In case someone finds this useful....
I didn't want to dismantle another phone to use as a subset, and early WE subsets for rotary candlesticks and D1 handsets etc seem scarce and expensive. So, I got a couple of Kellogg subsets on eBay cheaply and converted them to 634A equivalents.
I'm attaching 3 pictures: the first is the wiring schematic of the 634A, the second is the original configuration of the terminals and soldered connections in the Kellogg subsets, and the third is a description of the modifications I made to make it equivalent to the 634A (anti-sidetone). It was a matter of using the existing connected induction coil, renaming some terminals so there were enough connectors, and adding another capacitor.
It works! I could dial out and receive calls using a 151AL hooked up to this setup. I hope someone finds this useful too.
Otto
What is the number on your induction coil? Usually Kellogg stamped huge yellow numbers onto the wrappers.
Is it a 103A triad coil?
I have used a 103A similarly, and will check yours against my circuit. But to use a triad coil correctly in a WECo circuit, one actually has to cut a wire coming out the coil and reconnect the windings in a different sequence. Yet, indeed it can be made to work for laymen's purposes. But you cannot call it a 634A subset anymore.
I would think a good Kellogg subset is much rarer and harder to find than a WECo 634A, which can be found every week.
I could use a couple, for sure.
The only reason for my "conversion" was because the 103A coil was mounted inside a switchboard and I wanted to use a WECo 211 hangup handset mounting on the side of the PBX, and thus repurposed the induction coil.
The coil has 4 connections to the same terminals as the one in the WE subset, and is marked 99-A.
I've recently gotten 3 Kellogg subsets at about $15 each.... So I figured they are common and available. Are they actually rarer than the 634A?
Quote from: oyang on November 08, 2015, 05:46:22 PM
The coil has 4 connections to the same terminals as the one in the WE subset, and is marked 99-A.
I've recently gotten 3 Kellogg subsets at about $15 each.... So I figured they are common and available. Are they actually rarer than the 634A?
Aha! The No. 99-A induction coil is actually the completely wrong induction coil to make a 634A-class subset.
It is only a local battery induction coil, i.e. not even a common battery coil, and not an anti-sidetone type like that in the 634A subset.
It has only two windings instead of the three necessary for anti-sidetone operation.
I still haven't had a chance to go over your hook-up, but will do so.
Thanks for the review.
Maybe this is why the box doesn't ring for an incoming call? The striker does vibrate, but not hit the bells. I thought this was just a matter of adjustment to be made, but could it be because the coil isn't right?
Showing my inexperience, let me ask: does the induction coil play any role in sound transmission? I thought it was purely a converter of DC to AC, to allow the bells to ring because the AC current then alternates creating magnetic impulses on each side of the striker. Speaking of which, can one buy an appropriate induction coil from a modern source like Radio Shack?
With the setup I showed, the 151AL dialed out and received calls fine, with both transmission and reception of sound that seemed good.
Thanks,
Otto
Induction coils are used in the speech transmission circuit and they do not have anything to do with the ringing circuit. The TCI Library is an excellent source of instructional material regarding basic telephony.
You are not going to find a modern off-the-shelf source for a telephone induction coil, though some may attempt to hack an audio transformer. An inexpensive subset may be created out of an old 302 or 500-set base. In fact, Ray Kotke manufactures a cover that fits over a 302 base, making it look like a vintage subset.
Also, Automatic Electric "mini-networks" are prized by collectors since they often can be fitted inside of a telephone requiring a subset, though using them externally seems to be a waste of resources.
Quote from: oyang on November 08, 2015, 10:37:09 PM
Thanks for the review.
Maybe this is why the box doesn't ring for an incoming call? The striker does vibrate, but not hit the bells. I thought this was just a matter of adjustment to be made, but could it be because the coil isn't right?
Showing my inexperience, let me ask: does the induction coil play any role in sound transmission? I thought it was purely a converter of DC to AC, to allow the bells to ring because the AC current then alternates creating magnetic impulses on each side of the striker. Speaking of which, can one buy an appropriate induction coil from a modern source like Radio Shack?
With the setup I showed, the 151AL dialed out and received calls fine, with both transmission and reception of sound that seemed good.
