I am in the process of re-working my McIntosh mc240 power amp. I have some hum in only the right channel, I suspect some old capacitors, since I have changed out the tubes but, I want to insure that I do not change the original tonal quality of the amp when I chenge the bad ones out. The original capacitors in the amp are Sprague Bumblebee typ which are no longer made. Does anyone know of the brand of capacitors that I need to repace these with in order to keep the same original tonal music quality? Help!!!!!
First, it is unlikely that capacitors will cause a hum in one channel, with the exception of a coupling capacitor to the output stage. Does the hum continue with the inputs disconnected?
Second, it matters little what capacitors you use as long as they are the right capacitance and voltage rating.
Second, it matters little what capacitors you use as long as they are the right capacitance and voltage rating.
Electrolytic capacitors all need to go at twenty years no matter how good they were before the seals went bad. Look at the hours life rating of what you buy. Expensive electrolytics have better rubber and are rated 2000 hours up. Cheapo electrolytics can go bad in a year. Some manufacturers sell both kinds, like nichichrom. I can't beleive the stupid discussions on here throwing around brand names as some badge of quality, without specific part numbers.
Replacing paper caps with plastic requires more art. When I replaced the paper caps in my PAS2 preamps (that were causing channel imbalance, they had to go) it became excessively bright, too trebly. I'm handling this with the tone controls at this time, but perhaps a more scientific way would be to put some resistance in series with the caps. Kevinkr recommends the non-boutique cap the sprague orange drop, along with some pricier boutique brands. I've got 616 orange drops as grid drivers on my ST70 on one channel, and "arco" plastic caps on the other, which are a no-name brand, bought when I had more money to finish the project. I can't tell the difference between the channels. Many guitar amp shops have sprague orange drops. i bought some illinois capacitor .1 uf 630 V polyester caps for $.39 from newark.com, and think as an experiment they were very cheap and uniform.104MSR630K part number for 0.1 uf size. I'd have to think hard before I spent $10 each for one of the boutique brands or polyprophylene. If you do spend that much, buy some of the illinois 40 cent caps too, and let us know if you can hear the money talk.
Replacing paper caps with plastic requires more art. When I replaced the paper caps in my PAS2 preamps (that were causing channel imbalance, they had to go) it became excessively bright, too trebly. I'm handling this with the tone controls at this time, but perhaps a more scientific way would be to put some resistance in series with the caps. Kevinkr recommends the non-boutique cap the sprague orange drop, along with some pricier boutique brands. I've got 616 orange drops as grid drivers on my ST70 on one channel, and "arco" plastic caps on the other, which are a no-name brand, bought when I had more money to finish the project. I can't tell the difference between the channels. Many guitar amp shops have sprague orange drops. i bought some illinois capacitor .1 uf 630 V polyester caps for $.39 from newark.com, and think as an experiment they were very cheap and uniform.104MSR630K part number for 0.1 uf size. I'd have to think hard before I spent $10 each for one of the boutique brands or polyprophylene. If you do spend that much, buy some of the illinois 40 cent caps too, and let us know if you can hear the money talk.
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The electrolytic caps will likely have to be replaced since they're like 40 years old. That could contribute to the hum you're hearing, the filtering may be insufficient.
For the axial types people prefer Sprague TVA types, also Sprague Atom, and the bigger ones are multisection twistlock caps (also known as FP types). Some manufacturers still make them, like Authenticap, JJ, CE and F+T. You can get them at tubesandmore, thetubestore or tubedepot (they also happen have great BBQ sauce).
For coupling caps people use Sprague Orange Drops 716p, these have copper leads and an outer foil which you can determine with an oscilloscope. Or Illinois Capacitor MPW series, they're also metalized polypropylene with copper leads and are likely the most popular choice for restoring McIntosh (just look at pictures of restored units, it' the yellow caps). Mallory 150M metalized polyester are quite popular too. Fancy caps are Auricap and others, but they don't seem to bee too poular. What's quite popular though are Russian PIO caps, just search a bit here and over at audiokarma in the McIntosh forum.
If you're considering NOS Sprague caps read this: http://www.paintedpostcards.com/misc2/BlackBeauties.pdf.
The most important aspect for the coupling caps seems to be that they're non-inductive.
If you need to exchange resistors you'll want to use wirewound in the PSU (check for voltage strength!), metalfilm for signal (1W types have lower noise, higher voltage strength), maybe non-inductive wirewound for cathode resistors (Mils MRA5) and carbon composite or thick film for grid stoppers (they're noninductive, that's the important aspect here). Watch out for high voltage drops!
For the axial types people prefer Sprague TVA types, also Sprague Atom, and the bigger ones are multisection twistlock caps (also known as FP types). Some manufacturers still make them, like Authenticap, JJ, CE and F+T. You can get them at tubesandmore, thetubestore or tubedepot (they also happen have great BBQ sauce).
For coupling caps people use Sprague Orange Drops 716p, these have copper leads and an outer foil which you can determine with an oscilloscope. Or Illinois Capacitor MPW series, they're also metalized polypropylene with copper leads and are likely the most popular choice for restoring McIntosh (just look at pictures of restored units, it' the yellow caps). Mallory 150M metalized polyester are quite popular too. Fancy caps are Auricap and others, but they don't seem to bee too poular. What's quite popular though are Russian PIO caps, just search a bit here and over at audiokarma in the McIntosh forum.
If you're considering NOS Sprague caps read this: http://www.paintedpostcards.com/misc2/BlackBeauties.pdf.
The most important aspect for the coupling caps seems to be that they're non-inductive.
If you need to exchange resistors you'll want to use wirewound in the PSU (check for voltage strength!), metalfilm for signal (1W types have lower noise, higher voltage strength), maybe non-inductive wirewound for cathode resistors (Mils MRA5) and carbon composite or thick film for grid stoppers (they're noninductive, that's the important aspect here). Watch out for high voltage drops!
Oh, and don't go too big on the capacitances if you exchange the electrolytic caps. A 50% increase is likely ok, anything beyond that may cause irreversible damage to the power transformer wiring because of the larger inrush current. The coupling caps have standard values.
The thermistor will have to be replaced, I think a CL80 type fits.
The thermistor will have to be replaced, I think a CL80 type fits.
Add an inrush protection device or NTC to the amp AC input leads. They have them at Mouser. Quiets down the power on klank and protects devices such as diodes transformers etc for less than $5. And then you can use the bigger caps.
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