Converting a transistor Leslie to Tubes

[gsl]

Hi all,

Our organist plays a Korg BX3 through a Solton Twin-Jet 100W Lesley clone.
This setup is very fine indeed since the Solton is rotating and all but it sounds too clean.
Putting the BX3 through a guitar tube combo sounds exactly right but it obviously does not rotate.
Thus the object is amplifying the BX3 through a tube amp and passing the output to the rotating speakers and horns in the Solton cabinet.

I have opened the Solton cabinet and found 2 power amps (each 50W or each 100W???) one going into the lower cabinet and one to the upper rotating horn which are rated 4Ohm according to spec plate.

Now I am wondering what the best way to rig the tube-amp(s?) would be.

1) Do I need 2 amps (we do not need all the 100Watts. 30-50W tube power would suffice) or can I make do with only one?

2) Would I rig the wires going from the 2 Transistor PAs in series and connect to an 8 Ohm output? There are some RL Filters going into the upper horn cabinet that I do not understand (electrically - I know what they are for ;-)

Any ideas, suggestions, experience, schematics?

Thanks,
Michael.

[Albert Kreuzer]

Hi Michael,

I'm just in the process of converting a 760 Leslie into a 122. I took the iron from a trashed Dynacord Eminent II and built a 2 x 6550 Amp w/30something Watts.
Between the amp and the drivers is a 12dB/Oct passive crossover at 800Hz. The 760 has different drivers than the 122 (or the Solton), so you have to re-calculate the crossover.

Hope this helps
Albert

[Steve Conner]

I wouldn't risk putting the upper and lower sections in series. The treble horn crossover probably won't pass bass through to the woofer, and the effect would be an impedance mismatch to the tube amp with possible damage, not to mention a poor sound with no bass. Instead you should probably parallel them and drive with a tube amp that has a 4 ohm output. (I know two 4 ohm drivers in parallel is meant to be 2 ohms, but I'd expect the crossover to modify that)

[Brad1]

I modified a Leslie 825 (no top rotor...just the 12" JBL and drum) to bypass the SS amp. I pretty much just interrupted the speaker wire to have a 1/4" jack from the amp, and a 1/4" jack to the speaker. If I want run a guitar amp through it, I just plug into the speaker. If I want to use the internal amp, I just use a short jumper cable.
Since I have no Leslie control footswitch, I also had to build one and connect it to get start/stop and fast/slow. The only other thing I had to do was connect a power cable. I ended up hard-wiring one with a switch that will engage or diengage it, in case a footswitch ever gets connected through the umbilical to power it up.
Sounds kind of cool on guitar with a BF Bassman.
Since I need the power to spin the drum, I just let the SS amp idle away without a load. I GUESS that's all right? I haven't blown it up...yet.

Your's is a different thing, since you need to power both the bottom and top. Seems like there would be a couple of different options.
Build a passive crossover and plug one amp into it. In that case, you may want to consider putting some kind of pad or built-in attentuation on the high end to balance out the signal. One amp powering both may really push the high end, and barely work the low end. Of course, that may eat up power, so you may want to consider a bit more power just to get the lows up and make up for the loss the attentuation may create. Then again, if a 30 watt tube amp gets the overdrive and volume you want, then cool. And you may want to prepare yourself for the possibility that although the keyboard sounds good through the other amp, as is, if you use that same amp it may sound completely different. It could sound good, bad...whatever...when going through a different, slightly muffled 12" and a rotor.
Also, I'm not sure about the complexities of feeding a tube amp into a passive crossover, so better wait until someone can say HOW to do it, if it is OK.

A more expensive thing may be to use two amps.

An even MORE expensive and elaborate thing would be to use a preamp, an active crossover and maybe a dual-channel/output tube rack-mount amp. Just connect 1/4" jacks for the top rotor and the bottom speaker, adjust frequency split on crossover and volumes on crossover/amp.

Just some thoughts. Please correct anything I may have wrote that is stupid in my early-morning one-cup mode.

Brad1

[gsl]

Thanks for your valuable and enlighening input guys. I think I have made up my mind if you think it can be done this way. Since it is not my Lesley I have to be very careful!!!

I thought of leaving the top horn untouched powered by one of the Soltons PAs (for the high frequencies maybe it's even better to have them cleaner) and put a switched 1/4" jack to the bottom 2 speaker's cable which will interrupt if a cable is connected and leave the PA unloaded whilst leading to the speakers.

