Tube Amps / Music Electronics
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|11/17/2000 6:03 AM|
||Re: Illinois or Sprague Atoms?|
For what it's worth, the generic Illinois caps are the only ones I've ever seen that actually exploded in the amp!
For most applications I'd think you could get away with the IC caps but if the ATOMS are just a dollar or two more each, I'd use the ATOMS anyhow.
Make sure you have drained all the voltage from your old caps before removing them.
You probably won't need anything else, as far as installing them, well make sure the polarity is right when you solder them in.
You can do it one cap at a time to keep track of that.
|11/17/2000 3:57 PM|
|11/17/2000 8:35 PM|
Under normal operating conditions or was the polarity reversed? I've never had any of the IC caps fail on me and have been using them for 8+ years now. I have had a bias suply cap explode on me when I got careless about orientation.
|11/17/2000 8:43 PM|
||Variacs and Electrolytics|
is it really true about "bringing an amp up slowly" to AC line voltage using a variac?? ive heard so many varying opinions on this...
|11/18/2000 12:01 AM|
Here are the facts.
Electrolytic capacitors are designed by the makers to work properly when stored for some length of time before being used. This time is generally taken to be maybe a few years.
After unused capacitors are more than maybe five years old, they should be current limited while being charged up to working voltage for the first time. This current limiting "re-forms" the insulation inside the capacitor so it can once again withstand the designed voltage without excessive and damaging leakage currents. At some point, an unused capacitor is so damaged by aging that re-forming will not be sucessful. That's why you don't see more "NOS" capacitors.
The most straightforward way to do this is to temporarily solder a 100K resistor in series with the capacitor, then apply the intended voltage. The leakage current in the capacitor drops voltage across the 100K resistor, and prevents damaging high current flows. As the insulation re-forms and the leakage current goes down, the voltage across the 100K resistor goes down. When the voltage across the 100K is less than 10% of the working voltage, the cap is well enough re-formed to be used.
Re-forming may take seconds or hours, depending on the age and condition of the capacitor. If it goes on too long (days) without re-forming, the capacitor must be reguarded as potentially being permanently damaged. I wouldn't use such a cap in my amp.
The Gerald Weber Memorial Variac Award (a bronzed variac!) goes to Gerrie for the phrase "bring it up slowly on a variac". The problem with "bringing it up slowly" is that this doesn't usually work on marginal caps.
On tube-rectifier amplifiers, the rectifier heater is held to some lowered voltage just like the B+ windings are, so most of the "slowly" range does *nothing* because the rectifier doesn't conduct. On solid state rectifier amps, the diodes always conduct, but you have no good way of controlling the leakage current, so you can still burn out marginal caps.
What bringing amps up slowly with a variac *does* do is to allow you to spot telltale curls of smoke from backwards capacitors early before they explode or burn out your power transformer.
So - variacs are a good idea for making sure your wiring is OK. They are a bad idea for trying to re-form capacitors because they give you no useful control over the leakage currents.
|11/18/2000 2:51 AM|
||Re: Illinois or Sprague Atoms?|
I have to agree with RE on this one - I have never seen an Illinois cap fail. In fact, the only cap I have ever seen fail is a Sprague Atom, but only one time over the course of ten years of tube amp repair. Our different experiences are most likely just statistical anomalies.
The Illinois supposedly has a higher ESR than the Sprague Atom, but I have not noticed much difference between the two. I own over ten tube amps myself and have worked on countless others. I don't hesitate to use Illinois or Spragues in any amp. Yes, I do form the caps slowly with a Variac (always have), which may help "equalize" the Sprague vs Illinois chances of success. I know some folks seem to have a very low opinion of Illinois caps, but I have never seen or heard any justification for it in my experience.
Just another opinion (no disrespect meant toward anyone),
|11/18/2000 3:45 AM|
I agree with Bruce. I have even replaced two xx film caps in a HRD that leaked under use taking out the output tubes. It seems to me the IC caps are small for there voltage rating and the ones I measured had high ESR and were under the marked value new 18uf not 22uf. With caps I think you get what you pay for. Some older fender amps I looked at the B+ went over the voltage rating of the OEM caps when in standby one amp I think hit 500v and the oem caps were marked 450v and they were still good . I "think" the xx caps are voltage rated at the edge. FWIW I believe Bruce I myself use the same model cap tester. I find that caps with high esr don't seem to last as long ( I have tested lots of caps)
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