Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: octal on November 30, 2013, 04:30:47 pm
-
I've got a Jensen F12NS field coil speaker out of a Hammond organ. I'm using it for guitar and wondering what the voltage across the field coil should be. The coil measures 680 Ohms. I found a Hammond organ schematic showing 115V across a 1K ohm coil, so using a ratio, I'd guess I'd want to see around 78V across this speaker's field coil. Does anyone know?
Thanks in advance,
Nathan
-
I thought about this a little more. The first example of the field coil (1K, 115V) is dissipating 13.2W, so to keep the dissipation constant, you'd want to see 96V across the 680 Ohm coil. (96V, 137Ma) Is that right?
-
It's hard to say. Field coils have been designed for 3W and for 30W, depending of course on speaker size, power, and design.
If you feed too much, it slow-cooks itself. Much less is very low output, maybe flubby bass.
13W does not sound wrong for a large-room speaker. I'd probably bring it up to 10W, verify it even works, and let it cook a good long time. IIRC, they do run hot but not HOT. If it does not seem too toasty, sneak the field power up a Watt at a time. You should get more sound output. Don't get greedy.
-
Thanks, PRR. It works fine, I've run it up to 50VDC (the max my bench power supply can output.) I guess I'll keep hunting for a Hammond schematic which shows the in circuit voltage... I'd feel more comfortable doing it that way than playing the "what EXACTLY is too hot?" game. (Reaches for IR thermometer :icon_biggrin:)
-
If your speaker came from a Hammond M3 it will have a 700 ohm coil, those took 100 volts on the field coil.
-
Thanks! I feel pretty good about my calculated 96V then! The seller of the speaker was slightly vague about what the actual model name was- it was called "The Spinet," but if a 700 ohm field coil speaker in a Hammond organ runs at 100V, that's good enough for me. (I also feel the need to note that I was not personally involved in the destruction of a Hammond organ.)