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In Reply to: RE: Standby switch...pros and cons? posted by DAK on August 18, 2016 at 13:52:01
Just put a CL-90 or CL-130 inrush current limiting thermistor in the line between the rectifying SS diodes and the PSU filter.
It pays to remember that directly heated vacuum rectifiers, like the 5U4, 5R4, and 5Y3, turn on almost as fast as SS does. The thermistor will slightly slow the B+ rail "rise", when SS rectification is employed. IMO, that's good enough, especially in amp with "fixed" bias.
Eli D.
Follow Ups:
Eli, if the cl-90, cl-120 has yet to have a current draw through it how can it drop any volts? An extra ~200 ohms before the first capacitor will not keep the b+ from ramping up long enough to allow filaments to heat will it? JH
The NTC inrush limiting thermistor delays B+ rise for a few seconds. Those few seconds are enough, when an "instant on", SS rectified, bias supply is employed.
The 1st cap. can't charge up very quickly, working through the thermistor's cold resistance. Ohm's Law tells us that the charging current is limited.
Eli D.
I used 2) cl-120 in series after my shottky bridge and I observed b+ bang right on with a dmm. YMMV I suppose. JH
I would have chosen a device with a higher cold resistance and a longer time constant. A CL-140 (50 Ohms cold vs. 10 Ohms for a CL-120) would have been my choice. Also, I've never had any luck using them in series in the location where you have them. IMHO a single CL-140 would do much better for you.
is that it will also be in place with the tube rectifiers. I would end up losing some B+ when i use the tube rectifiers. That is why i was considering using a standby switch.
for controlled warmup, the CL is the right choice.
For standby, If you can, switch the primary side of the HV transformer rather than the B+ Center tap, but both do work ok. Most of the time unless you are running merc. vapor rectifiers, you don't really need a standby switch, at least not for any significant amount of time.
The CL-90 once warmed up has very little resistance. So not too much to worry about when it comes to voltage drop.
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