[HAM] Recapping again, was: Overdrive

[David Anderson]

Hi Mike,

I'm coming out of lurk mode for this one.

Wax-sealed paper capacitors had rather wide tolerances, on the order of 
+/- 20%. I have read that Hammond picked the odd .105uF value in order 
to get more accurate runs since .105uF was a military value.

That said, it's VERY easy to experiment with your own generator to find 
the capacitance value that will peak each filter. On a CV generator I 
have, I found the values between 92 and 116nF (.092 and .116uF). I 
think I paid $9 for an EICO capacitor decade box on eBay that worked 
just fine.

The problem with modern capacitor runs is that they often tend to be 
TOO consistent. I've argued that Hammond made a virtue of the range of 
wax paper capacitors for this reason: if Hammond had only had ONE value 
of capacitor, their filter inductors would have had to be VERY 
accurately wound. On the other hand, the tolerances in the capacitors 
could be used to offset tolerances in the inductor coils. Perhaps the 
labor cost involved in matching was less than the labor cost of winding 
precise inductors.

My experiments showed me that the matching isn't super-critical. I 
could move the decade capacitor switch in a 5nF range without seeing a 
lot of difference in output. But if you put a .105uF capacitor on a 
filter that peaks at .092uF, you will get reduced output. To my mind, 
trying to get a precisely matched batch of caps is barking up the wrong 
tree entirely.

I don't see why such an aura of mystery surrounds this matching 
process. It's quite easy to do--and kinda cool to watch the output of 
the circuit change as you dial in different capacitance values, if you 
have the geek gene like me ;-)

And, if you're worried about your organ sounding too bright, simply err 
consistently on the high side with capacitor values for that "aged to 
perfection" sound. Use .120uF capacitors or higher. Maybe .150uF or 
.180uF.

Of course, since Hammonds were originally designed to emulate church 
pipe organs, and pipe organs *can* be pretty shrill in certain 
registrations, it wouldn't surprise me if many of these wax-cap organs 
were rather brighter sounding when they came out of the factory.

I also think that wax paper capacitors improved over the decade of the 
1950s, but they are all vulnerable to extremes of heat and cold as well 
as temperature cycling. Water has a very high dielectric constant, so 
just a little will send the value of the capacitor upwards. In a DC 
blocking application, the water would cause leakage, but TG capacitors 
don't block DC.

But do the wax paper capacitors themselves add a certain euphonic 
coloration? It's a question to ask.....

David A.

On Sunday, November 19, 2006, at 01:35  PM, mike p wrote:

> I also am curious to know -  At the factory we have discussed that the
> matchers had bins of caps to choose from .  Speaking of the .105 type 
> now ,
> what was the range of their selection that they had to choose from ?  
> Maybe
> I'm not getting the picture here ,  did they have for example a .103 
> bin,  a
> .104 bin,  and so on ?   What did they range from ?
>
> If I had ordered from an electronics store  at a 10% expected 
> tolerance my
> range would be  from .090 to .110  in theory based on a .100 cap 
> because
> they do not make .105 .   If this is the case I would take any of 
> these as
> opposed to whats in there now.  I had a .303  and quite  few in the 
> .250+
> range .  Of course if you're going to go through the trouble of this 
> project
> then you are going to want as consistant a set of caps as you can get. 
>   It
> is not economical for me to buy a quantity of 500 bag caps to come up 
> with a
> set to do one generator but for someone in the business that does 
> generators
> every month of the year ,  they probably buy them in Qty of 1000  to 
> get a
> better rate too and lots of possibilities to make sets .  This is who I
> would rather buy from and thats what I did.     Going to work more on 
> this
> today,  will let you all know when the deed is done   - Mike P