[sdiy] Semi-OT: odd capacitor values

[Steve Carter]

Ingo

There have been several theories regarding how Hammond setup the tonewheel filtering and the batching of capacitors.  It seems for certain that the capacitor value doesn’t have to have a high tolerance as the slight attenuation of signal from an imperfectly tuned filter can easily be adjusted with the position of the tonewheel itself. However, even with the special tool that Hammond used, this is a lengthy procedure across the whole tonewheel generator. Apparently they bulk tested standard tolerance 0.1uF capacitors, established the mean capacitance value from the bell curve and worked out the tonewheel pickup magnet position according to that value. That way they got the highest yield of usable capacitors and could standardise a magnet setting of the tonewheel that would only require odd tweaks rather than a complete alignment - this saved a lot of time. So I’m guessing that the markings really are the target value for the caps.

That all sounds plausible but, having experimented with many tonewheel generators, the difference between a 0.1uF cap and a 0.105uF cap isn’t enough to be of any real significance on an individual tone but I suppose the profile over the whole generator would be altered with a 5% change of capacitance. I think nowadays the only thing that matters is that the capacitors are so out of spec that recapping with regular 0.1uF polyprop or mylar caps brings these old girls back to life in spectacular fashion and restores the bite that they lost over the years. That being said, I know of a few 1930s Hammonds with the original caps that still sound great - if it aint broke etc…..

Steve


> On 24 Aug 2016, at 20:23, Ingo Debus <igg.debus at t-online.de> wrote:
> 
> Well, this is not quite synth-related, but maybe some of the older guys here know…
> The capacitors on the tone generator of my Hammond organ are labeled 0.105MFD and 0.255MFD. Are these really 0.105 microfarads and 0.255 microfarads capacitors, or does the last digit 5 mean something else? I have read that Hammond hand-selected the capacitors back then to match them with accompanying transformers/inductors. Why would they have chosen such odd values? Does anyone know?
> 
> These are wax-paper capacitors and of course their values have gone way up over the years (the organ still sound nice though), so measuring won’t help.
> 
> Ingo
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