Leslie 251 amp mod/610 rebuild

[JimboJohnson]

Hi all. I've been working on a leslie 610 conversion; using an amp from a 251 leslie. The schematic is here: http://www.captain-foldback.com/Leslie_sub/Leslie_schematics/251.GIF
My intention was to get the same creamy overdriven sound of a hammond amp speaker level signal driving the leslie amp, when using a digital keyboard at line level.  The 251 seemed like an elegant solution; simply by driving the output of the 8ohm secondary channel straight into the rotary channel; and having a simple footswitch to drive the motor relays, removing the need for preamp pedals etc.
It works beautifully; however, being a novice, was wondering if anyone had an suggestions as to possible inprovements to this setup? 
Cheers, Jim

 

[Michael Tibes]

If you take away the loudspeaker as a load from the reverb amp output it might theoretically damage the reverb amps power amplifier. That's the theory with stronger amps, I don't know if the effect might be the same with the little reverb amp. I'd probably add a resistor across it's output to be safe. Same ohms and wattage as the speaker was...

Actually that's a cool idea, I gotta try it with my 251 

Michael

 

[ebel]

the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor.

Now, this is also dependent on how much energy (voltage) which is applied from the 6973 tubes to the output transformer, again dependent on how hard they are driven by the first stage. Most likely, your keyboard will not be able to drive the secondary channel to any peak level.

So, yes, pls add a 10w (125 ohm or less, 16 is good) from hot to ground. Since you're not driving a speaker, there is no big need to have an impedance match.

Notice that the secondary already has a feedback resistor of  4.7k + 3.3k to ground, but these will not able to take the energy off the tranformers.

If you want to be more fancy, remove the 6973 tubes, and wire the output from the 12UA7 (after decoupling) to the input pot of the rotary stage. See image. NB: replace the 10k Pot with 500k resitor (image does not show this)!

It might be enough to drive the rotary with the extra signal you're looking for. You'll have to experiment a little, but if you are not familiar with soldering and fiddling with high voltage electronics, add the load resistor and go a head.