Time to change some Filter Caps! YAY!!!

[DrGonz78]

Well I have finally got the only electrolytic 6000mfd 55v (50v on regular amp part) that I could find. It is one where is it 50mm (2" diameter) and I will have to modify the chassis to accommodate these new filter caps. This amp is the Randall RG80-112sc that likes to hum and now it is time to try new filter caps, since these ones are probably over 20 years old. I got these General Electric NOS 6000mfd 55v capacitors at Nebraska Surplus online. So, I am just checking in with some opinions about these caps and how they will match up w/ my amp. They appear to me to be computer grade as those typically have the screw in caps. Does anyone have any input to give me before I make the modification to this amp. I mean I think putting in these size filter caps will be a good idea since I have not found any that are smaller diameters like the original ones on the amp. Please can anyone give me their two cents about these caps and how they will match up to the originals??

Also any advice about soldering or not soldering the wires onto the terminals would be great appreciated too, thanks.

[Enzo]

You don't solder wires to screw terminals. You use a solder lug or an eyelet lug. The lug goes under the screw and wires are soldered to the lug.


The problem with searching for replacements for old filter caps like that is expecting to find the exact value. 6000uf is not a standard value. But something like 5600 or 6800 are standard values. And if you can't find 50v, 63v is a common rating. So instead of that exact part, use something close from standard values. You may not like going from 6000 to 5600, but really, it is a 10% difference.

Caps today are much smaller physically than they used to be, so a similar value cap today should be smaller than the old one.

I just looked in Mouser, and 6800uf 50v caps with wire leads, they show 8 choices in stock, most in the $4-$5 range. They are 22mm diameter and 50mm long. With snap-in leads, they have even more selection. At 6000uf even, they have none in stock. That is the difference between standard values and odd numbers like 6000. It sure won;t hurt that amp to have 6800uf instead of 6000uf filter caps.


Same thing happens with 1980s era Peavey amps. They all used the 5000uf 55v filter caps in a can mount. A non-standard value. Can't get them. Order one from PV and they will send you a 4700uf 63v cap. And that is how you find such odd parts.

[DrGonz78]

Just keep debating whether I want to modify this amp to fit these huge caps. Although these GE caps might be higher grade or something to make it worth while. Although the idea of just buying caps that fit in the already present mounting brackets is so much easier than drilling and even perhaps a bit of routing to make the terminals fit through the chassis.

As usual Enzo thanks for the speedy reply and great advice. This gives me a little more idea on what is possible for replacement parts now and in the future.

[Enzo]

Well, here is something to consider before you modify the amp for these. They may well form up just fine, but if you look at the bottom number in your photo - 1887502 - that is a date code. 188 is the General Electric code, and 7502 is the date of manufacture. Those caps were made in the second week of 1975, so they are 37 years old. That might push me towards a couple of $4 caps, and use these huge things in some shop project.

[DrGonz78]

Great point indeed! I definitely am going to buy new caps for this as you recommended. You mention 5600 or 6800uf... Which one is more optimal? I know I can go higher than 50v and get the 63v... Is there any reason I would not want to put 80v rated for replacements? Just in case I wanted go higher on the voltage rating for sake of selection and sizing.

Also, what sort of project could I build to use these GE caps? I was thinking something on those lines too, but have no direction of thought due to my lack of experience.

[Enzo]

The voltage doesn't much matter as long as it is high enough. 80v caps would work fine, however the higher the voltage, usually the larger the cap. On the other hand, even larger in that sense is probably still smaller than the original caps. Higher the voltage on caps also usually means higher price. If the original circuit was designed with 50v caps, 63v is no overkill, but you also gain no advantage with 80v caps other than maybe more selections than at some other voltage. But Mouser at least had a bunch at your 50v even.

5600 or 6800? I don't much care, I might find teh decision based on what someone has in stock. Going larger has POTENTIAL for extra filtration, but if the old caps did a good job, then larger ones probably won't do any better. And we are not moving from 6000 to 12000 or to 3000, we are just moving a little in either direction, so I doubt there would be a noticable change in "tightness" or anything.

[DrGonz78]

Well thanks so much Enzo!! I went ahead and ordered the 6800uf 50v caps from mouser, 35mm diameter which will fit just perfectly. Just trying to get the hum out a bit and maybe adding a tiny amount of filtering can't really hurt. I appreciate all the great advice as usual.

[Enzo]

Great!

I think having fresh caps will make more difference than the extra ufs, but they can't hurt.


