Size does matter....at least with capacitors.

[Axtman]

I knew that new caps were smaller....but I did not know they were that much smaller!

Is there something I can wrap around the new capacitor body to make it fat enough to fit the old capacitor clamp? The hole is too big to fit a new capacitor clamp. Thanks!

[52 Bill]

Is there something I can wrap around the new capacitor body to make it fat enough to fit the old capacitor clamp?

The neatest way that I've seen is white plastic plumbing pipe reduction adapters. See if there is one that will work for your situation.

[Axtman]

52 Bill,

I am not sure what you mean. Can you please post a link to a hardware store? Thanks.

FYI, the old capacitor is 1-3/4 inch in diameter and the new capacitor is 1-3/8 inch in diameter.

[bob p]

How stiff do you want the mount to be? I'm thinking that there are two ways to approach the problem:

1. Abandon the 1.75" clamp, cut a round sheetmetal disc to cover the original capacitor hole and screw holes, and cut a hole in that disc to mount the new cap with the 1.375" clamp. That would be the hard way, but it would look good.

2. The difference in radii on those two caps is 3/16". Wrap the new cap in something compressible that will form a tight grip when it's compressed down to 3/16" thickness, and use the original clamp. I'm thinking that there have to be 1/4" to 3/8" thick foam products that could compress down to 3/16" and give you an adequate purchase on the neck of the cap. Cut a strip and wrap it around the cap and use your existing clamp. I'd look for a product that's rated for high temps. Flame-retardant would be an added benefit.

[Mick Bailey]

For heat resistance, flexibility and easy availability I use cheap silicone cake pans as a donor material. Most have nice thick bases but can be doubled up with pieces cut from sides. The wall thickness is fairly consistent and the stuff grips pretty well.

[The Dude]

......I'm thinking that there have to be 1/4" to 3/8" thick foam products that could compress down to 3/16" and give you an adequate purchase on the neck of the cap. Cut a strip and wrap it around the cap and use your existing clamp. I'd look for a product that's rated for high temps. Flame-retardant would be an added benefit.

That's exactly what I have done. I use the stiffer foam-ish product that some use in packing materials and just cut a strip to wrap the bottom of the cap. It's stiff enough to be stable and flexible enough to allow compression within the mount.

[Axtman]

I like the idea of foam gasketting strips. Since the capacitor leads are screwed on (with lock washers) there should be no problem of metal fatigue if they leads jiggle around.

[xtian]

What amp does that go in? Bass amp?

[g1]

Another option would be gutting the old can and fitting the new one inside. This is more often done when appearance is critical.

[Axtman]

Appearance is not critical but I don't want it to look like some half-arse hack job.

The amp is a Fender 300PS. BTW, those caps are 490uf / 400v so you know I will be treating them with utmost respect!

[Axtman]

Look at the size of them trannies! I could barely lift the amp up to the workbench!

[Axtman]

Lead dress?! We don't need no stink'n lead dress!!

[bob p]

Smaller cap size is an added benefit to making things fit in tight spaces. Sometimes tight space causes serious problems. Here's an example.

I had a Carver C-500 come to me come to me that was having problems. (Think of the Crown DC-300A.) It would grunt like a bull and vibrate across the bench when the power was switched on. It ran hot and repeatedly destroyed it's output transistors. The amp had always eaten it's outputs, but it never started snarling until the original owner decided to re-cap it.

When the owner got tired of the snarling and the popping transistors he just stopped using it and blamed Carver for a bad design. Then he propagated some misinformation on the brand forum sites, claiming that that model of amp was a bad design and they all run hot and blow up their power transistors. Unfortunately, that disinformation became sticky on the internet and now people are believing that the amps all run hot, which isn't the case.

When I received his amp and plugged it in to idle before I set the bias, it got so hot that I thought I was going to burn myself when I touched it. I used my IR thermometer to take some measurements:



The amp was definitely WAY too hot at idle. (The term "power" in the chart legend refers to the curve fitting algorithm used.)

His amp had two problems. On one channel the bias circuit hard the wrong value resistor stuffed into the board, so that the bias on that side was too hot even with the pot at the limit of it's rotation. What amazed me was that the amp went back to the factory for service and they didn't catch the part-stuffing error. So the amp lived with a screwed up bias circuit for decades (until I got it and actually looked at the parts on the board). As evidence of how often amp ate it's output transistors, instead of having 8 matching devices it had 4 different types of output transistors populating one side!

Fixing the bias circuit and replacing the output transistors with a matching set solved that problem.

The other problem was that the caps were so darned huge that Carver had to use a threaded stud and standoff to clamp them to the chassis via the bridge recitifer mounting stud. Each side of the bipolar supply used a 6,000 uF/100V inverter grade cap. Unfortunately, the fit was so tight that when the original owner / recapping expert worked on his amp, he screwed a self-tapping sheetmetal screw right into the PSU cap:




Well, I guess that dead short from one side of the bipolar supply to chassis explains why the transformer sounded angry when the amp was powered on, and why it was vibrating enough to shudder across the table. The PT was also getting pretty hot.

What amazed me was that they guy was a "recapping expert" who had worked on dozens of amps for other enthusiasts, but he couldn't figure out his own amp's bias problem. And when he re-capped his own amp, he wasn't careful to notate where the screws came from and where they were supposed to go back to. The amp was designed with a combination of self-tapping sheet metal screws and blunt-tipped screws, the blunt tipped ones being used in the area of the power supply. When the owner finished recapping, he buttoned-up the amp without putting the screws in their proper locations and drove a long, self-tapping sheet metal screw right into the body of the bipolar PSU cap. He never figured out what a retarded thing he had done. Instead he pronounced the amp as being a bad design. Years later, the amp still has an undeserved bad rep for running hot.

I ended up replacing those huge Mallorys with modern caps with higher storage density. They're more compact and help to protect the amp from idiots with screwdrivers. After fixing the PSU short to ground and fixing the bias problem, the amp idles nice and cool, just like you'd expect it to.

See? Today's small, high-density caps are better in more ways that you might expect. I replaced those huge 6000uF / 100V caps with a pair of compact 3300uF CDE snap-ins and fabricated little boards to mount them directly to the PT mounting bolts. Lots of room in there now.

[Chuck H]

Lead dress?! We don't need no stink'n lead dress!!

[ATTACH=CONFIG]45479[/ATTACH]

Who brought the marinara?

[bob p]

Lead dress?! We don't need no stink'n lead dress!!

[ATTACH=CONFIG]45479[/ATTACH]

I'll see your PS-300...



And raise you a Super Twin Reverb!



What a rat's nest of wires Fender used in their high powered amps!