Rerurn to Romy the Cat's Site
In the Forum: Audio Discussions
In the Thread: SET amplifiers
Post Subject: Getting technicalPosted by JJ Triode on: 12/17/2014
fiogf49gjkf0d
Novygera:
If you have a transformer where the impedance primary-to-secondary is 5K:8, it means the turns ratio is (5000/8)½ = 625½ = 25. This is the ratio by which the voltage is stepped down and the current stepped up in going from the tube to the speaker. The output impedance is reduced by the square of the turns ratio. Typically the optimum loading for power output (not necessarily Sound) is for the output tube to see a reflected impedance about 4x its plate resistance. So 5K is optimal for a tube with a plate resistance of 1.25K. As I recall, the plate resistance of a 2A3 is 800R and a 6C33C, 200R.
I don't know if Romy is talking about a transformer with a turns ratio of 18 or an impedance ratio of 18. For sure though, the speaker driver should see the tube's plate resistance decreased, not increased, by the transformer. For example, a lot of SET amplifiers have an output impedance of about 2R, corresponding to the "optimal" loading, and giving a damping factor of 4 into a nominal 8R driver. For a turns ratio of 18, that translates to a primary impedance of about 2.5K which would be reasonable for, say, a 300B, so 18x is probably the turns ratio.
Rgds, JJRerurn to Romy the Cat's Site