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previous: Ryan Hi all,I'm looking to use 2... -- 5/28/2000 3:19 PM View Thread

Re: Max Rectifier Capacitance

5/28/2000 5:01 PM
Re: Max Rectifier Capacitance
A bit more information, from the Tube Amp FAQ:  
"Not recommended amp modifications - Adding massive amounts of capacitance to the power supply filters to reduce hum. Probably OK with solid state rectifiers, but in amps with tube rectifiers this can cause current spikes in the rectifiers that exceed the instantaneous current rating of the rectifier and wear it out quickly."
The first filter cap has the tough job of getting spikes of current every AC line half cycle and smoothing them out to DC. Those spikes are maybe 1ms long but contain the full 8-10ms of DC power, so they're 8-10 times as big as the DC that comes out of the filter cap. The bigger the capacitor, the shorter - and higher!- the spikes.  
Tube rectifiers have a hard and fast limit on the peak currents they can do without damage. This is why they rate them for only so much capacitance. It's the capacitor size that runs the spikes up, so tube makers did the useful thing and rated rectifier tubes for a max capacitor.  
Using multiple rectifier tubes might work, but as Bruce notes, it's a waste.  
You don't say whether you really, really need a lot of current for some mega-amplifier or instead you're just after a big hum reduction.  
If it's hum reduction, you can do that better by taking note that only the first filter cap is subject to a limit on size. Use a resistor or choke after a first 40uF capacitor at the rectifier tube and feed that into your 200uF bulk cap. The 40uF cap keeps the current in bounds for the 5U4, and the resistor/choke isolates the second filter cap from the first. The second filter cap knocks down the ripple.  
If you really, really need that much current, you shouldn't be using a tube rectifier at all, as Bruce notes.

Peter S I agree with what RG and Bruce said... -- 5/28/2000 5:34 PM