[HAM] Percussion & Matching Transformer

[Jordan Kersten]

Thank you for that detailed explanation, it was exactly what I was looking for.  I only have one last question: If I have a B2 with an AO-10 D, E, F or G, then my matching transformer is the same that is in a 3-series organ and the only mod I would have to make is to add the 3.9M resistor for the percussion circuit? 



----------------------------------------
> From: thermionic27609 at earthlink.net
> Date: Thu, 10 Jan 2008 16:36:38 -0500
> To: hammond at zeni.net
> Subject: Re: [HAM] Percussion & Matching Transformer
> 
> 
> 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-
> 
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