What do you think? Looking forward to your opinions.

@ Albert: I will try to make it to Neubauers on Wed. Can't promise though. Maybe we can discuss it in more detail there. I was going to use that Buoyer ST30 (EF86 -> 2x6L6) which is pending the Cap-Job to cure the popping for the rig. That's an 1958 cinema amplifier that sounds quite nice when connected to a guitar cabinet.

[Brad1]

A split signal out of the keys...one going to the Solton and one going to the external amp? With the external amp powering the bottom, and the Solton powering the top (I presume through a crossover)? May work, (with the blessing of the engineering-types...I just have an AA in electronics...I don't design, I just try to fix). Would be good for both to have some sort of level control to balance them. You mentioned a "cinema amp".... for use in this? I have a couple of those. Did you have to modify the input? Is a tube preamp out of the keyboard split to the amps (or even going to one, and not the other) even a consideration to adjust tonality and maybe add a little dirty-old-Hammond/Leslie grind?
Might be very cool to occasionally run a guitar through the thing? It's entirely possible to CAREFULLY construct a setup that would switch it from the keyboard to, say, a guitar, and have the keys then routed to something else for something like a piano sound, leaving the guitar free to use it (maybe through it's own preamp, etc.) while the keys ain't. I would ASSUME your guy doesn't wish to run a lot of piano through it? (Then again, maybe it's specifically and always on for the Hammond-type sound, and on almost all tunes?)

Just some more thoughts. Take'em or leave'em, if I suggest anything too stupid or dangerous, don't hesitate to call me on it! As long as you're messing about, might as well consider some of the possibilities, (and my twisted self can always think of something unorthodox...it's a sickness I caught from hanging around Zuzu, who NEVER wants ANYthing left stock)

Brad1

[Earl Norton]

If it were me, I'd try an overdrive pedal like a Boss OD-1 before the amp driving the horn. Tube effect will likely be lost on the bass side and the OD-1 might provide just the right amount of breakup you are looking for without rebuilding the leslie. Just a thought.

[gsl]

Hi Earl,

Thanks for the input. This was more or less what Albert said when we met face to face last Wednesday at a luthiers workshop (they build his pre-amps into their basses there). He also thought that taking care of the Horn would be more important and on second thought I think I agree.

Putting a stomp box in front of the Lesley, or just the horn sounds luringly simple but then we want to be able to experiment with different kinds of amps and stuff. So presently I believe I am going to try the rig that Steve suggested and parallel the cabinets into a 4 Ohm PP 6L6 output.

I will report as soon as I know but this project will have to wait till 2007...

Thanks all,
Michael Einem.

[Regis]

If it were me, I'd try an overdrive pedal like a Boss OD-1 before the amp driving the horn. Tube effect will likely be lost on the bass side and the OD-1 might provide just the right amount of breakup you are looking for without rebuilding the leslie. Just a thought.

I played with a guy that had a SS leslie and used an overdrive pedal like this, and it sounded awesome. Get a Bad Monkey, or a Boss SD-1 or DS-1, it would be cheaper and simpler.

[gsl]

Update

Hi All!

The project is finished now and I want to thank you for your help again and report the results:

We tried both approaches:
1) sticking an overdrive pedal in between was neither here nor there. It actually sounded worse than without. Nowhere near the mellow power tube distortion we were looking for.

2) I disconnected the speaker leads from the twin 100W SS-Solton amps and left them unloaded (allegedly this won't harm a SS power-amp) connecting the leads in parallel to the 4 Ohm output of the Buoyer ST30 (EF86 -> 2x6L6 AB) 30Watt 195? cinema amp.

Well, the results here were awsome!!! Exactly the beastly growl we had envisaged even with the preamp pentode in linear operation. Awsome and it gets even better:

In the 50es the mains voltages were not as stable as they are today, so this amp has a mains transformer with several taps on the primary winding controlled by the mains rotary switch and a meter which helps to find the correct voltage. Now if you use this to feed the amp with a lower voltage than supposed, it breaks even sooner and our organist's Korg C3 sounds decievingly like Mr. Jon Lords sweet Hammond C3. Wow!

The really surprising (to me) detail is that the 30W from the Buoyer are distinctly louder than the 2x100W from the solton. So although less power, the thing has actually become louder. Can you believe this? A very welcome by-product.

Our organist is "kissing my feet" at every rehearsal and I am also pleased as a kitten under a dripping cow ;-)

Best regard to all (especially Albert),
Michael.