Bypassing internal amp of Leslie 825

[phydauex]

Good Day All,
wanting to bypass the internal SS amp of a leslie 825, so if desired can run a guitar amp into the speaker and use the combo pedal to adjust speed.  can I parallel an ext speaker jack to it?  Or should I use a shorting jack to disconnect the speaker leads from the internal amp?  If i just disconnect the leads and the amp is on will it damage the power amp?  I'll try and attach the schematic here. 
Thanks,  Matt

[phatt]

Never allow the outputs of 2 amps to come together,, at least one will go bang

To be safe,, Completely Disconnect the speaker from internal Amp.
The internal amps will be fine without a load.

Of course you could just tap a simple 2 resistor voltage divider from first amp which would give a suitable line level to the leslie input?,, saves messing with the leslie internals. Just a thought?
Phil.

[phydauex]

Thanks for the idea.  I think that would be fine if he had a specific amp in mind, but I think the owner wants to be able to plug any amp into the cabinet, bypassing all of the internals.  He has the combo preamp which gives him 2 line input channels and controls the speed.  But I think I'll just mount a little jack plate with a DPDT switch and a 1/4" jack so he can disconnect the speaker from the internal amp.  I just wasn't sure if you could leave the internal amp hanging w/o a load on it without killing it. 
Thanks for the suggestion.

[phatt]

Sound good,,,,
but remember the speaker may have limited wattage so plugging a big 100watt rig *Direct* might make the novelty wear off pretty fast.
Phil.

[phydauex]

Yeah, got it up and running.  Used a heavy DPDT switch so the user can select the internal amp or plug in and get the external amp.  Works great and the owner has been advised about no more than 50 watts.  He's happy and now he's brought me a Leslie 16 to do a couple mods to. 
Thanks for the ideas, warnings, etc..
Matt

[phatt]

Hi phydauex,
                 Good to hear some positives come out of it all. :tu:
I actually use a rotory speaker from a Yamaha B5 organ. (no Amp)
It's only a mid type driver but by a simple Bipolar cap of 10uf/50Volt (in series with input) I run it from a 120 Watt Power amp with no issues.
(the original Organ Amp was only 20/30 Watts at best)

Now As the main 8-Ohm speakers are directly connected to the 120
Watt poweramp then the rotory speaker is just like a midrange in a HiFi crossover setup.
This allows you to pump far more wattage than is normally possible.

There is a hickup you may not have thought of:
Be aware that modern Guitar Amps tend to use speakers with stupid SPL ratings,,, i.e. 102/103 dB (1metre @1watt rating)

Whereas the old Vibro speaker is likely to be more like 87dB (1metre @1watt rating)
This will make the vib sound very subtle as the old driver will be way down in output per watt used.

For me I've found that a lot of the modern drivers very harsh and although they pump more SPL per watt used they are not as sweet as the older drivers with less SPL per Watt.

This stuff is not easy to put into words you really need to hear it with your own ears in an A/B test.
Trust me the moment you do you will understand EXACTLY what I'm ranting about.
Cheers, Phil.