Thanks,
Otto
We'll find out eventually what the problem with ringing is. The good news you are reporting is that the striker does vibrate. Since it doesn't strike the gongs, I suspect that it is located in the center between the gongs, and simply does not have enough amplitude to make a sound. This leads me to suspect that it in fact is what is called a frequency-selective ringer. These have the striker mounted on a stiff reed spring in the center between the gongs. The reed spring in combination with the weight of the clapper ball determines the principle frequency of vibration, the resonance frequency. The frequency of the AC current for ringing must match this mechanical frequency or the striker will not operate at full efficiency. Today's ringing uses 20 Hz frequency, which was historically called
straight-line ringing. The frequency ringers used a variety of frequencies between 16 Hz and 67 Hz. Look on the case of the subset or even on the ringer itself for any numbers in that range, most likely on the lower side. Perhaps 16 2/3, 25, 30, 33 1/3, 42.
As you know, a telephone line only has two wires, that serve for electronic speech transmission and reception at the same time, two-way traffic. A part of the art/science of telephony is to efficiently connect the microphone (transmitter) and the ear-piece (receiver) to the telephone line. Together, the receiver and transmitter have four wires, two each, one pair for reception and one for transmission. The induction coil performs the electronic function of converting the four-wires to the two wires of the line, by combining the reception and transmission paths, or when viewed in the other direction, by splitting the audio paths for transmission and reception. It does this in an efficient manner so that the least amount of energy that the transmitter generates is wasted in the receiver, and that the least amount of energy that is received from the distant telephone partner is wasted in the local transmitter. At the same time the induction coil also amplifies the signals produced by the transmitter through its action as a transformer.
I will let you know when I get chance to look at your wiring in detail.
In your pictures of your subsets you are showing us two different types, yet, you labeled them as "original" and "modified final". I am assuming you're simply using the first one to show the original state of the induction coil terminals, as you mentioned that you had multiple sets.
Picture "Slide2.jpg" shows a "normal" straight-line ringer, while the ringer in "Slide3.jpg" is the type of frequency ringer that I described earlier. This one must be the one that you described as vibrating only without striking the gongs. As can be seen, the clapper sits right in the middle between the gongs. In the other type, the clapper is pulled to one side by the bias spring.
You should have made the modifications to the other set, the bells might just have rung properly.
I don't seem to have the diagram for the 21068, I think this is the drawing number of the circuit. Are there printed labels with the circuit inside the cover or anywhere?
Considering your admission of little experience, you made an excellent choice for connecting your desk stand to coil and ringer. Good work, essentially.
What you couldn't know probably is that the black wire from the desk stand isn't needed when connecting the instrument to a sidetone subset. Its only use is for bypassing a set of switches in the instrument to provide the anti-sidetone compensation. As your subset cannot provide it, you simply don't need to connect the black wire at all. So, insulate the spade and store it. After doing so, you will have your BK terminal point (the second side of the 2 µF capacitor) not connected to anything. Connect it to your L2Y terminal, where the yellow mounting cord wire is already connected. Of course, you can just install a wire bridge from BK to L2Y.
What I can't tell for sure is whether you have used the two windings in the induction coil "properly". You can test and exchange L1--R with C--GN, perhaps one or the other configuration gives better audio performance. Perhaps you already discovered this in your experiments.
Your ringer is properly connected. I already stated the reason why it doesn't make a sound. Transfer your wiring to the other subset with the straight-line ringer and it might just work.
Thanks for these last 2 detailed posts!
You are correct about the two different ringers. The "slide 3" is the final tested rewired subset; "slide 2" is a different subset that was identically laid out and so I used that picture for convenience. I had not noticed that the ringer types were different, because everything else was the same. I was planning on rewiring the "slide 2" subset the same way, so sounds like it should work as intended.
The sound quality was quite good, so I haven't tried switching the terminals of the induction coil.
Question: Once I have added the second capacitor and there are now a 1µF and 2µF wired the same as the 634A subset, doesn't this now become an anti-sidetone configuration that can utilize the black wire from the desk stand? Or is the problem that the induction coil doesn't work for anti-sidetone?
You guys were right that this is addictive.... It's very zen to sit quietly studying a wiring diagram, stripping wires, crimping, testing my connections with an ohm meter, ending up with a physical object that functions. This contrasts with my job sitting in an office, staring at a computer screen, writing papers or grant applications, getting interrupted constantly by my students and lab staff to figure out why their experiments failed, going to meetings that accomplish almost nothing: the academic rat race!
