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I was wondering what the theory is about the preamp filter cap location. What is the advantage of putting the cap at the final terminal of that particular B+* string
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From Blencoe: http://www.valvewizard.co.uk/smoothing.html
Most amps supply the output transformer primary directly from the reservoir capacitor. However, the reservoir capacitor alone is not enough to provide the noise-free DC needed by the screen grids and preamp stages, so further smoothing is necessary. This is achieved with a chain of LC or RC (low-pass) filters, variously referred to as smoothing, bypass, or decoupling filters. These alternative names derive from the fact that there are really three interrelated jobs to be performed:
1: Smooth/filter out residual ripple voltage;
2: Bypass/provide a local energy supply for sudden current demands;
3: Decouple/isolate each amplifier stage from the rest...
...By chaining filters together we get progressively better ripple reduction. Less sensitive amplifier stages like the screen grids and phase inverter are fed from the earliest sections of the filter while the more sensitive stages receive the quieter supply, but are also subject to the greatest voltage drop. The input valve is always last in the chain."