help wiring in field coil speaker in place of a PM speaker

[Tom Phillips]

Tom,
Do you have and schematics I might could move forward with this on?
Thanks,
Jason

Jason I don’t have a schematic but I offer the following thoughts.

...Looks like it's dropping around 70V across the field coil in the organo circuit...

Given that and your 331Ω measurement of the DC resistance of the field coil Ohms law tells us that I=V/R=70/331=0.212A is flowing through the field coil. Let’s assume that this represents the maximum power that you will need to supply to the field coil of your particular speaker. Therefore, you need a power supply that is capable of continuously supplying at least 0.212A @ 70V. The supply also needs to be variable so you can turn down the voltage when you want the speaker to be less sensitive ala the Flextone approach. I don’t know what the lower useful limit of the field coil voltage will be but we can be sure that it won’t need to reach 0V. Maybe the useful range would only be something like 35V to 70V. In the end you will need to set the limits. If the field were to be set too low then the voice coil just becomes a heater. You wouldn’t hear any sound and eventually only smoke would be produced.

If you have access to a variable bench supply then you could experiment with that first and determine if you get results you like before you build a custom field coil supply. For information on building a power supply I’d Google “Experimenter’s power supply” or similar. The problem is that most hits are for low voltage supplies. You may want to look in older literature such as the Radio Armature’s Handbook old Popular Electronics magazines and the like. Edit: Here is one link http://www.eleccircuit.com/0-70-volt...-power-supply/

I don’t know your resources and capabilities but another approach is to cobble together a 70VDC supply our of junk box parts and then turn it down with a Variac at the line side or reduce the voltage to the field coil by inserting dropping resistors

Hope this helps or at least kicks off a discussion for others to add their ideas. I would think that we are not the first to think of using an old field coil speaker this way. There must be discussions out there on the same topic.

Cheers,
Tom

[g1]

I've been trying to recall who was recently dealing with the same idea, finally found it:
http://music-electronics-forum.com/t21491-4/#post370848

[nosaj]

Finally picked this back up . removed the transformer on the basket wired a 1/4 jack to the VC. Hooked a variable DC supply up to the field coil and wala it works. Now gotta find a suitable transformer isolation transformer would probably work well . I have 1k 50watt rheostat for adjusting, and 4 1n4007 should make a bridge rectifier. Will post pics when done with the supply.

nosaj

[nosaj]

Ok the only transformer I could find in my junk stuff has 120v in 50v out. so I wanted to build a voltage doubler to get the voltage up to a 100v.

It's built and working now is there a way to make the voltage not drop below 30v?
Thanks,
nosaj

[nosaj]

Here's the pics, sound pretty good just playing a small portable using the headphone jack, reasonable vol. When I get the junk box tweed Deluxe back (it's being passed around).

Right now voltage goes from 0-100v. I really need to find out if too low a voltage can be harmful to Voice Coil and if so how to prevent voltage from going that low.

[g1]

Not sure how the supply & rheostat is wired up, but you should be able to add a (suitable wattage) power resistor in series so that at the lowest setting you still have your 30V.

[nosaj]

Like this.

Thanks,
nosaj

[g1]

Set it for 30V. Shut down & measure resistance from wiper to C2 neg.
Add in a resistor of that value and appropriate wattage between low end of pot and C2 neg., but leave neg. end of field coil connected to C2 neg. like this:

[nosaj]

Played with the woodworking tools and put together a box with a white tshirt as speaker covering. Would love to wrap it in denim then shellac it. Plugged the junk box tweed deluxe in it, it doesn't change really the sound at all just the volume. So the power tubes let out all they can with it cranked and you dial the speaker down. Gonna try a closed back on it to see how it changes things. This was an experiment in can i actually do something with the junk i have.
Next is the vibrato scanner like the one analog outfitters has done.

nosaj