There is actually a very good reason for the 1k 25W resistor. There is a large thread on it (search for 1k 25W). The Express operates at a 6k6 primary impedance and at a large swing, that will cause the screens to draw some serious current. So the 1k resistor is there (plus the two 1ks mounted on the tube sockets) as a current limiter which is very important with that load line. Most importantly is the clean-to-mean mechanism and power supply sag which is also attributed to a significant voltage drop across that 1k which adds to the feel and response of the amp. Finally, that thing gets hot! A chassis mounted resistor such as those offered by Ohmite and Arcol will sink that heat to the chassis, but the old cement block types Ken Fischer used were airborne so overrating it was a good move. I am simplifying the response here for brevity, but dig into the archives and you can find much discussion it. Also look for the big thread by Katopan.solderhead wrote: ↑Wed Mar 05, 2025 9:53 pm I really appreciate the craftsmanship that goes into some of the boards/mounts. The attention to detail on many of these builds is very impressive.
as an engineer I'm going to ask a silly question:
Why does everyone use the ridiculously oversized cement resistor, and wirewound vitreous enamel resistors? Can anyone explain the reason for these design choices, or is that just part of the mystical secret sauce that gets copied over from the original amp builds just because that's the way that KF built a specific amp? I'm not saying that doing any of that is wrong and I'm not trying to belittle the decisions -- I'm just trying to understand why some of the non-conventional choices were made.
It's interesting that the amp was supposedly designed with the specific intent to avoid the inductance provided by a choke, but then inductive resistors get added into the mix -- big ones -- with ridiculous power ratings.
I'd like to be clear that I'm not criticizing -- I'm just trying to understand some of the design elements and choices that aren't blatantly obvious. If anyone can explain some of these odd choices, I'd really appreciate it. If anyone's got math to support these choices, that would be even better.
Hope that helps.
