Tone Wheel Recap - Voltage Choice of Capacitors ?

[RogerRabbit]

This is why I recapped my C3 the lazy way and swapped the entire filter tray from from my scrapped TTR-100 TWG, added the uncapped coils and it was done. Just had to adjust three pickups to be right on the curve.

[David Anderson]

Mylar is the original trade name for polyester made into a thin film -- as opposed to threads or fabrics.

[David Anderson]

Yes, you can add capacitance to detune the filter and reduce output.

There is an optimal capacitance value that will maximize the output of a single tone's frequency. It might be 95nF, 98nF, 100nF, 102nF, 105nF, 110nF, etc... Any capacitor value higher OR lower will reduce output. Think of it as tuning a radio with a signal strength meter to a particular station; it's essentially the same thing. At one point as you turn the dial, you will reach maximum signal reception. It's not going to get any higher than that. You are tuning a filter to let one frequency through and block others.

To put it another way, it's the combination of the filter's inductance and the capacitance that tunes the filter. That's what a lot of people don't understand. Hammond first installed the inductors on the filter tray and then selected capacitors in the manufacturing process at a station to tune the filters. Then the final calibration was done.

[Randall_Dibble]

Ok, I've been thinking this out and is it correct assume that if I had hot note, to strong of signal, that instead of screwing with the tone adjustment, pun intended, I could place in parallel a very low value cap 0.005uf or so to tame it.

Is there a way to do similar thing to bring up the signal strength ?

Don't want really screw around with calibration anymore then I have too.

As a Musician the Voicing of the tone generator is of great importance.

[David Anderson]

That's what I find cool about the series resonant circuit. Usually, when you put additional passive components in series a circuit, you lose some signal, but when an LC series resonant circuit is properly tuned, the output increases over the inductor alone. The AC impedances cancel out, and the only signal losses are the resistive losses in the inductor and capacitor.

[tpappano]

No argument from me on that! The filters are ‘series resonant', if you replaced the cap with a dead short you would still have a tone. Reduce the cap to zero uf, you'll have nothing 8)

[David Anderson]

edit: and I think what David is getting at is if a polypropylene cap produces a narrower bandpass curve, there is less forgiveness for variations in the actual L and C values, which in turn could make the signal levels coming out of the filters more variable from note to note. A wider filter response would tend to reduce these variations.

This is correct.

The "problem" with modern capacitors is that they tend to be pretty close to the nominal value. In the old days, Hammond took advantage of the range of values (centered around 105nF) to select capacitance values to tune the filter and center it on the filtered frequency. If you have to err, it's better to err on the higher side of capacitance. I did tests with a decade capacitor substitution box, and if the peak value is 105nF, but you only have 100nF and 110nF, 110nF will sound better. That's what my ears told me listening to a single tone at a time. As you drop under the capacitance value that peaks the filter, the tone starts to sound thinner. You can believe me or not on that. I'm not going to take the time to get into a long argument about it.

[tpappano]

Yes, that would be true. On my TGs, the three with the polyester caps sounded good without any adjustments. On the TG with the polypropylene caps (my first), I did adjust some of the pickups to get the levels within 1 dB of their neighbors but that was probably me just being a little OCD 8) the other three I just left alone because they tended to be within 2 to 3 dB, which to my ears sounded fine. I did a ‘waterfall' plot on my living room sound system which showed as good as ones speakers may be, what the actual room acoustics do to sounds once they leave the speaker is pure mayhem! I think the same is true with an organ 8)

edit: I'm curious enough about the bandwidth issue that I think I'll do some measurements on an actual TG filter and see what the two types of actually do

[Randall_Dibble]

So, the bottom line here is that using a Mylar/Polyester Cap will help reduce variations in signal strength and require less adjustment in re-calibration ? or even many not require re-calibration ?

[tpappano]

You are correct!

[Randall_Dibble]

So, does a Polyester Cap = Mylar Cap ?

Mylar being a brand name to describe a Polyester Cap ?

[tpappano]

I think one issue is that the wheel produces close to a sine wave, but not a perfect one. I think the intent of the filters is to clean up the tone by reducing unwanted harmonics and also perhaps reduce some crosstalk from other wheels.

edit: and I think what David is getting at is if a polypropylene cap produces a narrower bandpass curve, there is less forgiveness for variations in the actual L and C values, which in turn could make the signal levels coming out of the filters more variable from note to note. A wider filter response would tend to reduce these variations.

[tiredoldgeezer]

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.

So maybe help me out here, just what does all this do for the circuit?

I mean slope, peak, bandwidth, and similar terms I understand about filters. But the input to this circuit is a sine wave created by a rotating wheel. So the frequency is fixed with or without the filter. It's a sine wave, so there's nothing to speak of in harmonics.

What you're saying makes sense in something like a VCF or a full range filter, where slope and bandwidth would matter, but these filters really don't filter anything, do they? And if so, what could it be? And if nothing is being filtered, what is achieved with a selection of one type over another?

I suppose the need would be to achieve resonance via the tank circuit and in an ULZ circuit (16 ohms?) prevent crosstalk or back feeding. But that wouldn't be affected by mylar/poly/wax/etc. would it?

[tpappano]

There should be some inexpensive mylars, for example:

https://www.mouser.com/ProductDetail...A%2FbghBod8%3D

$0.145 each if you buy 100 pcs.

I'll defer to David's judgement on the polypropylene/polyester issue, he tested both. I used polyester on 3 TGs and polypropylene on one, but I have not compared their relative bandwidths.

[Randall_Dibble]

So, if I under you, David, correctly the choice of a Mylar cap will better reflect the character of the original caps having a gentler slop and wider bandwidth which results in less adjustment needed in re-calibration ? Will this also result in a wider range or effect in the use of the draw bars ?

Do you recommend a particular Mylar cap ?

The Mylar caps I've found are expensive !