1936 ES-150 tone pot and tone cap values, & 36 number in each bar magnet?

[murrayatuptown]

Hello:

I am looking at a local guy's ES-150 bar/blade pickup (call it CC or Oscar Moore, or whatever). The two bar magnets both have a number 36 below the surface (cast, I imagine, as I don't know if they could stamp or engrave that magnet alloy). No notch in the blade.

It is not installed in a guitar. The volume pot is huge and measures 530 k across the ends. The pickup measured 2567 ohms.

There is no tone pot or capacitor, nor is there a wiring harness. The leads from the pickup are floating.

I read somewhere that on the EH Hawaiian guitars from that year or 1937, the capacitor was 0.02 uF and the pot was 500k.

Is that a safe assumption for those tone component values on an ES-150 guitar?

Also, that the numbers 36 below the surface on each magnet indicate year, or coincidence?

The resistance and lack of pickup notch are both consistent with the 1936-38 pickup style.

Thank you

Murray

[bluenote61]

Here pictures of the harness from a '38 ES150. The pots read around 500k, the tone cap is .03 mfd.
Please note that the Vol pot is wired reverse as today, i.e. the pot's wiper is connected to the pickup wire instead of the output jack.
Hope this helps.

[murrayatuptown]

Thank you.

Disturbing what several decades of exposure to air (and whatever it bears) can do.

Murray

[ThatRhythmMan]

I don’t know if these will help, but I suppose they are good for general documentation in the least. These are in situ pics of the electronics in my ‘36.










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[murrayatuptown]

Thank you.

For anyone else's reference,

Approx. 2.4 kohms is said to be typical of this early version.

Measuring resistance & comparing just gives you a 'warm fuzzy feeling' that the winding is not open. I measured 2567 on two different meters (neither has any calibration certs but getting the same or very close is a good sign neither meter is crazy).

If resistance is unusually low (there is no stated tolerance = +/- some amount, common with marked resistors), compared to common knowledge, a partial-short is a possibility. It doesn't seem to occur very often with pickups (they usually open up completely), but it can happen with transformers that handle higher current and voltage.

Measuring the inductance, whether you know what it typically should be or don't know for a given pickup, gives you another comparative measurement to consider, especially if you encounter a suspect low pickup resistance.

A short across the two end wires of the full winding, or somewhere mid-winding, say between layers of turns, generally reduces the inductance drastically because a shorted conductor really messes up the magnetic flux in the inductor core, which is the blade or slugs in other pickup types. This causes severely reduced inductance, much more than slightly low resistance resulting from a partial short circuit. This is a way to check a transformer for an obvious shorted winding (but not full functionality), as you can typically only measure the leakage inductance with a shorted winding.

The 'messed up flux' I described is temporary and does no harm, once the short is removed (either rewound, or unfortunately, a subsequent open circuit).

Everyone can get reasonably comparable resistance measurements, but measuring inductance depends on many factors, like the frequency the measurement is taken at, and how much signal the test equipment applies.

I guessed I'd get a somewhat low inductance (wild guess 1.5-2 Henries) because that was common with some very early pickups, but I was wrong. It makes no sense to compare two totally unrelated pickups, just as comparing resistance between them isn't meaningful, for different reasons.

If you have an inductance meter, you often have minimal choice of frequencies to take the measurement at. Sometimes it is automatically decided by the meter, based on its design and it's initial 'opinion' of the impedance it sees. You need more sophisticated equipment to vary the test signal level.

At 1 kHz, I measured 4.756 H and Q factor of 1.363 (Q = inductive reactance/resistance)
At 120 Hz, I measured 5.777 H and Q = 1.311
AT 100 Hz, I measured 5.867 H and Q= 1.183.

So far, for anyone who cares, the DC resistance remains constant. The AC resistance (not impedance) measured by inductance and impedance meters/bridges/other instrumentation is usually slightly different, depending on the frequency. If you measure a speaker voice coil, the DC resistance is usually close to, and typically lower than the voice coil impedance specification.

Inductive reactance is proportional to frequency (XL - 2*pi*frequency*inductance [H]). 100 Hz is the lowest frequency I could select on this 'meter'. You might expect Inductance to drop with lower frequency, but the core (magnet and pole piece(s)) have unknown (to us) behavior. Typically, if you get to a low enough frequency, the inductance does reduce. I measured another very physically thin pickup on an impedance bridge with variable frequency and signal level that I no longer have access to since retiring, and found its inductance disappeared below about 70 Hz. I concluded it had a very tiny magnetic structure. The term magnetic applies whether there is a permanent magnet or a non-magnetized metal like iron, steel or nickel alloys.

The next higher frequency I could select was 10 kHz. The inductance had dropped to 3.112 H and Q = 0.786. This indicates the impedance is dropping at this high a frequency. At some higher frequency the inductance will decrease to 0 H (theoretically at the series self-resonant frequency, where the inductive reactance interwinding capacitive reactance are equal in magnitude (but of opposite phase angle and cancel each other, leaving just resistance) Higher yet in frequency , the pickup begins to look more capacitive than inductive.

At 100 kHz I measured some value in pF (picofarads) and didn't bother to write it down because it was not important (not even to the mad scientist).

Where the self-resonant frequency occurs, is not of much relevance in a musical instrument, where volume & tone pots, a capacitor and cable capacitance make the isolated pickup measurements meaningless (you can't interpret how much output signal or tone quality from the isolated lab rat measurements).

But I take measurements on any (electro)magnetic component I am trying to characterize (good or bad, or just whatever data pattern evolves). This is more useful with transformers. For example, I was given a Gibson BR-9 amp that someone had changed the output transformer in. It was no longer mounted on the speaker (a substituted PM speaker). It had one inked 4-digit number on it that made no sense as a date code (two digits larger than 52, and neither pair was in the right range for the years the BR-9 was made), and a handwritten penciled "6 V on the kraft paper insulation". The "6V" made me quite uncomfortable. I checked the turns ratio, and it was about 18:1, consistent with a 115:6.3 V filament transformer. This was rather disturbing, as the output transformer in a single-ended amp.

I did a frequency sweep and found the self-resonant frequency was about 32 kHz! That's not the same as measured frequency response, I was impressed with that but wondered how & why that was installed by the previous 'invader', as an output transformer.

Eventually, I communicated with the engineer at the company that made Classic Tone transformers, and he told me what was supposed to be in there (they had a folder of test data, having quoted Gibson, but were not selected as a supplier...3.5k:3.2 ohms, gapped to allow 60 mA DC SE...3.5k:4 was close enough). The original field coil speaker came with the amp but needed a new cone and voice coil. He didn't have such a transformer 'in their catalog' but I talked him into hesitantly agreeing their 7k:8 70 mA DC p/n would work as a 3.5k:4 OT.

Sometimes I get a bunch of data I don't know what to do with, but the unnamed thin pickup with the rapid reduction of inductance below 80 Hz appeared to have a bandpass response instead of low-pass, that others typically do. Some people find their archtops have too much bass plugged in (I don't know about acoustically, but assume also), and all this is surely total coincidence that I like that pickup on every guitar I try it on.

Hopefully someone found this interesting, or it helped with their insomnia.