Filter caps for a Silvertone 1482?

[bluzmn]

I'm going to be receiving a "for parts or not working" Silvertone 1482 that I bought on eBay in a few days; the first thing I'm going to do is replace the filter caps (and power cord). The schematic calls for 20, 10 and 5mfd filter caps; I ordered some TAD electrolytics from Mojotone, but they only sell 22, 16 and 8mfd caps. Is there any reason why I shouldn't use 22-16-8mfd instead of 20-10-5mfd for filter caps? I know the 22mfd and the 8mfd are probably OK to use, I'm just wondering about the 16mfd.

[Steve Conner]

The tolerance on electrolytics was typically -20%, +80%. So a 10uF electrolytic could be anything up to 18uF, hence 16 should be fine.

You should be able to get 10uF, though, it's a common value. Are Mojo just out of stock?

As a more general point, components aren't made in the 1-2-5 sequence any more. The preferred values for electrolytics are 10, 22, 47, 100, 220, etc.

33 and 68 are also available but not quite so common. 16 is unusual.

[bluzmn]

Mojotone doesn't sell a 10mfd, 450-500v cap by any manufacturer.

[Steve Conner]

Just as well they're not the only capacitor store in the world!
10uf 450v - Google Search

http://www.tubeampdoctor.com/de/shop...ator_axial_510

[Sweetfinger]

The 1482 uses a can cap with 3 sections IIRC. The other thing I'd look for in that is the power supply resistors are perhaps a teensy bit underrated. I've seen 'em nicely toasted so move those up to a higher wattage rating. Look for leaky caps in the trem circuit. I had one with a problem there. Those are pretty low-fi amps if you're not sure what they should sound like. Being a Danelectro made amp they seem to be voiced for lipstick tubes. If you plug humbuckers into one you'll just get mud. Good luck!

[Enzo]

Before ordering a bunch of caps for it, wouldn't it be prudent to wait until it arrives? That way you can see if the transformers are shot or even missing or perhaps other dire things wrong, which may affect your interest in the project overall.

[bluzmn]

@Enzo,
That's a good question, and there are 2 answers:
1. I know that even if I don't use the caps for the Silvertone, there will be another project on which I do use them, and
2. I'm learning by doing, so if one (or both) of the transformers is bad, great, I get to learn how to replace a transformer.

[km6xz]

Since this is a learning exercise, you might be better off repairing the amp first before doing any replacements of unknown-to-be-defective components. Get it working then restore it, doing it in reverse order will introduce confusion in causes and symptoms and possibly introduce errors that will be confused with defects. Experienced techs do it that way so that is a good thing to learn from them before it creates a problem for you.
You will benefit from having a baseline of performance of its repaired performance before deciding on what it needs for restoration.

[bluzmn]

@km6xz,
I don't understand what you mean; I've only said that I'm going to replace the filter caps. Isn't that always the first repair suggested for an old amp like this?

[km6xz]

It is on the internet but not by those who repair or design gear at a higher level. Repair and restoration are two very different tasks and in repair the goal is to reduce variables and generate an accurate diagnosis that fits all the observed evidence. Introducing more variables, by changing parts before the diagnosis if completed can but you further from the goal of an accurate diagnosis. It might be that the caps are causing the malfunction but you do not know yet. Treat the defect and after getting the unit working, and have a baseline of measured performance, feel free to rip it apart and restore if needed. Depending on the heat the caps were subjected to, they might be perfectly good. Also this applies to mods. Get it working and establish a baseline of performance before any mods or else you will not know if the mods are achieving the desired goal. Listening alone is a poor way of judging amplifier performance because the subjective evaluation varies a great deal with mood, environment and signal source. You will read that some player swapped out a couple tubes or changed something and claims dramatic improvements that objective tests or properly conducted subjective tests can't confirm. Hearing big differences usually is traceable to wishful thinking or anticipation, unless the prior parts were defective in the first place.
You mentioned it was not working, that is not likely to be caps unless the owner was referring to hum and all else working. If it is not working at all, get to making some measurements and compare them to what you would expect to find. A general understanding of what is going on can be arrived at with a very few logically chosen tests with a voltmeter, listening and, hopefully, oscilloscope.
What test instruments do you have to work with? To save time next time you need to get into the amp, by having the baseline measurements logged, you can save a lot of time determining if there actually is a problem with the amp. An amp is one part of a whole signal chain that any one part of that chain can influence the results of the system in ways that is not always obvious which part of the system is at fault.

