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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: Loading the taps... posted by hifipaul on November 23, 2015 at 06:31:25:
" The choke is a Stancor swinging choke 76ohms dcr and rated for 400ma and 4Hys. ... The original operating points for the KT88 were to be 400v at 75ma..."
Sorry, I meant to get back to this sooner. Can I ask why you're using a swinging choke? The amplifier is Class A, so average current draw doesn't change much when the amplifier is driven. That means the choke won't "swing." In any event, it's necessary to know the approximate inductance at the actual operating current (150 mA) in order to analyze this. For the purpose of simply taking a quick look, I'll assume it's 5H (the 4H rating is at 400 mA). If this choke is followed by 80uF, the following curve represents relative levels looking back into the supply:
As you can see, the curve is nice and smooth, with no irregularities that might indicate resonances "visible" to the amplifier. Just for contrast, here is the same supply with a 10uF output cap:
In this case, insufficient capacitance creates a much higher overall output impedance. Attenuation at 1kHz is degraded 15dB relative to the previous example. In addition, resonance between the cap and inductor produces an impedance bump. The supply practically "lets go" at 25Hz, meaning it won't be able to properly "anchor" the amplifier. Amplifier output in this area will almost certainly be distorted and poorly defined.
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