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replacing components on old amps

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8/13/1999 9:23 AM

replacing components on old amps

I'm about to replace the filter caps on my Fender amp. It doesn't use PCBs and uses the old type boards with eyelet type holes to secure parts.

Any special recommendations on how to remove the old caps? Do I just remove the solder, follow the lead until it ends then desolder and put the new cap in? Or do I simply snip off the old cap and simply solder on the new one???

Thanks,

Aron

8/13/1999 9:32 AM

It depends on which caps in what amp.

Most amps use the extra length of their respective ground(negative) leads to make the jump to the ground buss under the pots.

Some of them use their leads to connect to the other caps under the dog house on the bottom of the chassis where the transformers are mounted.

I suck all the solder out of the eyelet and pull the leads out of the ones that will come out and pull the lead hard over to the eyelet edge to make more room for a new lead on the ones I cut off before I insert the new one in the hole.

Bruce

8/13/1999 10:46 AM
BWilliamson	I wrote up a happy article for my snappy little web page on this. Pretty basic and step by step. Hope I don't insult anybody with the 1+1 attitude of the article. http://members.xoom.com/BoxerAmps/filtercp.htm enjoy and good luck bw

8/13/1999 3:28 PM

Hey, BW, that's a great tutorial. Is there a link to it on the Ampage resources? (If not, I think there should be.)

This is exactly what Aron needs to rebuild his 6G6-B power supply. That model uses a pair of (now very expensive) 600v caps in parallel for the first filter, but I recommended he modify it with the pair of 350v units in series. A picture's worth a thousand words.

And Bruce, thanks for mentioning the feedthru leads under the board. I clip the old cap off just above the topside of the eyelet, to retain these factory jumpers. New capacitors have shorter, thinner leads, and don't always reach all the way to the other eyelet.

Doc

8/13/1999 3:28 PM

Some comments on your article:

1) There is an error: that resistor in the bias circuit is 470 ohms, not 470K (silverface amps use 1K, which is fine also).

2) I really feel that the bias filter cap should be between 50uF and 100uF at 100 volts. A cap with a 50 volt rating will be operating at (or just beyond) is rated limit!

3) Either 80uF/450v or 100uF/350v caps are suitable replacements for the original 70uF/350v caps.

4) I'm glad you suggest replacing those carbon comp power resistors with metal oxide ones. Yes, those carbon comps can really drift and mess up the preamp plate voltages.

8/14/1999 12:08 PM
BWilliamson	Thanks for the feedback, I made the corrections and added the notes. Thanks Jim bw

8/13/1999 1:08 PM
Mook	With pliers (after draining filters), I like to heat the solder, and remove ALL the components from an eyelet. At this point, I "clean" the eyelet by "sucking the solder" (with a cheap Radio Shack solder sucker). Once the eyelet is totally clean of solder, I replace the components and apply FRESH solder. Mook

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