03-25-2015, 05:34 PM
The first problem is that your B+ voltage is way too high. If you used a 10 uF as the input filter cap, this would cause high B+ as the original filters were only 1 uF. The 71A's are only rated at 180 V max on the plate, so you are really pushing them way past their ratings.
This high B+ may not be causing your hum, but it should be addressed before you damage the tubes and possibly other parts.
The best cap to use for the input filter is a 1 uF film cap, which can take the high ripple current involved. An electrolytic will not last in this application. The power supply was designed for 1 uF and if you increase the capacitance, the DC output voltage will rise to excessive levels.
You can use a larger cap after the choke, as it will not affect the output so the 10 uf is ok here. The input cap is the one that is critical. Also the .13 uf cap should be as close as possible to the value, as it forms a tuned circuit with the choke to notch out the 120 Hz ripple. You can add a .03 uf across your existing .1 uf to get to .13 uF.
This high B+ may not be causing your hum, but it should be addressed before you damage the tubes and possibly other parts.
The best cap to use for the input filter is a 1 uF film cap, which can take the high ripple current involved. An electrolytic will not last in this application. The power supply was designed for 1 uF and if you increase the capacitance, the DC output voltage will rise to excessive levels.
You can use a larger cap after the choke, as it will not affect the output so the 10 uf is ok here. The input cap is the one that is critical. Also the .13 uf cap should be as close as possible to the value, as it forms a tuned circuit with the choke to notch out the 120 Hz ripple. You can add a .03 uf across your existing .1 uf to get to .13 uF.
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