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Old 5th Mar 2021, 5:20 pm   #46
knobtwiddler
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Join Date: Mar 2016
Location: London, UK.
Posts: 249
Default 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!
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