I recently parted out an old Baldwin 41P organ, and among the future amplifier parts is an OPT labeled 512-19234B. I did a ratio test at both 60 hz and 1khz, and the turns ratio is approximately 19.3 : 1. I did not think to check the speaker impedance or wiring (series or parallel) before disposing the of the unusable parts of the organ , so I have no idea what the reflected impedance would have been. Does anyone happen to know what the typical Baldwin organ speaker impedance/wiring scheme of that era would have been? My research seems to indicate that the speaker impedance may have been 16 ohms, and there were two of them powered by the 2-tube PP 6L6 amplifier section from which this transformer came. I also would like to know if a 6V6 PP power section would handle roughly 3K impedance on the primary from an 8-ohm secondary load. Thanks for any help anyone can provide!
PS, I did find this post about the same transformer (https://www.diyaudio.com/community/threads/baldwin-transformer-swap.168380/#post-2213258). It gives 6.4K to 8 as the impedance ratio which works out to a 28:1 turns ratio if my math is correct (square root of the impedance ratio 6.4k/8), but that doesn't match what I'm measuring out of my 512-19234B unit. Hmmmmm.....
PS, I did find this post about the same transformer (https://www.diyaudio.com/community/threads/baldwin-transformer-swap.168380/#post-2213258). It gives 6.4K to 8 as the impedance ratio which works out to a 28:1 turns ratio if my math is correct (square root of the impedance ratio 6.4k/8), but that doesn't match what I'm measuring out of my 512-19234B unit. Hmmmmm.....
1. For an estimate of the loudspeaker impedance, measure the voice coil DC resistance (DCR).
The loudspeaker impedance will be at least as large as the DCR.
At mechanical resonance it will be larger than that (one peak impedance at one frequency for an open box; one peak impedance at one frequency for a closed box; and two peak impedances for a bass reflex or ported box).
At very high frequencies, the voice coil inductance will increase the impedance from mid frequencies all the way to 20kHz.
Loudspeaker impedance specification is nominal (a generalization), it is not constant (there are some loudspeaker systems that have a fairly constant impedance; but a single driver with no R, C, and L parts is not constant across 20-20k).
2. Testing a transformer is tricky. Use a low impedance signal generator at 1kHz to drive the primary, do not terminate the secondary, and measure the primary and secondary voltages. The low impedance signal generator swamps out the primary impedance resonance when the secondary is unloaded.
Square (the primary voltage / secondary voltage you measure) and multiply that ratio x 8 Ohms. That will tell you the primary impedance when you do load the secondary with 8 Ohms (will be a little bit higher impedance than that, because of the DCR of the primary, and the DCR of the secondary).
The low frequency primary impedance will be less than it is at 1kHz, due to the primary inductance.
For low power output, if you really want to use push pull 6V6 tubes, suppose the primary impedance is too low for Beam Power (some say pentode) operation, then wire the 6V6 tubes in Triode Wired mode (100 Ohm resistor from screen to plate).
Oh, was that parallel, 2 push 6L6 and 2 pull 6L6?, or 1 push 6L6 and 1 pull 6L6?
If it was parallel 6L6 tubes (4 tubes), than Triode wire 1 push 6L6 and 1 pull 6L6 (100 Ohm resistor from screen to plate [one each 100 Ohm resistor for each tube])
Have fun designing, building, and listening.
The loudspeaker impedance will be at least as large as the DCR.
At mechanical resonance it will be larger than that (one peak impedance at one frequency for an open box; one peak impedance at one frequency for a closed box; and two peak impedances for a bass reflex or ported box).
At very high frequencies, the voice coil inductance will increase the impedance from mid frequencies all the way to 20kHz.
Loudspeaker impedance specification is nominal (a generalization), it is not constant (there are some loudspeaker systems that have a fairly constant impedance; but a single driver with no R, C, and L parts is not constant across 20-20k).
2. Testing a transformer is tricky. Use a low impedance signal generator at 1kHz to drive the primary, do not terminate the secondary, and measure the primary and secondary voltages. The low impedance signal generator swamps out the primary impedance resonance when the secondary is unloaded.
Square (the primary voltage / secondary voltage you measure) and multiply that ratio x 8 Ohms. That will tell you the primary impedance when you do load the secondary with 8 Ohms (will be a little bit higher impedance than that, because of the DCR of the primary, and the DCR of the secondary).
The low frequency primary impedance will be less than it is at 1kHz, due to the primary inductance.
For low power output, if you really want to use push pull 6V6 tubes, suppose the primary impedance is too low for Beam Power (some say pentode) operation, then wire the 6V6 tubes in Triode Wired mode (100 Ohm resistor from screen to plate).
Oh, was that parallel, 2 push 6L6 and 2 pull 6L6?, or 1 push 6L6 and 1 pull 6L6?
If it was parallel 6L6 tubes (4 tubes), than Triode wire 1 push 6L6 and 1 pull 6L6 (100 Ohm resistor from screen to plate [one each 100 Ohm resistor for each tube])
Have fun designing, building, and listening.
Last edited:
Sorry, I thought the speaker was useful (true, if it had a field coil magnet, a lot of people would think it was not useful).
One man's weed . . . another man's jewel.
One man's weed . . . another man's jewel.
Thank you for the helpful info! I will need to double-check my generator's output impedance, but pretty sure it is low impedance. The power stage will be a PP pair of either 6V6 or 6L6 tubes.