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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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28th Feb 2021, 4:20 am | #41 |
Triode
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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.
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28th Feb 2021, 7:47 am | #42 |
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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
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28th Feb 2021, 4:42 pm | #43 |
Triode
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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, 4:44 pm | #44 |
Triode
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Re: Sugden preamp WAL AC Farnell.
Back panel.
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5th Mar 2021, 3:26 pm | #45 |
Triode
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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, 6:20 pm | #46 |
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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! |
24th May 2021, 3:41 pm | #47 |
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Re: Sugden preamp WAL AC Farnell.
Just adding some info and readings to this thread, my apologies for the long radio silence.
Looking at these readings below, can anyone advise as to whether they look fine or are there any issues about which I should be concerned? Naturally, I don't want to risk any damge to my amplifiers or speakers (vintage Celestion Ditton 66's with hard to source replacements..) My intention is to try it out as a phono amp which will then be connected to a pre amp (NAD 3020A pre section) which will in turn be connected to a power amp. I realise that an RIAA circuit of some sort may have to be added to make this work satisfactorily. Does any one particular channel look to have better readings than the rest? And any to be avoided? MIC TAPE DISC OUTPUT CHANNEL 2.7 ~ 3.0 1.0 ~ -0.3 1.5 ~ 0.2 1 Mic left 1.0 ~ -0.3 2.5 0 ~ 1.2 1 Tape left 1.0 ~ -0.3 0.9 ~ -0.2 2.9 1 Disc left 4.8 0.7 ~ -0.2 1.2 ~ -0.2 1 Mic right 8.0~ -0.2 3.9 1.3 ~ -0.2 1 Tape right 0.9 ~ -0.2 0.8 ~ -0.1 4.8 ~ 4.5 1 Disc right The above chart displays the DMM DC readings in mV for each of the I/O sockets. The I/O socket from which the reading was taken are written in capitals at the top of the columns. The position setting on the rotary lever switch are written in lower case at the right hand end of the rows. When the rotary switch was turned a higher reading (90mV, 70mV, even a 170mV) would briefly flicker in between the selector registering in the new setting. Is this a concern? |
24th May 2021, 3:43 pm | #48 |
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Re: Sugden preamp WAL AC Farnell.
This is the same table, which may be easier to read..
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24th May 2021, 3:44 pm | #49 |
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Re: Sugden preamp WAL AC Farnell.
...
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24th May 2021, 5:01 pm | #50 |
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Re: Sugden preamp WAL AC Farnell.
I suspect this to be a re-laid out derivative of circuitry used in Sugdens then domestic hi fi range. IME of repairing and restoring such gear for customers the most likely issue will be noisy carbon composition resistors. They used these for quite a long time after everyone else had ceased to. They must have known about their dodgy noise performance (in a correctly functioning resistor rather than one "gone noisy") as in the most sensitive parts of the low level circuitry (RIAA amp) they used carbon film and even a few metal films for some production.
OP. I wouldn't worry about it damaging other equipment etc... VERY unlikely. If you have contact cleaner then a bit in the selector and sockets and then "work them" for a while and you should be good to go. If you find either initially or after a few days/weeks use that you get "bacon frying" noises, sometimes intermittently, it's probably those resistors. I've also known BC109's to go noisy in this vintage of Sugden equipment. |
24th May 2021, 7:21 pm | #51 |
Triode
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Re: Sugden preamp WAL AC Farnell.
Thanks Jez1234, appreciate your comments.
I took a risk and tried it out as a phono preamp (phono leads from a Systemdek 'bicuit tin,' Linn Akito arm and a Shure M55 cartridge) and it sounded very very nice 'as is.' Fortunately I couldn't hear, or smell, any frying bacon on this maiden voyage. If I wanted those mV DC to reduce, would the likely answer be to replace the elecrolytics? And would this likely improve sound quality or not? The Shure cartridge has a quoted output of 6.2mV. 'Tape' looks to be the best option in the table. Also, still anxious about this aspect, "When the rotary switch was turned a higher reading (90mV, 70mV, even a 170mV) would briefly flicker in between the selector registering in the new setting." I appreciate very much your reassurance but just want to double check that this particular issue shouldn't cause a problem in the other amps or speakers in my system? |
24th May 2021, 8:00 pm | #52 | |
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Re: Sugden preamp WAL AC Farnell.
Quote:
You can only use the phono input/setting for vinyl reply as it will be the only one with the correct RIAA equalisation. The Shure carts output is fine. |
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25th May 2021, 9:24 pm | #53 |
Triode
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Re: Sugden preamp WAL AC Farnell.
Thank you Jez, much appreciated.
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