I think the take-home message from this thread so far is "a little knowledge is a dangerous thing." I've learned just enough about telephones to mess around and think I've accomplished things I haven't (i.e. making a 634A-equivalent subset).
Quote from: oyang on November 09, 2015, 11:56:41 PM
Thanks for these last 2 detailed posts!
You're welcome!
Quote
You are correct about the two different ringers. The "slide 3" is the final tested rewired subset; "slide 2" is a different subset that was identically laid out and so I used that picture for convenience. I had not noticed that the ringer types were different, because everything else was the same. I was planning on rewiring the "slide 2" subset the same way, so sounds like it should work as intended.
The sound quality was quite good, so I haven't tried switching the terminals of the induction coil.
I found that the old catalogs provide conflicting information about the 99A induction coil. In 1928 it was reported as a common battery coil while 10 years or so later it was described as a local battery coil. Since I don't have one, I don't think, I cannot confirm one or the other. Since the quality you obtained is good, I suspect it is in fact a common battery coil. Only a resistance measurement of the windings can confirm that for sure.
Quote
Question: Once I have added the second capacitor and there are now a 1µF and 2µF wired the same as the 634A subset, doesn't this now become an anti-sidetone configuration that can utilize the black wire from the desk stand? Or is the problem that the induction coil doesn't work for anti-sidetone?
The anti-sidetone compensation requires a third winding in the induction coil. In the most common implementation, this third winding is wound or connected in a way so that the induced signal at its exit wires has the opposite phase relationship to the signal generated by the transmitter, so that when it is connected straight across the receiver it cancels the transmitter signal to high degree. Thus, no or very little energy is wasted in the receiver coming from the transmitter and more can be transmitted to the other party over the telephone line. This is the principle of anti-sidetone compensation. The capacitor has to be inserted into the feedback loop to prevent DC from entering the receiver. The value 2 µF is just large enough to not cause excessive signal damping. In theory, an infinite capacitor would be ideal, but until the 1930s capacitor technology was poor and a 2 µF capacitor would be larger than a pack of cigarettes. Automatic Electric used 4 and 5 µF capacitors for the same purpose in the Monophones in the 1930s, but by 1934 the sizes could be reduced with new materials.
In your adaptation, technically you don't strictly need the second capacitor, but since you already have it, I'll let you keep it there, because electrically it is a better choice than the 1 µF capacitor that would normally be shared between the ringer and the audio circuit.
Quote
You guys were right that this is addictive.... It's very zen to sit quietly studying a wiring diagram, stripping wires, crimping, testing my connections with an ohm meter, ending up with a physical object that functions. This contrasts with my job sitting in an office, staring at a computer screen, writing papers or grant applications, getting interrupted constantly by my students and lab staff to figure out why their experiments failed, going to meetings that accomplish almost nothing: the academic rat race!
I remember!
Quote
I think the take-home message from this thread so far is "a little knowledge is a dangerous thing." I've learned just enough about telephones to mess around and think I've accomplished things I haven't (i.e. making a 634A-equivalent subset).
You've done quite well, and this exercise is a good starting point to learn about the circuit theory of sidetone vs. anti-sidetone circuits, and telephone circuits in general. Dig into the TCI library and other websites and study the diagrams. There are also many excellent handbooks on the web from the 1900s to the 1930s that discuss early telephony better, certainly in more detail, than the scarce modern texts.
In Western Electric product terminology, the differences between the sidetone and anti-sidetone circuits is exhibited in the differences between the 102 and the 202 type telephone sets, discussed here on the forum many times. Here is a recent topic where I highlighted this circuit evolution, and it is directly applicable to the conversion you performed: 102 versus 202 (http://www.classicrotaryphones.com/forum/index.php?topic=4670.msg153048#msg153048)
Here are the wiring diagrams inside two of the Kellogg subsets, for documentation.
Ah, thanks for those!
From the little schematic in the upper left corner you see that the induction coil has only two windings and diagram also tells us that it is a common battery circuit, meaning the current to run the transmitter is supplied by the exchange.
Kellogg actually used the same terminal designations and wire colors here as did Western Electric, so this diagram is directly applicable to connecting WECo desk stand or desk top sets with a mounting cord having red, green, and yellow insulation on the conductors.
