It would be helpful to know the manufacturer, model, etc. of the problem equipment. I probably can't answer your question even with that info, but there may be others here who can.
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This might be a stupid question, but suppose that everything is in working order, all else constant. Does doing so change the character of the sound?
Yes this is certainly an old unit, and that could be a solution. So who manufactures an excellent filter cap? I was thinking of doing it myself. Despite being a beginner I have done a lot of soldering and work with basic electronics, like RC cars when I was a kid. I understand that I might be a little out of my league with tube equipment though. Perhaps this might be better left to an expert.
So suggest an excellent parts manufacturer, and I will check the specs on the unit and get the best part possible put in there. Thanks for the heads up. \
Does this mean you have actually heard of this unit? Very unusual. At the local shops when they ask me what I am using I ordinarily receive blank stares upon my reply.
You have bad filter capacitors in the power supply. This condition is called 'motorboating' and can be affected by temperature and the length of time that the unit is one.
It is something that needs service! I would not operate this unit until this is fixed. If left unattended, you run a good risk of frying the power transformer! Trust me, that is a very unpleasant experience.
Some nice, high quality caps can be purchased here(other manufacturers can be found, bottom of page):(http://www.partsconnexion.com/t/catalog/CapacitorsElectrolytic.html) or here: (http://www.percyaudio.com/Catalog.pdf) I've found both suppliers to be very reliable and prompt. The salesmen at your local shops probably used to sell sneakers or change oil for a living, and know little about audio(outside of what they've read on a component's blurbsheet). That's been my experience with the majority of salepersons I've encountered over the decades.
Dfelkai, most shops in the US will charge about $75-$80.00 per hour. That allows them to pay the technician and their overhead. Filter caps do not take a long time to change, and this is entirely off the top of my head, so unless there is something really unusual about this preamp I would guess that 2 hours is all that is needed.
Without even seeing the unit, that's the best I can come up with- diagnosing at a distance has its problems :)
What you may find is a "bench charge" or a diagnostic charge. Maybe $30-50 to be applied against repair if you go ahead. some shops will have a 1 or 2 hr minimum. Lots of variability in terms of minimum charges, etc. If $75 or $80 seems high, my plumber has a $100 minimum! 'Course his family is the biggest real estate holder in the most exclusive suburb in my region!!!