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
Arthur
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
Arthur
