Should I replace original filter caps in 1980 super bass 100

[Timbo]

I have a 1980 Marshall super bass 100 on my bench that a customer acquired from a friend. It belonged to his late father and had been collecting dust in the garage for over 20 years. He was about to take it to the tip……
I pulled the valves and checked the HT/ bias voltages (500V/ -45V), then I measured the capacitors with a fluke ( I don’t currently have an esr meter). I put the valves back in and fired it up. It sounds great, no hum at all, in fact it’s so quiet that at first I thought it wasn’t working.
so, is there something I can do to conclusively test the condition of the capacitors? Should they be reformed after 20 years plus in storage? Should I buy an esr meter??
I kind of expected that I would have to replace all of the 45 year old electrolytics, but this seems to be a very grey area. I guess they were made better in those days.

[Jon Snell]

The usual issues with old capacitors are, excessive background hum and physically leaking.
As from your description, they re neither, I would run the amplifier for an hour or two, monitoring the temperature of the capacitors.
If they stay fairly cool, I would leave them where they are as you will find, the quality back then vastly exceeded the quality of moder day components.

Play and enjoy it.

[Mick Bailey]

Take a very close look at the seals on the caps - there should be no sign of bulging or encrustation which is usually yellow/brown.

You're a bit late to reform the caps now the amp has been run.

Measuring the value of the caps only tells you something if the value is way off and a faulty cap can still read OK. Bear in mind that filter caps can have a -10/+20% tolerance. I measure ESR as an indication of condition, but this doesn't tell you everything as the characteristics of a capacitor under operating voltage are very different to a bench test at a few volts.

A failing filter cap will either leak, cause hum or get too warm, in any combination. You don't need anything other than your senses to check these. The worst that can happen is a cap shorts and blows fuses, or the can ruptures. Sometimes this happens when a long-dormant amp is fired up for the first time or the cap has been on its way for a while and maybe early symptoms ignored or not detected.

[R.G.]

I would vote for replacing every electrolytic in the beast. Yeah they look OK now. But all electrolytics decay with time. We just can't tell exactly when they will fail. You need to balance the cost to the customer to replace them all, and virtually guarantee that an electro won't fail in the next decade, versus deciding that they're fine and getting it back in a month with a bum electro.
Yes, they look good, no leaks, no bulges, voltage OK, etc. But they're going to go sometime.

Actually, Mick, I would be concerned that the worst that could happen is that a cap goes very high leakage, just enough not to blow the fuses. This endangers the rectifiers and maybe the high voltage secondary. Soft shorts are ugly.

[Chuck H]

I'm typically all for replacing filters while you're already in there. That said, these caps seem to be performing and holding up. That can't be said about evrery replacement alternatives!?! But I would probably replace the bias supply caps with a high quality product as a matter of course (remember they are installed + to ground if you do this). These are the most important WRT seriously detrimental failure. JM2C

[Mick Bailey]

My personal view is they'll need replacing at some point and you don't know how the amp will end up getting used or how often. Predicting failure is difficult and many times I've had amps for repair where a problem could have been averted because there were early signs of increasing hum over time that were ignored by the owner. The same thing with output tubes.

I had a very early Laney Supergroup in for a check that was as factory-fresh as I've ever seen. It had a note of the people who'd worked on it, signed by each one and pasted inside. It played perfectly. The owner wanted a full recap and the Mullard tubes replacing. I had my brief moment with it, re-learning all my old Sabbath tunes and getting to use my Rangemaster as it should. If that had been my own amp, I would have kept it exactly like it was and enjoyed it as a time-warp home player until it really had to be re-capped. A few years later I had another untouched Laney PA head with the same request and the same feeling when re-capping.

The difference there is if it was my amp, and it wasn't. I'd be fine-tuned into monitoring the condition of the amp in a home environment but a gigging amp is another matter.

[Timbo]

One of my worries is that I may be replacing a perfectly good component with a not so good one. I recently did a full recap on a DSL401. It was working ok, but several of the originals were bulging/ crusty and it was obviously well used. When I turned it on after replacing them, after less than a minute, one of the new Nichicons swelled up and popped. So, replacement can’t offer complete confidence. I will replace the two in the bias circuit as they’re looking a bit crusty, but I’m going to let the others stay.
My tried and tested rule of thumb is that if there’s nothing obviously wrong, then don’t try to fix it. I think I’ll stick with that.

[stokes]

I would replace them. The fact that they sat unused for 20 yrs is a big factor. Manufacturers give a ten yr shelf life, thats unused. If they are used regularly they will keep longer, but unused the electro dries out. Somebody replied that "The worst that can happen is a cap shorts and blows fuses, or the can ruptures". Ever had to clean the mess after a cap explodes? I did, once, and I will never do it again. If that amp came into my shop, I would recommend replacing. I've had people decline because "the amp works fine". Thats ok, but if the cap explodes, dont bring it back to me is always my reply. I dont want to clean that mess ever again. Another issue with old caps is that they may seem fine, but when you put fresh caps you will notice an improvement in the amps response, things tighten up,particularily in the bass response. About 20 yrs ago we had a long discussion on the subject on the Boogie forums board. I made the claim that if an amp is 20 yrs old or more and I replace the caps and you dont see an improvement I will replace the new caps with the old and no charge. I got about 50 takers on the offer over the next yr or so and not one wanted the old caps put back in. I would also avoid the Nichicon brands and go wit F&T or Sprague. The Spragues are quite expensive so I use F&T or if you can find them, BMI are made on the same machines in the USA as Sprague Atoms, but at half the price.

[DrGonz78]

I think it should be mentioned that we are discussing capacitors involved with high voltage. I have amps from the late 60’s that have original mains filter caps. But these particular units are solid state and lower voltage than tube amps. So I usually run an old lower voltage capacitor of say 60-80vdc with more confidence than a 450v one. I also measure the ripple to see how leaky it is getting. I have seen times where I read perfect ripple results on an amp and left it alone. I have also measured significant amounts of ripple and changed the caps. If caps measures good and visual appearance is normal then I leave them alone. If a customer wants it done or an amp will sell better then I just replace them.

[Timbo]

Thanks everyone, all great advice.

[mozz]

Never had problems with Nichicon, though i can't say i have used them over 350v circuits. I do use a lot of 105c in pedals or for bias caps.