Compare with this diagram from the topic "102 versus 202":
(http://www.classicrotaryphones.com/forum/index.php?action=dlattach;topic=4670.0;attach=131917;image)
All you need to do is flip one of them horizontally. The switches are different, but that is a minor aspect.
So, in hind-sight this topic should be renamed to ".... to WE 534A Equivalent". In fact, it doesn't need to be converted at all. For all practical purposes, it is the Kellogg equivalent. Simply connect the yellow, red, and green mounting cord wires to the same colors as indicated on the wiring diagram in the center.
Interesting.... so is the 2µF capacitor/condenser I added actually doing anything? Or is it just a nice decoration?
Quote from: oyang on November 13, 2015, 12:28:04 PM
Interesting.... so is the 2µF capacitor/condenser I added actually doing anything? Or is it just a nice decoration?
It is blocking direct current from reaching the receiver.
Without a capacitor, DC would flow from the line L2 through the secondary winding (S) via contact GN, the green lead into the receiver (RX), then the red lead via R and the primary winding (P) back to the exchange battery via L1 to complete the circuit.
Most receivers after a certain time were designed so that DC flowing through them had to be eliminated.
For this purpose the electrically ideal capacitor would be of infinite size, so that it does not affect the audio flowing through it. The value
2 µF is an acceptable compromise to keep it reasonably small in the late 1930s. Previous to that time, even a 2 µF capacitor was rather large, but was in fact used just before WW-I. Due to the war, materials were scarce and they decided to only use 1 µF capacitors to save material. After the war shortage, they just kept it that way, because it worked reasonably well and was cheaper.
In the case of the 534A subscriber set, as well as your Kellogg set, the capacitor can be shared between the ringer and the audio circuit. Looking at my diagram, the ringer and the induction coil (S) are connected at the point C, therefore C carries no direct current, because the capacitor (C1) is located between L2 and C.
Using a separate capacitor for audio, one would disconnect the induction coil from point C and use the second capacitor to make the connection to L2 from the induction coil. This leaves the existing capacitor working only for the ringer.
The benefit of the second capacitor is that it is a better size for the application, i.e. it has a lower impedance for audio. Also, since you are using a new capacitor, not a hundred year-old part, that it has no DC leakage, further improving results.
You can experiment and connect both the ringer and the induction coil to the existing capacitor, and if you rather have the original visual esthetics, you can remove the new one.
For an anti-sidetone subset, the second capacitor is a must, a single one cannot be shared with the ringer. That was the original premise of your addition of it.
As I mentioned before, I hit a bonanza of Kellogg subsets recently (4). Three came from a seller who knows nothing about phones and looked like they had not been used for many decades. A fourth came attached to a WE 202 phone, which appears to have been modified at some point. Just for your interest, I am attaching pics of all 4. The wiring diagrams posted earlier come from #2 and #1 respectively, which seem to be equivalents to WE 534 subsets, as pointed out. As far as I can tell, these two differ only in the ringer design. #1 had bad surface rust, so it is currently disassembled for painting. #2 is the one I modified.
All 4 are about the same size as the 534/634, in vented metal boxes that are held shut by a single screw.
Having some spare desk set boxes around is usually nice as you expand your collection.
I spent some time with the catalogs and it seems to me that these are F602 type subsets, as documented in Kellogg catalog No. 9, ca. 1928, on page 37 (attached). They are shown in the lower right corner.
| F602-SA: | 1000 Ω, straight-line ringer |
| F602-BA: | 1000 Ω, biased ringer |
| F602-HA: | harmonic ringer (16 2/3, 33 1/3, 50, and 66 2/3 Hz) |
| F602-HB: | synchromonic ringer (30, 42, 54, and 66 Hz) |
These are booster circuit (sidetone) subsets for three-conductor common-battery instruments.
The construction of these is indeed very similar to the construction of the WECo 534A, the only obvious mechanical difference being the attachment of the cover.
You have two types of induction coils, a No. 79A (visible) and the No. 99A (per your report). The mechanical difference is that the 79A does not have the connection plate integral with the unit, whereas the 99A is a combined unit.
79A: Primary 33 Ω, Secondary 17.5 Ω; 6 ounces, 4 5/8" by 1" square spool heads
99A: Primary 25.4 Ω, Secondary 7.8 Ω; 7 ounces, 4 1/4" by 1 1/8" and 1 3/8" deep.
It seems, these were intended to be used with the F118 manual desk stand and the F301 dial desk stands.
