[HAM] Percussion & Matching Transformer

[David Anderson]

Jordan,

The answer is a bit complicated, so bear with me.

Page 2-25 of the Hammond service manual reads that the early 2-series  
organs used "small core transformers" with AO-10 preamplifiers with  
codes A,  B, and C.

The later 2-series organs with AO-10s D, E, F, and G (plus all 3- 
series organs) used "revised small-core transformers with smaller  
stack and greater number of turns."

The manual says these matching transformers types are not  
interchangeable, but your mileage may vary. The two groups of AO-10s  
do have somewhat different input circuits.

The modifications to the matching transformer wiring to make it like  
a 3-series organ aren't too complicated, though you may need more  
solder terminals. What these resistors do is to lower the volume of  
the entire upper manual at certain percussion settings. Some people  
don't like this effect and jumper the 3.9M resistor permanently out  
of circuit to maintain full volume on the upper manual at all times.  
You might also want to add the special preset switch under the B  
preset key, present in 3-series organs, but not in 2-series models.

One effect you have to deduce from the wiring is that the cable  
capacitance of the wiring used to short the 3.9M resistor becomes  
significant due to the high value of the resistor. My calculations  
indicate it should be about 400pF, and the effect is that the volume  
is reduced less at higher frequencies since this cable capacitance  
bypasses the 3.9M resistor. My theory is that the 470pF cap across  
the 1M resistor is there to compensate for this cable capacitance,  
and you could tune this 470pF value to taste, especially based on the  
capacitance of the cable you have running from the percussion switch  
to the matching transformer assembly. Lower capacitance cable might  
make a smaller capacitor of 220pF-330pF useful.

Bottom line: your percussion circuits will work just fine without the  
extra resistors and capacitor. It all depends on whether you want it  
to work exactly as Hammond designed it for the 3-series organs.

David A.

On Jan 10, 2008, at 3:48 PM, Jordan Kersten wrote:

>
> Does anyone know exactly what the difference between a B2 and B3  
> matching transformer is?  I know that the B3's have some sort of a  
> resistor that has to do with percussion volume, and the B2's  
> wouldn't have this.  Are the transformers themselves the same? Can  
> you modify a B2's transformer with the percussion circuitry it  
> needs? Or would you have to replace the B2 transformer with a B3- 
> style one?  I am converting a B2 to a B3.  I now have tracked down  
> the plate with percussion tabs, the AO-28 preamp, smooth drawbars  
> and even a newer style vibrato line box.  I have herd that they  
> have different transformers.  Can anyone comment on this?
>
>
> -Jordan-