Hacking a Hammond (or two)

[ashcatlt]

My father recently purchased two vintage Hammond organs. I haven't seen them or heard them or even been told if they work at all yet. I'd imagine there may be some tubes which need replaced, maybe some old caps to swap out...

He was talking about wanting to hack in a Line Output, using an L pad off of the speaker outputs. This is fairly straightforward and should actually be pretty easy since I think the whole darn organ is wired with turret terminals. So we just go in and wrap our new wires around the appropriate turrets and hit it with some solder. Easy. We'll have to figure out what we want to to about mixing the Dry and Reverb channels together. These models actually have separate speakers built in for each, with an internal setting for the overall volume of the Reverb channel. I'm thinking we might need to find ourselves a blend pot for this. Still, pretty easy though.

Got me to thinking, though, that we had ought to also be able to hack in an input so's we can run a guitar or bass through the internal tube amplifier. From the description in the service manual, it sounds like there's a 1/4" cable which connects the preamp section to the power amp, and it's as easy as sticking a Y-cable in there to be able to jack other stuff in. That, though, bypasses the preamp section along with both the Vibrato (a crazy mechanical type of thing which I really want to play with) and Reverb. The Hammond Model M actually has a "Special Input" pin which provides a low-Z input point at the front of the preamp, but we don't have one of those.

We've got an L100 (manual with schematic) and an M100 (manual with schematic)

Comparing the models we have to the model M, I can see pretty well where to hack into the thing, and have a pretty good idea how to make it happen. I have a couple of concerns here, though:

1) Is there any meaningful DC voltage at the input to the preamp tube? Do I need a coupling cap? I guess it's probably better safe than sorry, and a good idea to protect the organ from poorly designed devices which might be connected to it...

2) I don't know how much noise is going to come from the tonewheel section when it's not actually playing. I'm thinking we might want to use some form of switched jack, or maybe just a switch to disable the organ itself when a guitar is plugged in here. But I'm not sure what to do about the end of the "matching transformer". I know from reading that tubes and output transformers don't like to go unterminated, so I find myself wondering what will happen if I just break that connection. Will it burn up like the output of a tube amp? What, if anything, should we do to avoid this?

I don't know exactly when we'll get around to this project, but I'm really pretty excited to hear what it does! There will, of course, be sound samples at some point.

[sumgai]

ash,

Well, your Nutz-heart is certainly in good working order!

The first thing I'll tell you is, make sure that whatever else you do, do not, under any circumstances, fool around with the ToneWheel section(s). Reason being, real and genuine ToneWheel sections are getting harder to come by these days. Reproduction units are being made, and can be had for not unreasonable prices, but most afficiandos can tell the difference, and they won't settle for anything less than the original.

Next up......

Inserting an L-pad at the speaker output is not really a Line Out, even when the dBv might be the same - doing so will also deliver whatever distortion was added by the power-amp tubes and the output transformer, distortions that are presumed absent from a "normal" Line Out signal. Not saying you can't do it, just saying that you might wanna keep this little factor in mind....

As for inserting a Line In jack, you're correct, you want to use that Pre-amp Out to Power-amp In connector - that's at line level, give or take a few millivolts. But plugging into the organ's pre-amp, instead of the connector, that's not gonna sound anything like what you're expecting for either a guitar or a bass. My guess is that you'd be in there for days, playing with cap values and such, trying to find a Tone that sounds good for both the organ itself and for whatever you plug in to the pre-amp. Not my cup of tea, but then again, my Circadian rhythyms not only don't match up every day, I don't think they've matched up at all since 1980!

It never hurts to insert a DC coupling cap at a circuit's input. Something on the order of 22µfd will do. That's usually an electrolytic, so put the plus side towards the input signal.

As for the ToneWheel section, unless there's an extremely noisy pickup, and/or a following tube, then there shouldn't be any problem with noise. However, if you do choose to disable the output, a simple 100Ω resistor across the transformer's output should do the job. (Make it a 1 watt unit, just for safety.) Not enough load to cause any harm to this stage, but definitely kills the signal as far as the following stage is concerned.

And lastly.....

I'm interested in hearing your impressions of Hammond's Vibrato. This particular circuit is often misunderstood in how it operates, but the fact is, it's the underpinning of several pedals made over the last 30 years. Such are the wonders of not-so-modern electronics.


HTH






sumgai