[Steve Conner]

It's a good point. I've seen quite a few troubleshooting threads on here where someone tried to do a cap job, and the amp "wouldn't work" afterwards because they miswired the replacement caps in some subtle way.

At least if you follow Stan and Enzo's advice, repair before restore, you'll know that it worked before you started your cap job, and if it doesn't work afterwards, you know you made a mistake and have an idea of where to look for it.

Having said that, if the amp has only one cap can and it's shorted, then you will have to do the cap job to get it working. Repair and restore become the same thing.

As for listening being a poor way to judge performance, I'd always supplement it by measuring the output power at least.

[bluzmn]

OK, maybe you guys are confused because I haven't told you the whole story (you're certainly confusing me). The amp was described as an "attic find" and the AC plug is cut off; it probably hasn't been turned on in years. I think I've read on this very forum that an amp like that should not even be turned on without replacing the filter caps first. Are you telling me that I should just install a grounded cord and turn it on (bringing it up slowly on a variac, of course)?

[km6xz]

Yes, bring it up on a variac which watching current after making power supply resistance measurements to determine if there is a short in the B+ supply or load. Bringing it up slowly is mostly to be able to catch any unexpected changes in current. Check for heat rise with the caps, the result of ESR increasing and leakage. That mains idle current measurement in your baseline data will be useful in the future if there is a problem which can be identified by a change in mains current draw.
I assumed from the original post that it was non-working and so it definitely is a repair project that should be treated as one until it works as expected.

That amp was probably built in the mid 60s to late 60s which is about the time when capacitor construction and dielectric formulation was advancing where caps became much longer lasting and more stable. The conditions of storage and temperature they were subjected to will have a big impact on ultimate life but that open chassis of the Silvertone was able to keep heat a minimum.

You have likely read a lot of things on this and other forums but most people on forums are hobbyists who are repeating what they have read by other hobbyists or wacko esoteric audio types. You did not read that from the few electronic engineers and advanced techs on the forums, however. Replace parts that you prove to yourself are no longer up to that task they are being asked to perform. Wholesale parts replacement is usually an indication of the tech never really figured out the problem or mechanism of fault. That, and shops wishing to run up a repair bill. Shotgunning a repair can be very expensive for the customer because the parts added to the bill in an attempt to hit on the defect by pure luck, are paid for despite being completely unnecessary. So the customer looses out twice, paying for and not having an accurate diagnosis of their problem.

[Steve Conner]

This How-to guide is aimed at restorers of vintage tube radios, but I reckon it applies equally to vintage guitar amps that have lurked in an attic for years.
UK Vintage Radio Repair and Restoration - Initial Checks and Tests

[hasserl]

(you're certainly confusing me)

Understandable, because in the end a 40 - 50 year old amp that's been sitting unused for years is probably going to need new filter caps anyway, especially if you plan to actually use it. You may end up chasing ghosts trying to isolate other issues as you get the thing running that can end up being traced to the caps. And the amp will never sound as good as it can with decrepit old filter caps.

I think what the guys are trying to say is, just get it working first, take some baseline readings of the operation and check it over thoroughly. Then make your plans for refurbishing it. You may get lucky and find it needs nothing but a new power cord. Some guys dislike new filter caps in amps, and having them changed out devalues the amp to them. Give the old girl a chance first before making big plans.

[bluzmn]

OK, now I understand. Thank you all for your advice; I'm going to install a power cord and bring the amp up slowly on a variac.
@km6xz-
As I am watching the ammeter on the variac, what am I watching out for? Also, the power supply resistance measurements you suggest; do you mean measuring the dropping resistors to make sure they're within spec, or is there some other measurement you're referring to?

[Enzo]

If I may analogize...

WHen we say repair it before restoring it, think of someone in a hospital with a swollen appendix (however you spell appendix). While treating the appendix is not the time to be getting their hair done, sitting in a tanning bed, and having a nose job. Once cured of the problem, THEn do the extra things.


I may see a 40 year old amp, and think to myself, this one will be needing all new caps. But I always fire it up as is first to see what I am up against.