
Very few gain, but quite a lot of hum nevertheless!
Voltage on the pickup is 150V (@1MegOhm). The filter for this voltage 
should suppress ripples to 500uV,
and indeed disabling the 150V supply does not change the background 
noise at all.

I suspect that all the hum comes in directly from the large pickup bar 
itself, and would be
increased or attenuated together with the signal whatever preamp 
changes I make (?).
So I doubt if increasing preamp impedance + lowering polarisation 
voltage would make
much sense. Rather the contrary ...

So maybe a toroidal transformer would help to keep the overal 
electromagnetic dirt low,
which might help to get the pickup cleaner. A collegue of mine 
suggested that the mechanical
hum of the transformer would also have an influence on the pickup ... 
makes sense, but
I don't know how much of an effect. I think I'll just unscrew the 
transformer and hold it in
my hand, and see if anything changes.
If I go toroidal, I might use a 18V x 2 one, and then use a tiny 
230V/30V transformer backwards
(from the 18V) to get the 130V AC for the polarisation network.

Funny thing, btw: The *distortion* was most probably caused by a wrong 
speaker! The previous
owner had replaced a defective one by a 4Ohm type. Either that was too 
low for the pwer amp,
or the speaker had problems itself. Anyway, with only one speaker 
connected, most of the distortion
is gone.
Speaking of the power amp, it's a bit largen than on the 200 models, 
as well:
pnp differential pair gets its tail current from the (regulated) +15V 
preamp supply; the rest
is powered by an unregulated +/-22.5V supply: one npn gain stage, 
complementary output
stage with these npn/pnp "quasi darlingtons" (what was the right name 
for that ??),
and a npn for level shift for the output stage. Bootstrap elko for 
high output swing.

JH.


"As the dust settles, see our dreams all coming true ..." (GG)