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Need Help With a Scott 208!

Just wondering what the consensus is here for how many caps need to be replaced? Drew_t mentioned the ceramic caps, it was my understanding that these rarely fail. After caps, should I check every resistor individually? And how exactly do I go about that if so? And how about the can caps, how do I know if they've been stuffed? Going to fire up the amp in the next couple days to see if the immediate problem follows the tube or stays with the socket. If it follows the tube, how safe would you guys think the amp is to operate if retubed with EH or Tung-Sol as is? And if it stays with the socket should I first try cleaning and bending the pins a bit to eliminate a faulty contact as the problem (read where Craig at NosValves recommended that on another site). Sorry for all of the questions but I'm trying to learn as much as I can in a short time. I'm one of those unusual people that started life as a math and science whiz but then took a radically different direction and now am trying to exercise some long dormant muscles.
 
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Actually I see where trainbuftony also suggested replacing the ceracaps. Don't know that they are actually ceramic and have put the bottom back on, I guess the name threw me off. I've seen in other threads that coupling caps are important to replace. I'm reading a lot of other sources while this thread is ongoing but the info is just starting to seep in. I'd like to have a fair understanding before I pick up a soldering gun and start ordering parts.
 
Coupling caps can become leaky (which can cause major problems)
Some types of old coupling caps seem to age much better than others.
People with a lot of rebuilds under their belts
(I don't feel I have enough to qualify in that regard) have a sense for which brands typically are ok. You'd need to test for leakage at rated voltage to see) For the cost of a few good quality film caps (make sure they physically fit and match C and V ratings, it's cheap insurance.
I used to test resistors individually.
Testing only requires a reasonably accurate ohm meter (multmeter?)
It might be necessary to lift one side to test. Other components attached to it can change readings.
For the cost of resistors , shipping for them, and the fact that you're lifting one side anyway, you might want to just change them (they're relatively inexpensive) the old carbon comp resistors were fine 50 years ago (and might in fact be ok today but they tend to drift. Also, they absorb moisture and are noisier)
If you replace with 1% metal films you tighten up the specs (probably not that you'd notice), you lower the noise floor, and you're good for another 50 years.
All that said.
I'd address the PS,
The big cans, and any other elytics.
If you know (or think likely) those ceraps are known to go bad, get rid of them now.
You might be perfectly happy with the result after just that.
Oh , watch the wattage ratings on the resistors.
Also don't be surprised if the modern one appears smaller than the original.
 
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The 4 Cera caps (the white tubular looking ones) are connected to the grids of the output tubes. They should have a few hundred volts DC on one side and something like -15 to -25 volts on the other. This negative voltage is what biases the tube. If the capacitor is leaky the DC from one end will cause the grid to become less negative, or worst case scenario have a positive voltage in relation to the cathode of that tube. You would have a bright red plate in no time under those circumstances. (I just stocked up on capacitors from a popular online retailer and paid about 30 cents a piece for these types of caps.) Consider this: If the socket pin which is supposed to be held negative by the bias supply is being held negative, but is not making contact with the pin of the tube for some reason, you will have the same result. Loss of bias voltage to the grid, and a runaway red plate tube. Or as I like to call it "A total thermonuclear meltdown" I kid... There are several reasons a tube could red plate and finding the reason or reasons on your specific amp will require some poking around. I would start by re tensioning the pins on the sockets for the output tubes, then fire it up, upside down wit the bottom removed. If the unit comes up to power check the coupling caps and make sure they all have some negative voltage on one end. If one of them is closer to 0 or 0 or positive in any way, and the associated tube is red plating, then you found your problem, and you didn't guess about it.

First I would be really darned sure that you don't have a shorted tube. By checking for grid to cathode shorts as I described in an earlier post, or better yet find a tube tester locally and have a pro check your tubes.

If you find that the coupling caps are leaky you should probably order the 2 aluminum can caps at the same time, from the same convienient online retailer, which I can point you to in a PM if you'd like.
 
To be clear, "Ceracap" is not an abbreviation for ceramic disk capacitors; it is the brand name of those white tubular capacitors which can be seen in your photos.

As to the can caps, if they don't have any kind of modern looking labelling on them, and you can't see any sign that the body of the can has been sliced through, or that the rim at the base looks like it has been disturbed, you should probably just assume that they're original. Even if the tech who worked on the unit checked them and found them to be working, they are too old to be relied on.
 
Sorry not in Portland, so can't answer where to get tubes tested. Here are some general suggestions. Best place to find people with tube testers, buy electronic equipment like soldering irons, meters, variac's etc. are Hamfest's. Do a search, find out when the next one near you is, and go. Next suggestion, fix your problem first, then consider doing additional updates.

If you are going to work on tube equipment get a variac. It allows you to bring up the voltage slowly so you can determine what the problem is before damaging anything. I have bought variacs at Hamfests for $20 - $25. Even if you don't have one, tube equipment is forgiving, I didn't read all of your posts, but if your problem is still tube red plating and it is one of the outputs (7591) simply swap it with one of the others, turn it on briefly. If the same tube in new location red plates, it is a bad tube. If the same location with a different tube red plates it is probably the coupling cap to that socket. Less likely is a resistor close to that tube. You can leave the amp off for hour or so to allow caps to discharge and check the resistors with an ohm meter.