MOuser usually has dimensions on their parts selection page. And it may not matter so much in a guitar amp, but in a thin, single-space rack piece of gear, there is limited space above the board, so leength matters. The selection page lets me see that dimension.

ANother dimension that sometimes matters is lead spacing. You usually find that most or all of one value/voltage cap will have the same lead spacing - 10mm is common for larger caps. But it pays to know what your spacing needs are, especially on older gear whose original caps may have been larger. Because sometimes you have a choice.

[DrGonz78]

On this amp the caps are held up vertically by clamps through the holes on the bottom of the chassis. So in this case it was mainly getting the right diameter to make sure that they fit the clamps. Then there is all the wiring going on from AC power to switch & transformer. The bridge rectifier is bolted straight down (legs pointed up) on the chassis with the two "~" sides connected to the transformer and the "+" and "-" connected through each side of the filter caps. There are just wires that connect all of those main power components together and no need for select pin spacing. I had ran into lead spacing a while back working on some Crate amps that were there for the tinkering.

So basically I will be wrapping some wire firmly around each terminal pin and soldering to hold. I do like the idea of those computer grade screw tops in a situation like this, but can't have your cake and eat it too. I like how this amp has the BR secured to the chassis as the terminals are easy to remove if needed, unlike many BR's that are soldered into PCB.

[Zer09]

Gonzo,
I had to do this recently with my brother's Peavey bass head. I ordered a couple Illinois axial caps to replace the cans and was able to mount them right on the board (they are that much smaller now) with just a little stick-on foam pad under each to make sure they didn't constantly vibrate when on top of a cab. Worked like a charm and had just enough lead to fit them. I also put shrink tubing on the leads just in case the the foam somehow detached and the cap was able to move enough to short to anything. There was no way the foam was coming off without being scraped but I had to have the tubing for my own peace of mind.

[DrGonz78]

Yup Zer09 the more I get working on amps I find there are so many solutions to make something work. That's why I usually will ask for advice on MEF before doing something I might later regret. Thanks for sharing your tips.

[Enzo]

Whatever works is the solution, often more than one.

Those old PV blue cans were in everything back in the 1980s, now when I install a replacement on like a 400BH board, I wind up drilling a hole in the pc board to accomodate the smaller cap lead spacing.

[DrGonz78]

Well I got the caps in now and it sounds great!!! I love playing Ramble On on this amp as it has that nice clean tone and then I can add some crunch w/ the mixed channels and Whammo Slammo Ala Ca Zammo!!!

But this thread is in relation to figuring out why this amp is way more susceptible to bad ground hums. (<<< refers to another thread about this particular amp when it is on a bad grounded line is more susceptible than any other amp I own). The thing is way noisy in that situation and it is not the amps fault, but something tells me the hum is so loud that it might be something component wise to lower it a bit in those situations. There is a thread on here about the RG80sc112 hum problem... and I think so far the hum is lighter after changing filter caps. However, I have not tried it out on my not so grounded outlet in the spare room...>>>(where it hummed really bad when the guy came over to sell it). I then returned from work and tried it out in my main room and it was way way better! I can take any amp I have plug it into the bad grounded line and it is noisy(hum) but not like the beast(RG80). I want to get this amp to the point where you can feel comfortable bringing it gig to gig without the worry of the power hum problem.

I am now gonna start looking at some more cap replacements on the board. Also, there are a many ceramic disk caps that are probably leaky on this unit... Hence it has some cracked legs... They are definitely in need of replacement. There is a cap on the power switch that I think is going bad too...

I noticed on this amp that the treble pot when I pull(booster) it up that it is not working now? This was not caused by my work on the caps but by something else relating to wire replacements to the potentiometer wires that lead to the board. Maybe something in the circuit there is bad or just the pot is not working correctly now. Not sure why but will look at the schematic some more and even test the pot itself.

[DrGonz78]

Okay I just tried this amp out in a room that made it hum like a 1000 monks when I first bought it... Now it is quiet as a mouse!! Now just gonna get that treble circuit to function again and it is all set for jamming! Treble pot works until it is pulled up for the boost mode, so we'll see how that goes...

Edit: Well the response from the outlet is not how it was before but that tiny hum is still present. Mostly the other thread on this issue really came to a conclusion that maybe these amps just are designed in this way where there will be a hum. However, there is a 35v 2200uf axial cap that could be replaced and many others on this amp that might add to this noise/hum. Also, like I mentioned the ceramic disk caps have a few that are cracked around the legs and they feel a little bit brittle to me. Another clue to this is that when I changed the main filter caps, there is now some slight popping noises when it powered on or off. Nothing major but popping makes me think of caps...