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Usually power tubes working in Pentode Mode, don't require exceptional filtering, because the screen grids are additionally filtered, but when the power tubes are working in Ultra-Linear mode, the only thing that separates the screen grid from the plate voltage is a given percentage of the primary winding. It acts as a choke, but the filtering is not as good as we need it to be, because chokes work best when the core is highly magnetised, but push pull amps tend to cancel any DC magnetization of the core. The solution is to use a better filtering scheme, than just a capacitor. A simple LC filtering is sufficient to attenuate the ripples to a reasonable value (less than 5mVpk-pk in this case). The spike you see at start up, shouldn't occur in reality because EC doesn't accompany for the magnetization current the transformer needs at start-up. The output is usually not abrupt, but smooth. The choke you can salvage from an old transformer. Just cut out the terminals for the secondary winding, and use the primary as the choke. Depending on the transformer it should be in the range 1H-20H. There is one peculiarity that one must consider with these filters. If not properly dampened, the filter will resonate at a given frequency, so the amplifier may cause ringing of the power supply rail, at some frequencies (usually bass). The solution is to critically dampen the filter at a frequency outside the audio range (less than 20Hz, best less than 10Hz). This filter is dampened at around 4Hz. In order to dampen it you need to add a series resistance to the choke, which will drop some voltage, so careful consideration of the input voltage and the power dissipation of the resistor is mandatory. You can figure it out experimentally, because figuring out the voltage drop after rectification is a nonlinear process, and simple math won't do you good. The 60ohm resistance is the internal resistance of the choke, and will sometimes be enough, to critically dampen the filter, but not in this case.
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