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Another solution - filament bias

Posted by andy evans on January 11, 2017 at 03:09:08:
This is more of a theoretical solution, since it requires something of a re-build. But it's the solution I found and have been happy with for a few years now. As said, cathode bypass caps affect the sound negatively. I tried all kinds of types when I used 2a3 outputs and just gave up - could not find anything I liked.
So instead of a 2a3 output, I used two DHT 4P1L trioded in parallel which is pretty much the same thing, other than that you get a welcome increase in gain from around 3 to around 11. Similar plate to cathode voltage of around 250v and OPT requirements.
But this gives you the opportunity to set up the tube properly with a low value cathode resistor of around 15 ohms. You need clean DC, so Rod Coleman regs and preferably a choke input DC supply to those. But it works perfectly - very clean and transparent.
It's a re-build because you need 2 more sockets and a heat sink for the regs, and also the cathode resistor needs to be a large wirewound type. But it's worth mentioning because it is a solution for this problem that works well. Filament bias won't work for a 2a3 or 300b, because the resistor would be too big and hot, but it suits the 4P1L perfectly.