Tweaker's Asylum

Posted by Doc B. on July 13, 1999 at 08:45:00:

In Reply to: Re: Hehehe....... posted by Tom S. on July 13, 1999 at 05:44:09:

I'm kinda stumped here guys. What is this deal with SS rectifier reliability and heat dissipation? With tube amp HV supplies it's pretty negligible. We've had one rectifier that was bad on all the kits we've sold over the years, and it was apparently bad before it ever fired up.

In an SE tube amp a typical SS rectifier on the B+ supply drops maybe .7 to 1.7V. If the amp is drawing say 75 mA, typical of one of my own amps, anyway, the dissipation is about an eighth of a watt.

Power down a running amp with an SS rectifier, let the HV supply bleed down, and unplug the amp from the AC mains. Then touch the rectifier. It won't be very hot. If a diode goes, it is usually taken out by a shorted filter cap that is drawing way more than the rectifier's rated current.

Now that failure mechanism may be more likely to occur if the first cap in the filter string is not rated to handle the peak V that the cap charges to before the tubes warm up and draw current. This is more likely to be an area where a designer may miscalculate the ratings required. In this case a tube rectifier will have an advantage as it limits voltage surge during its slow warm up and usually the peak V the first cap charges to is lower than with a SS rectifier.

But - There is also a limit to how much capacitance that first filter stage can have with a tube rectifier ( usually around 40 uF), so one can screw up a tube rectifier/filter just as easily by putting in say 100uF at the first node. The current surge created by the increased capacitance will greatly increase the wear and tear on a tube rectifier, but a SS rectifier won't care as long as it's current rating is not exceeded.