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Single Ended Triodes (SETs), the ultimate tube lovers dream.

How to use the 4P1L

The 4P1L is microphonic. This matters in small signal positions, but you can pretty much forget it if you have 2v in IF, and this is a definite if:
- you starve the filament a little. I run parallel filaments at 1.9v or 2v.
- you have a heavy and robust chassis. I'm talking 4mm aluminium top plate.

What you want to avoid is a flimsy steel chassis. I have 2v in from my DAC and I'm good with a 2 stage amp - 4P1L into PSE 4P1L. No suspended sockets or rubber decoupling, but a solid chassis.

I realise Jeff has his own way of doing things. Otherwise I would strongly suggest filament bias. Small cathode resistor like 10-20 ohms. No cathode bypass caps. Rod Coleman filament boards, really good filament supply e.g. choke input. One of these to each tube. A lot of care goes into the filaments - and better to use an outboard power supply. Interstage coupling with a really good interstage. Very simple circuit, but quite a big build with all the filament supplies.

I've never heard of AC filaments for the 4P1L. Even if you want to direct couple, be sensible and get some Rod Coleman boards. We've been working on this for years with the 26 as the initial tube and there are monster threads on DIY Audio on both tubes. The knowledge is well advanced and many are building with them and loving it.


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