This type of subset was also made in an anti-sidetone variety with the model number 610.
Subset 1: F602-BA
Subset 2: F602-HA or HB
Subset 3: F602-SA
The subset (4) in your last picture has the guts, i.e. the induction coil and the ringer of a Western electric 634 set. It's hard to see, but I think the (wrapped) condenser is WECo-made also.
Quote from: unbeldi on November 09, 2015, 04:00:21 PM
Picture "Slide2.jpg" shows a "normal" straight-line ringer, while the ringer in "Slide3.jpg" is the type of frequency ringer that I described earlier. This one must be the one that you described as vibrating only without striking the gongs. As can be seen, the clapper sits right in the middle between the gongs. In the other type, the clapper is pulled to one side by the bias spring.
Is there a way to make the frequency-selective ringer work, some type of adapter device to generate the right signal? The posts I've seen on this subject have suggested simply replacing them, but of course I don't seem to like being practical or doing this the simplest way....
Quote from: oyang on November 24, 2015, 05:20:42 PM
Quote from: unbeldi on November 09, 2015, 04:00:21 PM
Picture "Slide2.jpg" shows a "normal" straight-line ringer, while the ringer in "Slide3.jpg" is the type of frequency ringer that I described earlier. This one must be the one that you described as vibrating only without striking the gongs. As can be seen, the clapper sits right in the middle between the gongs. In the other type, the clapper is pulled to one side by the bias spring.
Is there a way to make the frequency-selective ringer work, some type of adapter device to generate the right signal? The posts I've seen on this subject have suggested simply replacing them, but of course I don't seem to like being practical or doing this the simplest way....
I don't blame you! I don't like doing that either.
In fact, it should be a crime. :o
Building a frequency specific ringing supply is actually not that hard. But easy is to use one of the few VoIP ATA's that can actually be set to those frequencies. Asterisk can also be hacked to produce them on the TDM400 cards.
Quote from: unbeldi on November 24, 2015, 07:04:58 PM
Building a frequency specific ringing supply is actually not that hard. But easy is to use one of the few VoIP ATA's that can actually be set to those frequencies. Asterisk can also be hacked to produce them on the TDM400 cards.
Do you have a source of information on how to build one? Parts from RadioShack? I tend to like leaving things as original as possible, which is why I started this thread in the first place: I don't like the idea of tearing down a complete phone just to have a subset, even a common one.
A second question: is there a list somewhere of induction coils describing which are sidetone versus antisidetone? I was thinking of updating some old ringer boxes to be complete subsets by adding induction coils and capacitors. The capacitors are easy to get, induction coils seem to turn up frequently. It seems like adding these to ringer boxes in the circuit configuration of 634A subsets would be straightforward. Some of those small ringer boxes are really attractive quartersawn oak and would be nice sitting next to a candlestick.
Quote from: oyang on November 24, 2015, 07:37:26 PM
Quote from: unbeldi on November 24, 2015, 07:04:58 PM
Building a frequency specific ringing supply is actually not that hard. But easy is to use one of the few VoIP ATA's that can actually be set to those frequencies. Asterisk can also be hacked to produce them on the TDM400 cards.
Do you have a source of information on how to build one? Parts from RadioShack? I tend to like leaving things as original as possible, which is why I started this thread in the first place: I don't like the idea of tearing down a complete phone just to have a subset, even a common one.
A second question: is there a list somewhere of induction coils describing which are sidetone versus antisidetone? I was thinking of updating some old ringer boxes to be complete subsets by adding induction coils and capacitors. The capacitors are easy to get, induction coils seem to turn up frequently. It seems like adding these to ringer boxes in the circuit configuration of 634A subsets would be straightforward. Some of those small ringer boxes are really attractive quartersawn oak and would be nice sitting next to a candlestick.
Well, Radio Shack probably has some useful parts, but you have to look elsewhere.
I have a Panasonic PBX that I have adapted to ring frequency ringers. It only involved feeding the frequency signal from a signal generator into the right spot, and using the ready made mechanism in the PBX to do the rest. A signal generator can be bought for a few dollars.
I also have built some standalone ringing generators with PAM8403 class D digital audio power amplifiers, available for pennies almost from China, driving a low-voltage transformer in reverse.
Western Electric induction coils are documented in Bell System Practices. Search for Section C31.401 Issue 1 1933-11-22, for the early induction coils up to that time.
