Measuring T/S parameters on old Jensen C15 woofer

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Measuring Thiele – Small parameters on an old Jensen C15 woofer from a Leslie 822
I bought an old not working Leslie 822 and ripped the motors and speakers from it. It was build in Denmark like many of the ´newer´ Leslies we see here, but it had 117v motors. A Leslie 960 I have, have 230v motors.

It had an original Jensen V20 or V21, 16 ohms that works, and the woofer is 4 ohms. That didn’t matter at that time because it had 2 transistorized amps that could be tuned individually. I am going to make it passive, hopefully in a smaller cab, so I can hook up a valve amp top, and just have a pedal for the motor controls.

I have not been able to find any T/S parameters for the old Jensen’s anyshere, and I have for a long time had an interest in trying to measure T/S parameters myself. I have the necessary equipment to do it, and I found a very good practical introduction on the ESP web site. It has a thorough explanation how to do the measurements, and a well working spreadsheet to calculate the T/S parameters afterwards (be aware it is using 2 sheets).

I used the Added Weight method to measure Vas, because it is easier and because I have a weight that is precise down to 2 decimals in grams.
I suspended the woofer in free air and used 1 volt output and I hooked my Fluke meter over the amps output to be sure about the output at all times, and to my surprise I had to adjust the output slightly at every frequency. The amp is a small Class T ´50w´ amp. Does it mean that the amp is unstable? And how does Class T amps distort- pleasing or not pleasing in a MI sense?
All in all I took great care in the whole process to measure correctly, and the sanity check in the spreadsheet were very close (58.54 vs. 59 Hz), but I got some quite weird results:

Re 2.9 ohms
Fs 59 Hz 32.99 ohms at resonance
Qms 5.33
Qes 2.29
Qts 1.6
Vas 178.7 liters

I suppose the old 16 ohms ceramic woofers have similar values, except for Re. Anyone that have the parameters from one of those?

A quick calculation gave a vented cab at around 179,000 liters – that is a lot!!
And a high Q like that does not work in a closed cab. Leslie seems to have been very random with cab tuning. Sometimes they have one triangle cut out, and sometimes they have two, and they use different, probably not similar, speakers in the same cabs.

Looking back it was designed 60 or more years ago, and open cabs and open baffles were often used. Could it be that the parameters are correct?
And what to do? Probably make it kind of Open Baffle? I did simulate the parameters in Loesch´s Open Baffel spreadsheet, and it seems it can deliver some ok results in an open box (Open Baffle with sides) W50xH50xD20 centimeters. That would be very usable for a smaller Leslie type box, and probably very open sounding, but very critical with placement. Any opinions / good advice?

Best regards

The parameters are typical for an old driver to be used in an open baffle.
High Q does not work sealed, which makes vented a complete no-no.

Any sort of open backed cabinet would be suitable, and don't be scared
of oversizing it to give some more bass boost - if that is what you want.

rgds, sreten.


Joined 2003
Jensen C15? Jensen models normally began with a 'P'. University with a 'C'.

Regardless, back then these drivers were 'all of a same', so they could be cross loaded with each other and to the 'class leader', Altec's 803; ergo an Fs in the low 50s [often closer to 60], Vas in the 6-7 ft^3 and Qts in the 0.2-0.28 ranges, so 'batting' two out of three.

Even if this one was made long after SS amps became the norm, the Qts should still only have risen to the 0.4-0.7 range depending on the date code, so at >2x this, no clue why, what with the others being ~spot on.

BTW, what is the date code?

Hi GM,
Jensen P were Alnico and C were Ceramic, or actually still is.

It has a Leslie part number which is 031070, and another number which is 1098839.

They were OEM´d by Leslie.

There is also a number on the backside of the cone, but it is totally smeared, so it is not readable.

Best regards


Joined 2003

Interesting! 'Dating' myself as I don't recall ever seeing one, but in a search found this one from an organ and if its magnet size is similar to yours, then it would indeed have a very high Qts: Vintage Original Jensen C15N Ribbed 15" Speaker 8 OHM Tested Very Nice | eBay

With your specs, a stuffed to 'taste' Fs/Qts TL with the driver down around 1/3 - 2/5 is a viable alignment. Use Hornresp or similar to find optimum [or at least minimum acceptable] net volume [Vb] as it will be quite large by today's standards.

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Somehow I allways have problems attaching pictures.


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Forget Thiele Small here, at least to design a tuned cab, these old speakers had way too high total Q for that, even too high for a closed cabinet, go figure.

Does it mean they are unusable?
Not at all, simply that low frequency response will be neither flat nor controlled, but who cares?

A 6dB bump at actual resonance in cabinet and terrible damping is bad for Hi Fi but cool in a Musical Instrument cabinet where it passes as full/warm sound, go figure, and in any case is part of the sound signature.

The holes cut in old cabinets were just that, holes, not vents, the idea was simply to relieve a little of the boxiness.

Just curious, how do you know it's a Jensen speaker? (and not a Rola / Oxford / Utah / CTS / Eminence / EV)
Please post all codes found on it.

Jensen in general would have stamped a 6 digit code (220 something) in white ink somewhere on the frame, usually the frame edge (where the mounting holes are) , sometimes with black ink on the magnet, very few times on black ink on the (light grey/brown) gasket edge, I see none of that, only a 109 something.
031030 is the internal Hammond/Leslie code for that speaker, actual manufacturer might have been anybody who won the bid that year.
there are no more codes on the frame, just doublechecked.

I dont know exactely, but read that Leslie allways used Jensen P15 and C15 for woofers, and Jensen V20 and V21 for treble drivers.

And I have had a Jensen C12 smooth cone that exactely looked like this one, just smaller.

I wanted the T/S parameters to decide if it is reasonable to think that I could make a good sounding smaller cab.
Thanks for helping me out :)

Thought for sure it were a Jensen speaker.

Regarding Q measurements I just blindly used the ESP spreadsheet to calculate values, so now I have a new situation.

The spreadsheet seem locked so it is not possible to se the formulas. But of course I could try to calculate things by hand.

Best regards
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