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Tips for replacing can caps etc.?


 
12/21/2005 10:10 AM
kluson guy Tips for replacing can caps etc.?
Any tips for replacing filter supply caps etc. in old amps? I have several amps that need a cap job and I am fairly new at this. I have plenty of soldering experience and am familiar with how to work safely on tube amps.  
 
The things I am interested in knowing are how to do a good job of replacing can caps with standalone caps. (Should I add a barrier strip to keep things neat?) How to secure the caps? Or should I be looking for a direct replacement can cap?  
Any suggestions appreciated.  
Thanks!
 
12/21/2005 3:17 PM
Greg Simon
Usually it is cheaper to use discrete caps instead of can caps, and you also have a wider selection that way. Good brands to use are Xicon, F & T, Sprague Atom....stay away from Illinois. There are different ways to secure the caps inside the amps...zip ties, silicone, putting in tag boards, etc. If its a vintage piece and worth money, then putting in tag boards and drilling holes will lower the value. You'll find that most amp builders use axial caps, but radials are cheaper and even more readily available.  
 
If you must use a can cap, then there are some standard values available like the usual dual 50uf caps, and you can also get custom cans made from Ted at Webervst, or Zack at Vibroworld. Both are excellant quality, with Ted's being a little cheaper pricewise. The only problem with Ted's is that he only offers the 4 ground lug FP style, whereas Zack offers 3 ground lug FP style in addition to others.  
 
If its a situation like in a Sunn where you need at least a 600v rating on the first couple cap sections, going with discrete caps wired in series is your best choice, because then you can get a 700v rating with using two 350v caps. Make sure if you do this that you also use a 220k 1 w resistor in parallel with each cap section. The can caps usually only have a 450v max rating, and that isn't enough with high voltage amps nowadays.  
 
Greg
 
12/21/2005 3:26 PM
George
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" Make sure if you do this that you also use a 220k 1 w resistor in parallel with each cap section. "  
 
I've seen that in some power supplies but don't understand why. Might you explain the benefit of the power resistors in parallel with the caps? Are they just to bleed the caps to ground when power is off?  
 
Thanks,  
 
George
 
12/21/2005 9:16 PM
Greg Simon
George, in a series connection with caps, if you start with a pair of 220uf 350v caps, what the circuit actually sees when they are in series is one 110uf 700v cap. If you don't put a 220k 1w resistor in parallel with each cap, one cap or the other can try to take ALL of the voltage, so if you have a DC supply of 600v on a 350v cap, you can see that would be a bad idea! The 220k 1w resistor(s) allow each cap to share the voltage between them, and an added benefit is that they will drain the high voltage out of the caps when the amp is switched off within a minute or two.  
 
Greg
 
12/21/2005 10:12 PM
Bruce /Mission Amps
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This is and has been a bit of a misunderstood function for quite a while and I offer an auxillary explaination.  
The two resistors do divide the DC voltage across the caps fairly evenly but not just so they both see the same DC voltage.  
Remember that the idea of using a capactior in the DC rail is to smooth out AC ripple....  
Other then to allow a DC path to ground in order to bleed off the caps when not in use, the other reason the two resistors are used like this is because the two caps can (and probably do) have two different capacitances and thus, different impedances to that very same AC ripple, the very thing the "filters" are trying to smooth out.  
If one filter cap had a rediculous amount of reactance while the other had very little, then the voltage would not be equal across the caps and one would be doing all the work.  
Two caps in series can be a voltage divider at AC.  
The resistance of those two "equal value" resistors swamp out those cap reactances.  
I'm sure there is much more on this topic somewhere on the Net but I'm too lazy to go look for it right now... ha ha... this is a good job for Ray or Steve A. ;)  
 
Bruce
 
12/22/2005 4:24 AM
Greg Simon
Hey Bruce, thanks for posting that! I didn't think about the caps having a different reactance, but now that you mention it, it does make sense.  
Greg
 
12/23/2005 2:21 PM
Rick Erickson
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Bruce,  
 
The reactance of a 50uF cap at 120Hz is around 26 ohms. Let's assume one cap has a reactance of 25 ohms and the other has a reactance of 100 ohms. (pretty messed up) Assuming a pair of 220K resistors, the resistors will be in parallel to these reactances forming a divider of 24.997 ohms and 99.95 ohms. Still not very equal. It doesn't get much better at 100K for the resistor value. As you can see, the resistors will have little or no effect at the ripple frequency, be it 50, 60, 100 or 120 HZ. The resistors are there to keep leakage currents from creating an unequal static DC voltage drop across the caps. They also will discharge the caps when the amp is turned off - and that's a good thing.  
 
RE
 
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