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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: Choose: 300b SET or 2a3 PP? posted by andy evans on May 26, 2012 at 02:31:20:
Probably a misunderstanding.. The grid of the input tube in that amp is at -150 volts bias. I can hang the battery heater in there like that, floating at -150V and it doesnt mind at all. I woundnt battery heat a 26, it draws too much filament current.. and I wouldnt reference a DC mains heater supply to -150V.
On the 01A, intersting things happen to distortion when you lower the voltage across the filament. Mr Bench wrote some useful info on this.
If your input tube grid is at 0V and its plate is at 90-120 volts, the driver cathode need to be at that voltage plus bias voltage, so lets say 130 to 160VDC. With reasonable plate current (18-30mA), and using the best case scenario with those numbers that bias resistor is 4000 ohm, plus and needs a bypass cap. Sure you can increase the plate load to accomodate the increase in plate resistance (say driver tube is Rp 2k, that unbypassed resistor is a major increase in Rp - 3 fold) and if you can get a decent choke (or any CCS) that drive impedance is increased substancially, affecting quite a few things. The Cin of 2A3 or 300B is far from negligable.. and paralleled 2A3 doubles the Cin figure.
If one uses a split rail supply for the input and direct couples to a grounded filament return driver, he avoids not only the bypass cap, but that resistor also and the drive impedance is back where it ought to be, which is important with PSE.
Regards,
Shane
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- RE: Choose: 300b SET or 2a3 PP? - shane. 05/26/1223:18:58 05/26/12 (0)
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