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Old 16th February 2012, 01:47 AM   #1
Leland is offline Leland  United States
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Smile Leslie 720 used with stereo keyboard

I have a Leslie 720 in good condition that I purchased on ebay. I also have a Yamaha CP300 stage piano. I have been using a Fender bandmaster as a preamp for the leslie but it is bulky and only a mono amp. Since the 720 has essentially two different sets of speakers; one down firing 15' woofer with 8 mid range fixe speakers and a 10 inch down firing with spinning baffle below
and the rotating high frequency horn above. I would like to purchase two used hammond organ (or other make) amps and put them in the Leslie cabinet and have them power the two sets of speakers. The lower keyboard notes would be handled by the 15 and 8 inch speakers and the midrange and high notes by the 10 inch and spinning horn. Does anyone have a suggestion on how I could do this. I know that the 720 amp is solid state but the fender bandmaster I have been using is tube type and produces a very warm sound combined with the leslie. Any help would be appreciated. I don't have a lot of money as my wife has been in and out of the hospital for the last 8 months, but I can spend up to three hundred on this scheme.
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Old 16th February 2012, 02:48 AM   #2
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Hammond tube amps are easy to get, people are scrapping out M's and L's all the time. You'll pay more if you buy them on E-bay, than going to someones house and picking up their garbage before the new owners take possession.
Any such garbage amps will need the electrolytic caps in them replaced, in my opinion. Rubber seals are garbage at 40 years. To do that, read the high voltage for newbies sticky at the top of the tube amp thread. Do it right or not at all. After that, make some power checks. Some old amps need paper grid and plate caps replaced, most don't. Some high hours amps need the rectifier and power tubes replaced, but most M's and L's are not high hours. Don't buy an AO28, they are a preamp, not a power amp. AO39 and 40 are good. Look for the two 6BQ5's on them, about 13W a pair.
You'll need a preamp from the keyboard to get up to the 7 VAC expected by the AO39, usually driven by an AO28. The integrated amps out of M's and L's run off the TG and expect about 25 MV.
I suggest you fuse the amps, which hammond didn't do, about an amp each. Goes in the blue wire. Jumper blue to yellow on the back of the 4 pin amphenol to bypass the on switch in the organ.
You'll need some kind of filter to split the highs and lows for a bi-amp setup. Read wikipedia on filters, a cap and a resistor is what you need in most cases. Do it at the low level places to the currents are low, that is before the inputs to the amps. You can also buy commercial bi-amp filter units from Peavey and other suppliers.
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Last edited by indianajo; 16th February 2012 at 02:51 AM.
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Old 16th February 2012, 05:49 PM   #3
Leland is offline Leland  United States
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Thank you so much for your help. I will try to follow your recommendations.
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