Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: osing on September 09, 2021, 11:14:53 am
-
Hi there,
I recently acquired a Leslie 18. Unlike the Fender Vibratone and Leslie 16, the 18 did not come with a crossover but instead had an adapter with straight through wires where the others had a capacitor and inductor. I had started a separate thread on this a few weeks ago:
https://el34world.com/Forum/index.php?topic=27887 (https://el34world.com/Forum/index.php?topic=27887.new#new)
I am thinking of building a crossover for my Leslie 18 just to see how it sounds. I have a few questions I am hoping someone can help clarify.
I found these installation instructions for the connector kits for Leslie 16 & 18. The crossover network (part# 012377) for the Leslie 16 suggests a 2.0uF capacitor and 3.2mH inductor (I attached a screen shot as well - see yellow circle):
https://bespin.federalproductions.com/mycloud/Organ%20Documents/Owners%20and%20Service%20Manuals/Leslie%20Kits/Vibratone%20Kits/7016-7019.pdf (https://bespin.federalproductions.com/mycloud/Organ%20Documents/Owners%20and%20Service%20Manuals/Leslie%20Kits/Vibratone%20Kits/7016-7019.pdf)
In addition, the Vibratone and presumably Leslie 16 also had a 200uF capacitor in line with the speaker - it can be seen in the manual (link below, screen shot attached as well, blue circle). Note that the manual indicates that the crossover network is part #012377, but it does not specify the inductor value (it just says "L1").
https://www.fmicassets.com/Damroot/Original/10001/OM_leg_gtramp_Vibratone.pdf (https://www.fmicassets.com/Damroot/Original/10001/OM_leg_gtramp_Vibratone.pdf)
1) Any google searches I do for the crossover design for the Vibratone suggests that the inductor value was 1.2mH. This differs from the 3.2mH for the crossover network (part# 012377) above. Does anyone have an original Vibratone or Leslie 16 and do you know for sure what value was used (I have included come calcs below which might help draw a conclusion as well)
2) Any thoughts on what specs should I be looking for w.r.t. inductors for this application? Since there is speaker current going through this, does it need a high current rating? I looked at some inductors but they seem to only show DC current rating?
3) Unrelated to the crossover, does anyone know the purpose the 0.01uF 1kV capacitor across the trem speed foot switch (circled in red in the second attachment)? When the switch is closed, the speed is slow, when open, it is fast. Is the capacitor some sort of voltage spike suppressor? Is the 60Hz secondary AC signal not getting through this cap because its value is low enough?
Based on posts I have seen from PRR, Sluckey, and others, I believe the math for the band reject filter in the crossover might be as follows:
With a 1.2mH inductor and 2.0uF capacitor in the crossover network, the center frequency of the band reject filter is 3,249Hz (1/(2*PI*SQRT(LC)). With a 4 Ohm speaker (which is what the Vibratone evidently came with), the dip would start at 531Hz (R/(2*PI*L) and end at ~20KHz (1/(2*PI*R*C). The 200uF capacitor (blue circle) limits frequencies below 200Hz from hitting the Leslie speaker (based on this table https://www.diyaudioandvideo.com/Tables/ButterworthCrossover/1stOrder/ (https://www.diyaudioandvideo.com/Tables/ButterworthCrossover/1stOrder/))
If instead a 3.2mH inductor is used, the center of the band reject filter becomes 1,989Hz, and the range would be from 199Hz to ~20KHz. Does this perhaps give clues as to whether a 1.2mH or 3.2mH inductor was used? With the 1.2mH inductor, the range of 200Hz to 531Hz would make it to the main speaker without being attenuated. With 3.2mH, it seems like hardly any frequency makes it to the main speaker without being attenuated?
If anyone has played around with different inductor and capacitor combinations, any suggestions would be appreciated.
I do realize in my case, since I have an 8 Ohm speaker (that is what the Leslie 18 came with), I would need to double the inductor value, and cut both capacitor values in half relative to the math above in order to achieve the same filtering effect.
-
I finally have some free time again and have started looking at a potential crossover for my Leslie 18. I am struggling with the components.
Not having a sense for how much current might actually flow through the speakers run by a 2x 6L6GC amp, I ran a 100mA signal through the amp at various frequencies into a 4 Ohm dummy load. As an example, with the amp on 3, I was measuring ~13 Vrms around 400Hz using an oscilloscope. I assume therefore that the current flowing through the speaker wires into the dummy load was roughly 13/4 = 3.25A at that frequency/ input voltage.
In the attached photo, the cylinder at the bottom left is what would have housed the notch filter capacitor and inductor that would have come with a Leslie 16 or Vibratone. Based on the size and shape of this cylinder, I kind of assume the inductor must have been an axial lead inductor? Has anyone seen one of these crossovers in person or seen a pic of the inside? I cannot seem to find anything online.
Regardless, I cannot seem to find any inductors in the 1.2mH range that are relatively small and that have a current handling capability anywhere near 3 Amps. I realize that the actual current flowing through the inductor might not approach 3 Amps since it will only pass low frequencies (the rest of the signal being split between the high pass capacitor in the notch filter and the high pass capacitor for the Leslie speaker), but how would one go about deciding what would be a reasonable current rating to use?
Thoughts?