Bieworm wrote: ↑
Mon 06/21/21 1:35 am
I'll post a schematic with changes this far.
You're right about the tail thing. Just been reading what merlin wrote about cathodyne PI
Different authors often describe the various topologies with focus on characteristics most important to their own applications. This is understandable, but it can produce omissions that are critical in understanding and applying the circuit under varying conditions. I have long been a fan of the cathodyne for the reasons pointed out on Merlin's page. Lately, however, I have gained a strong affinity for the long tailed pair. Ironically, it has nothing to do with the 36W I just built, which would have indeed employed a cathodyne if not for the fact I wanted to clone the original Marshall sound as much as possible.
I won't go into the reasons why I believe the LTP is superior, because the advantages it brings to the table are mostly (but not entirely) relevant to achieving low distortion hi-fi reproduction. Nevertheless, I believe it is so much better, I will probably not build another amp with anything other than a long tailed pair for the PI. Low power amplifiers (such as your 1W) are probably the only exception.
I'm only mentioning all this because I think it's important to study a design from different perspectives before reaching hard and fast conclusions. Many articles have been written about the cathodyne going back decades, and many of these are available online. Each one you read will open up your understanding of the circuit and how it operates (and just as important, now it integrates into the surrounding amplifiers).
As a side note, the arc protection Merlin recommends is a technique that's needed in other circuits as well, when solid state rectification is used. Cathode followers with close grid-cathode spacing usually need it (I think he mentions this on a different page), and so does the long tailed pair if its input is DC coupled. I'm generally allergic to the inclusion of solid state devices anywhere in the signal path of vacuum tube audio gear, I use a NE2 bulb, rather than the diode. I seem to remember that Merlin also shows an example of that technique somewhere.