I think you asked about tube sockets. It is helpful to spray them with contact cleaner, and if you need to tighten them, use a dental pick. Best way to determine if tube socket is loose is to fix your problem then with amp on, wriggle the tube a bit, if it makes noise, stops glowing or in any way acts up, tighten it.
 
I believe those Ceracaps are actually a film capacitor inside that ceramic tube. I changed some out before testing them and sadly I cut the leads too short to do so later but the epoxy seal at the end makes me think they aren't just paper jammed in a tube. Unfortunately without the ability to test them for leakage its hard to justify leaving them. I do leave old film caps alone for the most part, I've not had one test bad yet but I do test them if I don't plan to change them.
 
My experience with Ceracaps has never been one of leakage, but one of them going intermittent -- particularly in the smaller physical sizes. I've seen that problem a bunch of times -- and also with some other cap brands of that day as well. But leakage isn't a problem I've encountered with this brand.

Dave
 
Hello again everyone. I've been a bit bleary-eyes reading about tube amp safety. So I think I have an understanding about how to discharge filter caps (assuming these are the can caps). Have a wire attached to two alligator clips, with or without a resistor in the middle (depending on my tolerance for potential sparks) and attach one end to the chassis and the other to the positive end. I know this won't be necessary until I start changing caps but just want to make sure I have this down.

What I am less clear about is how to test capacitors safely while the amp is on. I have a healthy respect (fear!!!) or electrical voltages and am proceeding with an abundance of caution as I don't have anyone to show me in person and am stuck just reading, watching youtube, and asking questions here. If anyone would care to impart a little wisdom in a 'tube amps for dummies' kind of way I would be greatly appreciative. I really have only the vaguest idea of what I'm doing and constantly reading about lethal voltages has me a bit gun-shy.

I've definitely decided to take on this project and will be reading and rereading this thread before I do, so even if I don't get to it til after the holidays everything said here is like gold to me. I feel like I'm gaining a pretty good understanding but being hands-on (especially with the amp on!) has still got me nervous. I don't want to do anything stupid, and much of the info I've read online has at times been murky or contradictory.
 
It's a much better idea to discharge caps through a resistor.
I don't remember the science, but dead shorts don't do them any good.
I've heard some mac techs say run amp on load (btw this was for a SS amp until clipping) to test PS caps.
I think the only accurate way to test them is out of circuit with a leakage tester.
50 year old caps (electrolytics) are a bad bet.
Well past their expected lifespan. A tester that could accurately measure them would likely cost 15-20x what the caps cost.(leakage at rated voltage)
It seems (to me) a no brainer. Toss them and get new.
The coupling caps address separately. My 2c
 
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The Ceracaps are film and foil, I’ve broken one open and have easily been able to unwind separate foil and clear film. American Radionics which were in my 340B are also constructed the same way. Break an old one open, they can easily be unwound.
 
Yes, use a resistor, or just let it sit. The voltage bleeds off. To err on the side of caution, measure the DC voltage which will tell you how much voltage is left, if any. You wrote regarding discharging caps, "I know this won't be necessary until I start changing caps". This isn't true, the voltage is in lots of places in the amp, so if you are going to be touching components etc., let it discharge or speed it up with resistor to ground. I can't remember the last time I discharged a cap, because they usually bleed down pretty quick. Hooking up a meter to B+ (DC voltage hot side of PS electrolytics) and powering down the unit, you will see how fast it bleeds down.

Electrolytics, fail two ways, they dry out, less capacitance, higher ESR. Or they (electrically) leak, which isn't very common. If they leak they may heat up. If you hear hum, replace them. Or lots of people just replace them due to age and in most circuit applications replace with additional capacitance which stiffens the power supply, tighter bass. ESR meters aren't expensive, they are handy, but not required.

Like someone said, ceramics rarely fail. Coupling caps, test for leakage in circuit by measuring down stream DC voltage. Some types are more prone to failure than others. I usually replace them, not because they fail, but to improve the sound, but the majority of the time it isn't necessary. I tend to use a mix of film and PIO's in the same unit, but it is a personal preference. There should be a lot of posts about coupling capacitor preferences.

If you are worried about getting a shock, you can wear rubber gloves. Concerned about making a mistake and damage something, mentioned before, get a variac, bring it up slowly.
 
Jensen makes a model of capacitor with a ceramic tube. It is their most expensive model within the same line.
 
fredtroy said:
Yes, use a resistor, or just let it sit. The voltage bleeds off.
Click to expand...

One vote against "let it sit". I recently opened up a CD player with a tubed output. Last time it was plugged in was six months prior. Forgot to discharge the caps, got a pretty big spark on my pliers, fortunately it wasn't my finger.
 
dead shorting caps isn't good for them. Its a massive amount of current you're asking for. Also, it tends to blow the tips off screwdrivers and such. A 10K resistor to ground is plenty to discharge any cap in a fairly short time without harming anything. Might eventually burn up the resistor if you use a 1/2 w but its easily replaced.
 
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