Rivera-era Fender Concert

Cjsinla

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I bought this amp new in the 80’s. I’ve had a love-hate relationship with it ever since. I always wanted it to sound like a Marshall but it was either too clean or too buzzy sounding. But, it was the first amp that I was finally able to eventually appreciate a clean sound. It’s been sitting in my closet for the last 10 years as in favor of my Marshalls. I did drag it out recently to cannibalize some tubes for an amp I was building. I turned it on first, it was dusty and the pots were scratchy but it sounded fine.

Today I decided to clean it up. I’ve never opened it up in 30+ years but I needed to spray some Deoxit in the pots. I have seen pictures of the inside of Concerts before but I was still surprised by the way the amp was laid out. I couldn’t really make much sense of what I was seeing. I’m going to have to order a few more 6L6 tubes to get this thing running first.

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For years I thought that this amp did not have a bias adjustment but I swear that I see a bias pot, am I wrong?

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The caps seem to be in decent shape...

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I’m actually thinking that I should move this amp on as I really never use it. Does anyone have any ideas of what this amp really excels at?
 

cmjohnson

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That pot is a bias balance pot. Not a true bias pot. Leave it be, it's usually just fine.

Those caps are due for replacement. At 30 years old they're not the caps they were made to be. If they should fail, and they couid at any time, the cost to repair the amp can increase dramatically. By now their ESR is higher and their capacitance is reduced and the second worst thing you can do for cap life is to not plug the amp in and run it on a regular basis. Those caps need voltage applied regularly to preserve the condition of their anodized aluminum internal plates. Just parking the amp in the closet for years is BAD for the caps.

Every electrolytic cap should be replaced at this point.


The layout of the board in that amp is a NIGHTMARE. I've never seen one of those before. I hope I never see one in person.
 

Cjsinla

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That pot is a bias balance pot. Not a true bias pot. Leave it be, it's usually just fine.

Those caps are due for replacement. At 30 years old they're not the caps they were made to be. If they should fail, and they couid at any time, the cost to repair the amp can increase dramatically. By now their ESR is higher and their capacitance is reduced and the second worst thing you can do for cap life is to not plug the amp in and run it on a regular basis. Those caps need voltage applied regularly to preserve the condition of their anodized aluminum internal plates. Just parking the amp in the closet for years is BAD for the caps.

Every electrolytic cap should be replaced at this point.


The layout of the board in that amp is a NIGHTMARE. I've never seen one of those before. I hope I never see one in person.
I looked at buying some replacement caps but I can’t find those 70 mfd 100v ones. I found some at 68 mfd 100v, would those be comparable?
 

cmjohnson

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Yeah, 68 uF is so close to 70 that it makes absolutely no difference.

The general rule of thumb is that when substituting capacitors, always use capacitors that have the same or slightly higher voltage rating. Never go DOWN on voltage ratings as compared to the design's original caps. As for capacitance value, you want the same as the original value (and 68 is close enough to 70 to call it the same) or you can go a bit higher. If you can't find 70s or 68s but 100s are all you can find, use 100s. Going up in value is OK if you don't go crazy. Go up by as much as 50 percent more and it should be just fine.
 

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