What to do with OT with 500 ohm secondary?

[lacrebob]

I have a Stancor A-3841 single plate to line output transformer. The secondary is 500 ohms. What can I do with this? Can I use this as my OT if I get a field coil speaker matched at 500 ohms, or am I better off getting a Line to Voice Coil transformer to take the 500 ohms down to match a speaker at 4, 8, or 16?

[tubeswell]

You say the secondary is 500R. What's the reflected load? (That is what really matters. If the primary was 250k (ridiculously hypothetical guess), then you could get it down to 4k with an 8R speaker - if I did my math right )

[lacrebob]

Primary Impediance: 7,000/6,000/5,000/4,000/2,500 ohms
Secondary: 500 ohms

[tubeswell]

Well that transformer might be good as a reverb transformer driving a pan with about a 600R input. If you had another one, you might find they work as a 1:1 isolating buffer if you wired them back to back. I guess it could also be used as a low impedance signal send for a Direct Box/Line Out circuit. Not much good as a geetar output tranny tho - unless you had a 500R (or thereabouts) geetar speaker.

FWIW as far as reflected load goes, impedance ratio Pri:Sec is the square of the turns ratio Pri:Sec. The turns ratio is the same as the voltage ratio Pri:Sec. So if you put a 10VAC source across the sec and (carefully) measured say 287VAC across the primary, that is a voltage ratio of 28.7:1, which when you square it (i.e.; 28.7:1 x 28.7:1), would give you an impedance ratio of 825:1. Then to apply that, you multiply both numbers by the number of ohms you are loading onto the secondary with the speaker. So for an 8R speaker 825:1 x 8 = 6600:8, which would be a reflected load to the output tube(s) of 6k6 (with an 8R speaker load), (or 3k3 with a 4R speaker load etc).

[Ripthorn]

That could also make an awesome tube microphone transformer. Of course, building mics and building amps are different beasts.