Sugden preamp WAL AC Farnell. - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

wirewithgain · 28th Feb 2021, 3:20 am

Very tempted to see how the the Sugden sounds as a phono preamp, or even a pre-preamp. However, no ground screw is present on the unit. Any suggestions as to the best way to go about remedying this, perhaps just as a temporary measure in order to see how it sounds.

Radio Wrangler · 28th Feb 2021, 6:47 am

There's no photo of the back panel in the set posted, but does it have a screw you could replace with a binding post, or with a thread sticking out and a terminal nut?

David

wirewithgain · 28th Feb 2021, 3:42 pm

Thanks David.
The previous interior images 2 and 5 show a fixing for the mains cable.
Detaching the grey cover at the rear reveals the holding screw. Might there be a way to utilise this as a ground?
Back panel photos attached.

wirewithgain · 28th Feb 2021, 3:44 pm

Back panel.

wirewithgain · 5th Mar 2021, 2:26 pm

Latest update.
Delighted to hear that the manufacturers are sure that it was designed by Jim Sugden.
Advised that a variac can sometimes cause instability issues.
Plugged it in to the mains and no smoke or pops to report.
However, can anyone suggest some DMM readings that I could perhaps take from inputs and outputs that might offer me some reassurance prior to connecting it to source, amplifier, and speakers?

knobtwiddler · 5th Mar 2021, 5:20 pm

As we've ascertained that the preamp is built using what appears to be good quality parts (i.e. Mustard caps), and most, if not all of the I/O are likely to be cap-coupled, I reckon there is little likelihood of it hurting anything. However, just to be safe, put your DMM into the DC range and look for anything above about 50mV DC on any I/O. If there is DC present, it ought to go down with the small loading the meter presents it with (totally normal).

Ideally, you ought to test it with an oscillator + scope (or analyser), but if you haven't got one, it'll take less than a minute to check I/O for DC (which may not be 'DC' in some designs, but high-frequency oscillation, that looks like it to a meter), and it'll give some peace of mind.

NB - I ought to have said that the variac should be used without audio connections. If the reservoir caps have started to pass DC, simply turning it on will put a load on the transformer and rectifier. Ramping up with a variac is not as good as reforming them externally with a DC bench PSU, but it's better than nothing. An alternative is the light bulb limiter method. The variac method is not recommended for valve designs, and designs that have logic circuits might object to it. But in this case, I think there is a fair reason to suspect that the preamp has a simple, conventional PSU, probably with a single rail. Ramping it with a variac after a layoff, potentially decades long, is better than nothing.

edit - seeing as you've already powered it, the last paragraph is redundant!

28th Feb 2021, 3:20 am	#41
wirewithgain Triode Join Date: Feb 2021 Location: London, UK. Posts: 16	Re: Sugden preamp WAL AC Farnell. Very tempted to see how the the Sugden sounds as a phono preamp, or even a pre-preamp. However, no ground screw is present on the unit. Any suggestions as to the best way to go about remedying this, perhaps just as a temporary measure in order to see how it sounds.

28th Feb 2021, 6:47 am	#42
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 17,139	Re: Sugden preamp WAL AC Farnell. There's no photo of the back panel in the set posted, but does it have a screw you could replace with a binding post, or with a thread sticking out and a terminal nut? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Feb 2021, 3:42 pm	#43
wirewithgain Triode Join Date: Feb 2021 Location: London, UK. Posts: 16	Re: Sugden preamp WAL AC Farnell. Thanks David. The previous interior images 2 and 5 show a fixing for the mains cable. Detaching the grey cover at the rear reveals the holding screw. Might there be a way to utilise this as a ground? Back panel photos attached.

28th Feb 2021, 3:44 pm	#44
wirewithgain Triode Join Date: Feb 2021 Location: London, UK. Posts: 16	Re: Sugden preamp WAL AC Farnell. Back panel. Attached Thumbnails

5th Mar 2021, 2:26 pm	#45
wirewithgain Triode Join Date: Feb 2021 Location: London, UK. Posts: 16	Re: Sugden preamp WAL AC Farnell. Latest update. Delighted to hear that the manufacturers are sure that it was designed by Jim Sugden. Advised that a variac can sometimes cause instability issues. Plugged it in to the mains and no smoke or pops to report. However, can anyone suggest some DMM readings that I could perhaps take from inputs and outputs that might offer me some reassurance prior to connecting it to source, amplifier, and speakers?

5th Mar 2021, 5:20 pm	#46
knobtwiddler Pentode Join Date: Mar 2016 Location: London, UK. Posts: 234	Re: Sugden preamp WAL AC Farnell. As we've ascertained that the preamp is built using what appears to be good quality parts (i.e. Mustard caps), and most, if not all of the I/O are likely to be cap-coupled, I reckon there is little likelihood of it hurting anything. However, just to be safe, put your DMM into the DC range and look for anything above about 50mV DC on any I/O. If there is DC present, it ought to go down with the small loading the meter presents it with (totally normal). Ideally, you ought to test it with an oscillator + scope (or analyser), but if you haven't got one, it'll take less than a minute to check I/O for DC (which may not be 'DC' in some designs, but high-frequency oscillation, that looks like it to a meter), and it'll give some peace of mind. NB - I ought to have said that the variac should be used without audio connections. If the reservoir caps have started to pass DC, simply turning it on will put a load on the transformer and rectifier. Ramping up with a variac is not as good as reforming them externally with a DC bench PSU, but it's better than nothing. An alternative is the light bulb limiter method. The variac method is not recommended for valve designs, and designs that have logic circuits might object to it. But in this case, I think there is a fair reason to suspect that the preamp has a simple, conventional PSU, probably with a single rail. Ramping it with a variac after a layoff, potentially decades long, is better than nothing. edit - seeing as you've already powered it, the last paragraph is redundant!