Lumley ST70 Output impedance change and other questions

[Colin151]

Not sure if I'm posting in the right place... but,

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. It’s 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.

Cheers,
Colin

[Ali Tait]

Should be pretty straightforward. Take the bottom off the amp and have a look at the output transformers.There should be a spare set of terminals or wires on/from the trannies going to the speaker output terminals. It would just be a case of swapping over to the other set of wires, or in the case of a spare set of terminals, use a soldering iron to swap over the wires going to the amp outputs.

Be aware there are potentially lethal voltages in a valve amp. If you are not confident you can do this safely, I can put you in touch with someone who's an expert in these matters.

[Colin151]

Many thanks for the tip. That's great. I'll have a look when I get the amp (in a couple of days).

I am no DIY expert, but was aware of the lethal voltage inside valve amps. I would obviously never have the amp on when looking at this partiuclar issue, and would try to discharge nearby capacitors with a fat resistor before looking. I always wear rubber gloves when working on amps too, just in case.

Would be interested to know of a expert anyhow, as I would not mind getting the amp serviced and assessed for KT90 or KT120 output valve use. If you could let me know that would be great. Thanks for your help.



Should be pretty straightforward. Take the bottom off the amp and have a look at the output transformers.There should be a spare set of terminals or wires on/from the trannies going to the speaker output terminals. It would just be a case of swapping over to the other set of wires, or in the case of a spare set of terminals, use a soldering iron to swap over the wires going to the amp outputs.

Be aware there are potentially lethal voltages in a valve amp. If you are not confident you can do this safely, I can put you in touch with someone who's an expert in these matters.

[Ali Tait]

No worries. I will PM the details.