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Old 23rd July 2011, 06:13 PM   #1
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Default Tube amp / Leslie speakers

I have a Fender PA-135 head (tubes) that I want to power two custom Leslie speaker cabs with. The problem is one is 8 ohms and the other is 16 ohms. The 2 output jacks on the amp are labeled 8 OHM MINIMUM. Each Leslie has a passive crossover, a high frequency horn and a 15" woofer. I want to run both cabs at the same time. What can I do to make the 8 ohm cab a 16 ohm? Could I put an 8 ohm dummy load in line to the 8 ohm cab? I just put new tubes in the Fender and I'm afraid an unbalanced load would not be good for the amp.
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Old 24th July 2011, 02:30 AM   #2
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Some leslies have amps in them and don't load the organ. Which one have you got?
If your leslies don't have an internal amp, you could buy a 16 ohm leslie woofer to replace the 8 ohm woofer, and put a crossover network in that leslie to keep the woofer and tweeter from using current at the same frequency. Series impedance=R+sqrt((1/2pi*F*C)+(2*pi*F*L). So inductors (chokes) in series block high frequencies. Capacitors in series block low frequencies. Most of the early leslies used 16 ohm speakers.
Another problem between guitar stuff and organ stuff is that leslie amps expect about 6 VAC for input (from a Hammond). Guitar amps put out 1.6VAC on the line output, and up to 95V AC on the speaker output, depending on the wattage of the amp. I don't know the specs on a PA135. A 35 W amp designed for 8 ohm speakers would put out about 17 VAC on the speaker output.
You'll find a lot more leslie experts on organforum.com, the leslie thread of electronic organs. They are always hassling about this kind of custom hookup, and there is somebody there that owns every leslie made, including the ones salvaged from obsolete organs. Some people build them out of wood, motors, and sewer pipe.
Or you could build or buy an amp to drive the second leslie. Usually the line out of a head will drive the line input of another amp.
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Last edited by indianajo; 24th July 2011 at 02:44 AM.
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Old 24th July 2011, 02:57 PM   #3
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Thanks for giving it some thought. These Leslies have no amps of their own. My problem is how to power one 8 ohm cab and one 16 ohm cab with the Fender PA-135. There are two output jacks on the Fender labeled 8 OHM MINIMUM, but I'm worried about damaging the amp or the tubes by having mismatched loads. If that's not a problem, there's still a volume difference at the cabs. Could I simply put a volume pot on the 8 ohm cab?
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Old 24th July 2011, 05:08 PM   #4
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You are not going to get maximum volume by putting a resistor (pot) in series with the loud speaker. If you have enough volume, the resistor (pot) would help unload the amp from producing excess current.
If your amp has a tube output stage with transformer coupled output, then it is hard to damage it. But again impedance mismatch (too low) impedes ideal energy transfer. If your amp has a direct coupled transistor output stage, then excess current may blow something up. Some "tube" amps have a tube input for distortion or crunch when overdriving, and a transistor output stage for a popular selling price. Good output tubes are $30 a pair and a tube output transformer is about $50 up, so going transistor on the output stage saves a lot of money- and sells a lot of "tube" amps. Again, I don't know what you've got. You might try looking up the schematic on eserviceinfo.com if you are really interested.
8 ohm high wattage stereo pots are sometimes sold as "L-pads" and are designed to fit wall plates: light switch size boxes. I got mine at R**** S**** years ago.
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Old 24th July 2011, 05:23 PM   #5
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The amp is all tube. It puts out 135 watts. I'm two doors down from R**** S*** right now... gonna go see if they still sell any electronics. Thanks!
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Old 24th July 2011, 11:10 PM   #6
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Quote:
Originally Posted by Leslie Hammond View Post
The amp is all tube. It puts out 135 watts. I'm two doors down from R**** S*** right now... gonna go see if they still sell any electronics. Thanks!
The speaker outputs are simply parallel connections to the OT, and according to the schematic (see link below) can operate with "total load impedance 4-Ohms min", so you can just connect both of your Leslie's to the amp, which will give you ~5 Ohms load, the PA-135 should have no problem driving them.

The connectors on your amp are labeled "8 Ohms Min", just means each jack should be connected with speaker(s) of 8 Ohms or more, so the "total load impedance 4-Ohms min" from the schematic still applies.

Good luck, btw, how much did you pay for the amp?

Jaz

Schematic

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Old 24th July 2011, 11:54 PM   #7
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Thanks, Jaz. I'd still like to reduce the volume of the 8 ohm cab, to be more in line with the 16 ohm. R**** S***'s wall mount speaker volume controls are only rated at 25 watts, though. I bought the amp about 25 years ago. I think I paid about $350 at the time... about what they're going for now!
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Old 25th July 2011, 02:12 AM   #8
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Quote:
Originally Posted by Leslie Hammond View Post
Thanks, Jaz. I'd still like to reduce the volume of the 8 ohm cab, to be more in line with the 16 ohm. R**** S***'s wall mount speaker volume controls are only rated at 25 watts, though. I bought the amp about 25 years ago. I think I paid about $350 at the time... about what they're going for now!
Yes, the volume level will be different... Found this on flea bay, more than double what you paid (not counting inflation), anyway I think you should definitely keep it...

Jaz
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Old 25th July 2011, 01:51 PM   #9
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Okay try this 100 watt one. OHMITE|RKS10RE|RHEOSTAT, WIREWOUND, 10OHM, 100W | Newark.com $56 + freight from NC. You'll also need a box to mount it in, a knob, and possibly input and output jacks. Save freight and get the other stuff on the same order. If you don't have at least a WP35 iron or equivalent with screwdriver tip, you'll need one. I find R**** S**** soldering irons to be garbage.
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Last edited by indianajo; 25th July 2011 at 01:54 PM.
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Old 25th July 2011, 10:59 PM   #10
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Thanks, guys!
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