Here is an excerpt I use in my documentation. I have also more extensive tables for my use. There is also another BSP from the 1950 or 60s for induction coils.
I don't know of any charts for other manufacturers. Usually it is necessary to sift through the catalogs.
Quote from: unbeldi on November 24, 2015, 09:29:00 PM
I also have built some standalone ringing generators with PAM8403 class D digital audio power amplifiers, available for pennies almost from China, driving a low-voltage transformer in reverse.
Thanks for the info. Would you be willing to share specifics on how to build the PAM8403 ringing generators and calibrate to the ringer?
Otto
Quote from: oyang on November 25, 2015, 01:27:08 AM
Quote from: unbeldi on November 24, 2015, 09:29:00 PM
I also have built some standalone ringing generators with PAM8403 class D digital audio power amplifiers, available for pennies almost from China, driving a low-voltage transformer in reverse.
Thanks for the info. Would you be willing to share specifics on how to build the PAM8403 ringing generators and calibrate to the ringer?
Otto
This is my basic design and the only formal diagram right now. It's really just three components, generator, amplifier, and transformer. The added resistor just functions as short circuit protection to limit current draw.
The frequency generator can be implemented in many ways. A ready made function generator is the easiest, but I also have used a little Arduino board supplying a waveform using pulse width modulation.
These Class D amplifiers are widely used to drive low-impedance speakers (<10 ohms) directly, in lap-tops and all kinds of small portable music/audio players. Cost ~ $1. The transformer is used in reverse, to transform a 5V AC signal to ca. 100V.
Don't take the circuit inside the box too literally, it's just to indicate an amplifier is involved, the actual implementation is rather different on the board from an op-amp. The board is only about a half inch on each side. Not shown in the diagram is its 5V power supply.
PS: Here is a brief introduction and view of a modified Panasonic PBX for frequency ringing:
http://www.classicrotaryphones.com/forum/index.php?topic=11948.msg127016#msg127016
I think you markedly overestimate my skills! My ability to implement will need more concrete instructions like x is connected to y on your Kellogg subset ringer...... Hopefully it's not too much trouble to ask for that guidance. I'm going to track down the PM8403 class D part, and beg you for help from there....
Embarrassedly,
Otto
Quote from: oyang on November 25, 2015, 05:01:00 PM
I think you markedly overestimate my skills! My ability to implement will need more concrete instructions like x is connected to y on your Kellogg subset ringer...... Hopefully it's not too much trouble to ask for that guidance. I'm going to track down the PM8403 class D part, and beg you for help from there....
Embarrassedly,
Otto
I didn't want to be the one spreading discouragement, lol.
You asked for the principle, and I shared it. I think that's about as far as I can be of help. And as shown, it is only for ringing a telephone otherwise not connected to the telephone network.
That's where the PBX solution is much easier to work with.
Ha, thanks! So excuse my ignorance, but the signal going to the ringer from the circuit in the subset should be known, and what the ringer needs is known.... there is no simple electronic way to make the conversion with one transformer that you simply insert in series between the two?
Not sure what you're suggesting.
You can certainly plug in the subset (or any phone) into the output of my transformer, but that doesn't make the telephone useful for talking.
For a complete configuration, to use this in a normal telephone hookup, one has to detect ringing from the telephone company, or from some other source such as an ATA, and replace this ringing signal with the output of my generator, typically with a relay or electronic switch to prevent ringing current to be sent back to the line. This is exactly how electronic key systems work, such as the Panasonic type I referred to.
So, it is easy to build a ringing generator to demonstrate the sound of a frequency ringer. But from there on it gets more complex.
Oops; guess I wasn't clear! I was asking whether the frequency-specific ringer could be made to ring. Right now the clapper in the ringer in the Kellogg subset vibrates weakly because it's not getting the right type of current. My question was whether somehow I could add an electronic element to the subset to make it ring, some type of adapter.
It depends how far the frequency is off from 20 Hz. I am not sure we established the frequency of your ringer. Are there any markings on the ringer anywhere? I got a box full of Kellogg ringers, and IIRC most or all have markings on the armature that pivots during ringing.
If it is not too far off 20 Hz, say 30 or 33 Hz, it may be possible to raise the value of the ringing capacitor which would lower the electrical resonance a little. I haven't tried it with Kellogg ringers.