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Max Rectifier Capacitance

5/28/2000 3:19 PM
Max Rectifier Capacitance
Hi all,  
I'm looking to use 2x110uf caps (in parallel) on my B+ line right after the rectifier tube. Now, the 5U4 is rated for a maximum capacitance of 40uf, a 5AR4 (GZ34) is rated for around 60uf I believe. So, if I were to run 2x or 3x 5AR4's in parallel would that put less stress on the Rectifier tubes than using just 1 tube??? I'm really confused here, so ANY help would be really great.  
5/28/2000 3:41 PM

I don't understand why you are going through all the trouble of using multiple 5AR4s/5U4s just to use large value filter caps?  
That's a huge amount of wasted power through an extra 5v filament tranny....  
Just use solid state diodes and the two 110uF filter caps in parallel and be done with it!!!  
However, if your design needs that much filtering and you need the voltage drop of the rectifier tubes, then use a different PT or use the solid state rectifier put some high wattage zener diodes in the center tap of the PT to ground to drop a little voltage off the B+ rail.
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5/28/2000 5:01 PM

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.
5/28/2000 5:34 PM
Peter S

I agree with what RG and Bruce said, but I'm still very curious as to why you want to use a tube rectifier. If you want the sag of a tube rectifier, using 2 of them in parallel will defeat that because you will get half of the voltage drop accross them, and coupled with all the extra capacitance of the larger cap, you wont get much sag at all. The solid state rectifier would be cheaper and more reliable. I use tube rectos in all my amps, but I want that sag. Truthfully,my experience has been that on a typical 50 watt amp, I have found that using a 40 or 50uf cap for the first 2 filter caps with a tube rectifier is, as far as the ears can hear just as quiet as using 100uf caps with a solid state rectifier.
5/28/2000 7:17 PM

Well, my the reasoning behind using a tube rectifier and soo much capacitance is...  
I'm building an SLO-ish clone; high gain, a good amount of filtering is required. Soldano uses 2x110uf caps right after the rectifier (solid state). I've built an SLO clone before and the noise is far less than my Mesa Rectifier which only uses 1x110uf cap after the rectifier (ss or tube) so I decided that I wanted to keep the 2x110uf caps for that issue. Next, I really, really like the softer / saggy sound of a tube rectifier (I use my Mesa with the tube rectifier all the time); I play with moderate high gain and there IS a very audible difference between the ss and tube. So, I decided that I'd use a tube rectifier in this next project.  
Well, that's how I arrived at my decision of tube recto and 2x110uf caps. But so what you're saying is if I do use multiple rectifier tubes and the large input capacitance I'll loose the sound I'm after?  
5/28/2000 8:04 PM
Peter S

You won't lose the sag entriely, but keep in mind that if you parallel 2 recto tubes the current draw through the tubes will be split between the 2 tubes and therefore your sag will be roughly cut in half.....add some extra capacitance to that and you will have even less sag. All of my amps use tube rectos, or ss I can stick in an ss recto if I want. Using a GZ34 rectifier tube and a 50uf input cap I get a small amount of sag because the GZ34 has a low internal resistance, if I change it to a ss rectifier, there is not very much difference in sound because most of the power supply sag from either the ss recto or the GZ34 is actually coming from the voltage drop accross the power transformer. If I use 2 GZ34's in parallel, then I get a voltage sag that is nearly exactly the same as with a ss rectifier. If I use a 5u4.....then I get noticably more sag.
5/29/2000 7:08 PM

....and if you want the extra filtering that's fine, but if you want the filtering and the sag, then you have to change the power supply design a little and try something like a high wattage low value resitor (135 to 240 ohms) in the B+ leg to the center tap of the OT.  
More current flow through that resistance would cause a power tube related voltage drop in the B+ rail to the power tubes, simulating the rectifier tube sag.  
You could rebuild the supply with a FW solid state rectifier and 220uF to 440uF caps and the big power resistor to the OT and have high voltage DC and extra DC filtering with some sag at high volumes.  
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