31-10-2016, 03:17 PM
(31-10-2016, 02:48 PM)Diabolical Artificer Wrote: I had in mind something that would amplify high line voltage devices like CD/MP4 players so used 1v AC RMS as my input "parameter". I used a 3u cap and a 50k pot on the input which should form a high pass filter, with a 1hz approx roll off, to block DC. However I'm not sure about this as the "sums" are correct if 50k is put in the formula, but the pot doesn't sit at 50k, it varies, so I couldn't find out how to calculate this.
Spot on here...
It would be different if something else followed on from the pot, but as a valve has a very high input impedance, there's no problem with the calculation.
Just for background, let us pretend that the following stage was something else - doesn't matter what - that had an input impedance of 10k. In this case, the input impedance - as seen by the source, and as seen by the input capacitor - would vary from 50k (pot at min) to 50k||10k (pot at max). 50k in parallel with 10k being 8.33k. The capacitor needed to deal with that would be more like 20uF.
I'm (patently!) no expert with valves, so others will hopefully chime in, but my initial thoughts might be aimed at the output transformer. SE amps obviously put a DC current through the primary, which puts flux in the core. As you try to swing either side of that quiescent position, the core will saturate at some point. So if there is distortion on the peaks of a sine wave corresponding to maximum transformer current (lowest voltage at the anode), that might be a clue. Of course, the opposite happens if you don't have enough standing current. But all that applies irrespective of the technology, and is probably blindingly obvious (sorry!)
