Alembic Guitars Club
Connecting => Miscellaneous => Topic started by: tncaveman on July 04, 2012, 05:43:07 AM
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I have a 1973 Gallien Krueger GMT300B amp that I have had since around 1982. It's been a good amp but doesn't sound very good anymore. It lacks bottom end and some of the switches don't work any more - one that doesn't work is the contour switch. I have a tech friend that thinks a re-cap (replace all the old electrolytic capacitors) would probably restore it's sound, since they are known to dry up and loose capacitance with age. This is something I can do myself. However, the caps are about $125 for 19 capicitors, which seems like a lot - possibly more than the amp is worth and not a guarantee that it will sound better. However, it would be a fun project to do with my son. Anyone have experience with re-capping an old amp? And, is there a better/cheaper place to get good quality caps than Mouser? They have everything in stock and a great web search site. Other sites were very difficult to navigate through or did not have what I need. also, if any of you are a tech and have components, I have a list of what I need. Here are a few pics of the amp. Stephen
(http://club.alembic.com/Images/449/135133.jpg)
(http://club.alembic.com/Images/449/135134.jpg)
(http://club.alembic.com/Images/449/135135.jpg)
(http://club.alembic.com/Images/449/135136.jpg)
(http://club.alembic.com/Images/449/135137.jpg)
(http://club.alembic.com/Images/449/135138.jpg)
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You probably don't need to replace all your electrolytics. Start with the big blue Spragues (you can up the capacitance if you want to tighten up the bottom end of the amp) and any that show obvious signs of leakage, like bubbling, corrosion, or oozing goop). You should also take a good look at the solder on the PC boards--solder joint do degrade over time and that may be part of your problem. Most importantly, don't do any of this unless you've got a quality soldering iron--the cheap ones will only frustrate you and potentially create more problems than they solve. Lead free solder is a pain to work with because of its higher melt point--I strongly recommend the leaded variety. Mouser is probably your best bet for capacitors for this kind of project--there's better sources for high voltage caps as used in tube amps, but for solid state stuff Mouser is pretty much my default source. Plus they will ship USPS Priority, which is generally cheaper and quicker than UPS or FedEx.
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Oh, and if you need a schematic, GK is very helpful. Send them an email and they'll send you a pdf of the schematic. Thankfully they've still got a solid archive of their legacy stuff.
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The it would be a fun project to do with my son part is of significant value in and of itself.
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Yes Dave, that is one of the things about doing this that is appealing. And I did get schematics. I actually thought about trying just the big ones. None of them show any leakage? I actually had never heard of them drying out, but it makes since that they don't last for ever.
Stephen
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I love old gear, it always has history about it and it looks so cool and retro, look at the case, it seems artillery proof!
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I love old gear too; I love my Acoustic 370 so much I got another one as backup, that I'll never need...if no caps look bad, start with the switches as they could be source of craptone, and clean the pots of course...and really check the soldering as that fixes SO many amp problems it's not funny...
Tony
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I've not read all of these posts, but have you contacted GK, and shared with them your situation? I'd include the pics as well.
Just a suggeston....
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That was the first thing I did. They said that they don't work on them anymore. But they sent me the schematics very quickly. Since the big caps are screw terminals, I might just take them out and get my tech friend to test them. I cleaned the pots at first and it did help some. I'm going to test and clean the slider switches. That might be what's wrong with the contour circuit.
Stephen
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Just be sure to discharge those big electrolytic capacitors before your try to remove them. They can pack quite a wollop if they are still carrying a charge.
Keith
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Oh yeah - I got zapped by a camera that I was taking apart way back in college - brings a real appreciation for what even a small capacitor can do.