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I have dropped 7 volt ac to 6.3 DC, via a bridge rectifer and a resistor for the heaters on my 6SL7's on my amp. I put in a fifty volt 2200 uF cap after the rectifer, one lead to positive off the bridge rectifer and the other to ground (the cap is polar and wired right) but the capacitor has blown once. The second cap started doing the same thing. I wired this per the manufactures recomendation (Cary).
My question is what am I doing wrong and is this smoothing the voltage enough?
Thanks, John
Follow Ups:
it looks like the bridge rectifier was bad. I replaced it and used some more caps and everything is fine. The amp sounds great.
Thanks again for all of the help.
John
OK, assuming you have wired the cap correctly (is there +ve Volts on the +ve terminal and -ve Volts on the -ve terminal?), then the next possibility is the ripple rating of the cap. Look for a really high quality cap, with a high ripple rating.A few more questions:
Is the cap near the bridge rectifier?
Bridges will typically run quite warm, when used in such applications. Heat will accelerate damage to caps, significantly.Is the Voltage ON THE CAP the correct polarity?
What brand of cap are you using?
Is is a good quality component?
Trevor
Trevor:
The cap is hooked up correctly. It is near the bridge but this is a 25 amp 200 volt bridge that shouldn't run too hot. also there is space between the cap and the bridge to allow for ventilation. The manufacture (of the amp) recommended putting the cap across the + and - point on the bridge rectifier. The cap is 35 volt but I do not think it is specifically a high ripple cap. Cary Audio stated that they use many different types of caps in this circuit and with a voltage of 25 volts or more they never have a problem (they do not use high ripple caps here. The cap is a good quality but I don't know the manufacture).
I could place a cap at each of the tubes to get them away from the bridge but anywhere under the amp is warm. There are a couple of other electrolytic caps that function fine and are under a much heavier load.
Thank you so far for you help.
John
I'll just betcha the bridge IS running hot. Nevetheless, if the orientation of the cap is correct and it is not near a hot running object, then the problem will, most certainly, be that of poor ripple current ability. You may need to seek out a cap, with the appropriate ripple current ability.You mention other caps, which are under heavier load. Do you know that, for sure? A filament supply for an amp, or preamp, is quite a tough load, in terms of ripple.
Trevor
I have checked the cap and it is marked with a - on the lead that I have connected to ground. I agree that a 50 volt should not blow on a 6 volt circuit. I checked the voltage before the caps went bad and with a 1.5 ohm resistor on the 7 volt DC, and at the tube measuring the pins 7 and 8, I had a voltage of 6.24 DC. This is the voltage I want. Does any capacitance increase what the DC voltage will be?
I don't understand how capacitance will increase the DC voltage by about 50 percent. Is this a constant or does it depend upon the amount of capacitance?
Also do you know what the best design for this simple circuit would be? All I want is to have DC to the filaments rather than AC.Thank you for your help.
John
The voltage measured on a voltmeter is the average (or rms sometimes) of the pulses from the rectifiers. The cap will charge to the peak of the rectifier output, eg 1.414 of the rms value. The rectifiers prevent this from discharging via the power supply, and a light load does not cause the cap to discharge much. As the cap is recharged each 1/100 sec (1/120 for yanks) by the peak voltage pulse, the resulting current is the RIPPLE CURRENT
One other question. For this type of circuit what is the optimum amount of capacitance and in what form (the best way for the capacitors to be placed)?
Thanks,
John
So do you go by what you measure at the pins of the tube or do you calculate the 1.41 increase over what you are measuring and go by that? If you measure after the capacitors what is the true voltage?
Thanks,
John
John, what you describe is typical of an electrolytic capacitor connected reverse polarity. Are you absolutely certain you have the polarity correct? Some manufacturers mark the negative lead while others mark the positive. A 50V Cap connected across 6.3VDC should not blow. Also if your voltage measures 6.3VDC at the output of the rectifier without a capacitor, the voltage will go up when you connect a capacitor to its output. The exact amount of this increase depends on the load current, and to a lesser extend on the impedance of the voltage source. With no load it would go up by a factor of 1.41. While this is not enough to blow your cap, it is something to consider since it would affect your tube life negatively. You may have to increase your dropping resistor to compensate for this.
Jack
1) The cap is connected 'round the wrong way. Take note of the polarity, when connecting electrolytic caps to DC supplies.
2) The 'Ripple Rating' of the capacitor is too low for the intended operation. High ripple rated caps, tend to cost lots more than low ripple rated ones.
Trevor
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