Lumley ST70 Output impedance change and other questions - diyAudio
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Old 31st January 2012, 11:16 PM   #1
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Join Date: Dec 2007
Default Lumley ST70 Output impedance change and other questions

I am just about to pick up a Lumley Reference ST70 power amp to use wth my Magnepan speakers. It was made in the 1990s. It has switchable Triode/Ultralinear mode. I am not sure of the bias method, - manual or auto. It's supposed to deliver around 70 WPC in UL mode with 6550 valves.

Apparently it comes with only the 8 ohms transformer taps hooked up to the speaker terminals. There are also 4 ohms taps on the transformers but they are not hooked up to the speaker terminals as standard. To run my Magnepans optimally I ideally need that setting to be on 4 ohms. Itís a long shot but was wondering if anyone might have any info on how to make the change from 8 ohms to 4 ohms internally?

Any schematics of this amp, instruction manual, or any other info at all would also be really welcome if anyone has anything.

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Old 3rd February 2012, 01:49 PM   #2
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Join Date: Dec 2007
I just got inside my Lumley ST70 and am looking at the output transformers. I want to change the output impedance setting to 4 ohms (its now set on 8 ohms).

There are four connections coming off each output transformer. One is labelled "8", one "4", another "4" and another "C". The two labelled "4" have a jumper connecting them . Currently the one labelled "8" is connected to the speaker +ve terminal, and the one labelled "C" is connected to the speaker -ve terminal.
I am not sure what to do, - obviously I need to disconnect the 8 and likely put that on the tap labelled "4" next to it, but do I leave the other connect labelled "C" as it is? And what do I do with the jumper - connect it between the other 4 and the 8? Perhaps I don't need the jumper I'm not sure. If anyone had any ideas it would be much appreciated. Attached is a pic of one of the output transformers.
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Old 3rd February 2012, 02:18 PM   #3
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Join Date: Aug 2008
From first sight, you should simply reconnect speaker terminal to transformer tap labeled "4", the one closer to tap "C" (Common), jumper between "4" and "4" can be removed, but if you keep it, it should be fine.
Looks like secondary made with 4 windings connected in parallel.
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