by **EWBrown** » Fri Jan 27, 2012 1:30 pm

If you were to replace the C354 with a resistor, use a 50 to 60 ohm, 10 Watts WW resistor. (Or use two 100 or 120 ohm, 5W WW resistors connected in parallel).

The DC resistance of the C354 is nominally 56 ohms, but that can vary +/- 10%. Its (AC) reactance at 120 Hz is about 1130 ohms, which is a lot more effective at reducing the B+ voltage's 120 Hz ripple content.

FX = 2PI X F X L; XL = 2PI X 120 X 1.5; (6.28 X 120 X 1.5) = 1130.

Applying this to the 1H choke, its XL = 754 ohms, and for the 0.5H choke , XL = 377 ohms.

Both of which are a better choice than the more basic resistor CRC filtering

Th resistor will work, but there will be less effective 120 Hz ripple filtering. May not be a real problem, with a PP amp like the ST70 - The ST35 does not use a choke, just a 50 ohm, 5W WW resistor, and that has no ripple issues.

THe ST35 draws about 160 mA total B+ curent, so the voltage drop across 50 ohms is about 8 VDC. The ST-70 is about 220 mA, and across 56 ohms, the voltage drop is about 12.5VDC, hence the larger sized 10 Watt WW resistor should be used in that application

Chokes are a lot more important in single ended amplifiers, as the PP output stages will cancel out any 120 HZ ripple, but there is no such hum / ripple cancellation in an SE amp.

HTH

/ed B in NC

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