Lumley ST70 Output impedance change and other questions - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 31st January 2012, 11:16 PM   #1
diyAudio Member
 
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.

Cheers,
Colin
  Reply With Quote
Old 3rd February 2012, 01:49 PM   #2
diyAudio Member
 
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.
Attached Images
File Type: jpg IMG_9298.JPG (737.6 KB, 31 views)
  Reply With Quote
Old 3rd February 2012, 02:18 PM   #3
diyAudio Member
 
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Thinking of buying a Lumley Reference ST70 with attached passive pre ringo_the_hifi Tubes / Valves 3 9th March 2010 07:45 AM
Transcendar Output Transformers ST70 Wharf_Creek Tubes / Valves 3 27th November 2007 03:55 PM
Can An Output Transformer Change A Voltage Amp's Output Impedance From 0.1 To 47 Ohms kelticwizard Everything Else 11 25th March 2007 05:17 AM
Output section impedance change query KBK Tubes / Valves 7 5th August 2006 01:47 AM
How to change impedance with two 8 ohm speakers? Urloony Multi-Way 8 17th February 2005 02:22 PM


New To Site? Need Help?

All times are GMT. The time now is 12:57 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2