Driving a Leslie 120 With a Salvaged AO39

[tpappano]

I had a junked AO39 chassis missing the output transformer and a few other parts, being somewhat compulsive I had to fix it use it for something. I was pleasantly surprised to find that Hammond Mfg. makes a whole range of vacuum tube output transformers. I patched the CV, rebuilt AO39 and 120 together and was pleasantly surprised at how well it sounded. As social distancing drags on, I've now discovered I'm perfectly fine with making a fool out myself, so here are some sample sounds:

https://www.youtube.com/watch?v=S3cr0DgGOSQ

[Drawbar Dave]

Well done Tom, you got a nice grind out of that. I forgot it was a single rotor!

[tpappano]

Thanks Dave! I had not heard one of these until now, from what I've read I was expecting it to sound kind of tame or dull. I thought it produced a fairly impressive amount of bass too, considering the small cabinet size. Considering how this one sounds If I was Leslie-less I could see one of these as a fairly decent "bridge" until a dual rotor unit came along 8) I think I might go ahead and mount the little amp inside the box.

[David Anderson]

Where are you getting your distortion? We've had a few discussions on here about CV preamps not having quite enough drive, but we've also discussed some mods for them, including changes to the rheostat box.

[tpappano]

Hi David, I basically deleted the hi frequency rolloffs in the preamp and swell box, in addition I increased the shunt resistors on the matching transformer secondary (in the swell box) and bypassed the swell box inductor. The resistance increase substantially increased the drive available to the preamp. I need to scope G-G to see how clean or dirty the preamp actually is. I'll do that shortly 8)

[tpappano]

I recall these mods were mostly Kon's suggestions.

[David Anderson]

Wes and I had a couple of long threads about optimizing the gain of the CV preamp, but, yes, after you've done that, increasing the drive to the preamp by reducing the signal loss in the rheostat box is the next step to greater output.

[tpappano]

A quick check with the scope on the CV's preamp (as modified) from G-G:
The output is pretty much clean up to 6-8 volts rms, a lot of headroom for sane activities.
Things start getting interesting from there, as one increases the 'spectral density' by bringing in more fingers the preamp continues to distort until it reaches a maximum output of about 12 volts.
With this particular lashup of the AO39 and Leslie 120, the AO39 is mostly clean up to about 2 volts input, so fun things start happening from 2 to 7 volts, then still more from 7 to 12 volts 8)
I took a quick look at the RT3 (still a virgin). The AO28 seems to behave in a similar fashion, but not quite as pronounced. The distortion starts at a little higher output but has a similar look. As the peaks start to flat-top, there is a downward slope. The AO28 seems to have a smaller transition zone than the CV.