I added the CL140 to the secondary side in the SSE amp board back in 2011. This was a point in time where all three of the major new production tube makers were selling poor quality 5AR4 tubes. The rectifier tube must try to fill the empty filter caps in a tube amp while it is warming up. This requires a uniformly coated cathode concentrically mounted in the plate so that all the start up current does not happen at a single point. A thin spot in the coating will heat faster and start to emit electrons first. A point where the cathode is closer to the plate than normal will flow more current than the rest of the cathode. If the current density is too high in eeither case the tube will arc further damaging the coating. Many SSE's built with modern cost reduced (less wire for less resistance) pushed the 5AR4 near it's spec limit and some brand new tubes sparked out on first power on. The CL90 on the primary slows the inrush caused by the low cold resistance of the tube heaters. The CL140 on the secondary side doesn't begin to act until the rectifier tube starts to conduct, blunting the current surge which saves marginally constructed tubes.
In the case of silicon diodes the CL140 keeps the peak charging current down making life easier on the diodes, transformer and first capacitor. I generally only used the CL140 in SSEs or other applications where a tube rectifier is pushed near the spec.
I have also seen a CL140 burst into flames if the total peak current is high enough. Here I was using a 480 volt industrial control transformer (very low DCR), a bunch of series diodes, and big polypropylene cap. There is a maximum spec for these things, but serious math is required to use it and most use cases for tube amp builders are within the spec.
In the case of silicon diodes the CL140 keeps the peak charging current down making life easier on the diodes, transformer and first capacitor. I generally only used the CL140 in SSEs or other applications where a tube rectifier is pushed near the spec.
I have also seen a CL140 burst into flames if the total peak current is high enough. Here I was using a 480 volt industrial control transformer (very low DCR), a bunch of series diodes, and big polypropylene cap. There is a maximum spec for these things, but serious math is required to use it and most use cases for tube amp builders are within the spec.
My third and final SPP.. I decided to go with an offboard rectifier and I really like the layout of this one. First filter cap is next to the rectifier for a short loop.
These are custom Sowter 8K transformers with 25% Ultra-linear taps, their frequency response is flat from lower bass over 20khz (measured with ARTA).
That said, with the 4 independent secondaries, it was an absolute pain to wire for switching 4/8 ohms output.
These are custom Sowter 8K transformers with 25% Ultra-linear taps, their frequency response is flat from lower bass over 20khz (measured with ARTA).
That said, with the 4 independent secondaries, it was an absolute pain to wire for switching 4/8 ohms output.
