Tone Wheel Recap - Voltage Choice of Capacitors ?

[David Anderson]

And keep in mind that Hammond did tune the filters via selecting capacitors based on capacitance value. They reportedly tested the capacitors, separated them into several ranges based on capacitance, and chose a value that best tuned that filter to its associated frequency.

[David Anderson]

ESR in this circuit is pretty much not a factor.

With respect, in this particular instance, this is not exactly correct.

I would strongly suggest using Mylar and not polypropylene here. Usually, I prefer polypropylene capacitors, but this is a rather unusual application. The DF/ESR of the capacitor used does affect the filter's bandwidth. This is a case where you have to analyze the circuit and do the math, which I have done on a spreadsheet. Paper and Mylar have approximately the same dissipation factor, but polypropylene has about 1/10th the DF of paper or Mylar. Polypropylene will cause the filter to have a narrower bandwidth (sharper slope) and make the tuning of the filter more critical; in other words, as you move away from the capacitance/inductance combination that peaks the filter, the slope will fall off faster with polypropylene, magnifying any mismatch. Mylar should get you closer to the original design parameters.

It is, in fact, the increasing DF of the paper capacitors as they age that widens the bandwidth and makes it so the filters still work at all, considering the drift in capacitance value.

In most audio coupling applications, the DF/ESR of a film capacitor is not a significant parameter but it matters here.

[tpappano]

Pretty much any polyester or polypropylene is good. I read that many seem to be fans of the old "orange drops" but in my humble opinion as a manufacturer, they should have no advantage over any other polyester cap. If one was really wanting to split hairs, some capacitors can have lower (better) ESR (effective series resistance) which provides higher theoretical performance of the capacitor but since they are connected to coils that already have substantial resistance, ESR in this circuit is pretty much not a factor.

[Randall_Dibble]

SO, other then getting the correct uF value, lead length and enough voltage to cover the signal strength are anything to selecting capacitor material for a good sound ? Are orange drop caps something to consider ? or just about any type will be better then the original waxes?

[tpappano]

Or is it simply getting caps of the correct uf and long enough leads to span the distance between the terminals ?

You got it! Back in the day, 200v was about the lowest voltage rating available for that type capacitor. Most any polyester (Mylar) or polypropylene caps will work great. For example, Panasonic 'ECQ' series are inexpensive and compact. Be careful when ordering because various lead lengths are available, I was in a hurry one night and wound up with short "pcb mount" leads 8). The .255 are not real common, I used two caps in parallel for those filters.

[tiredoldgeezer]

Oh boy! "Cap Talk."

Ordinarily, a capacitor in series blocks DC. But where is all this DC coming from? It's a sine wave.

I could offer some meaningless speculation, but I'm sure someone has a better story.

Like you, ain't seeing the 200V either. But there it is.

[Randall_Dibble]

Ok, While researching recapping my tone wheel I discovered that the voltage of the old caps to be replaced are valued at 200volts. Give that the cap are in fact are handling millivolts what are alternate voltage choices do I have ? Or is it simply getting caps of the correct uf and long enough leads to span the distance between the terminals ?