Mismatch between output transformers and speakers

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El Sid

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My vintage partridge OPTs have taps for 3.75Ω and 16Ω speakers. The impedance is set by changing the links on the right according to the scheme on the left:

58zXJde.jpg


But now most of my speakers, including the Philips 9710 fullrange I recently bought to use with my valve amps, are 8Ω. So i looked a bit more into the OPT windings to see if I could configure it for 8Ω. Sadly, I don't think it's possible  - it seems there are two windings, connected in parallel for 3.75Ω, series for 16Ω:


If there was a way of doing it, wouldn't it be marked on the transformer?

(Although as I type this I'm thinking what if only one winding is used? Resulting impedance 8Ω? Effect on efficiency and power? Studied this stuff too long ago, too lazy to think...)?

So i got to thinking if it would be possible to bias the EL84 power valves in such a way that an 8Ω impedance would work on either the 3.75Ω or 15Ω connections? Given the specified 8kΩ anode-to-anode impedance of the transformer, 8Ω would reflect as 16kΩ (approx) on the 3.75Ω connection and 4kΩ (approx) on the 15Ω. I drew the loadlines for the 3.75 case  (8kΩ and 4kΩ) and the 16Ω case (2kΩ and 1kΩ) at my expected Va voltage of 320V. (The thick pink line is the original bias). It seems to me that the 3.75Ω case (8k/4kΩ) is very anaemic and also will stay well below the knee, so it looks like that's out. As for the 16Ω case, the 1kΩ load line (when in class B) is so deeply outside the max power dissipation envelope that it's not feasible. (incidentally this exercise shows why the 8kΩ impedance works so well with an EL84)
5TYE4kC.jpg


But then I remembered that the power transformer also has 250V taps, so assuming a B+ of 220V (say), i looked at the 15Ω case again, and it looks like a 45mA bias could work?
fJ3uDm1.jpg


Of course I could just go out and buy 4Ω or 16Ω speakers....




 
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