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diyAudio Member

Join Date: Feb 2004
Location: Silicon Valley
How many rectifiers?

Greetings,

I'm trying to figure out my first tube-rectified PS and can't reconcile between the two pieces of info that I have. These pieces are in the drawing below.

The upper portion depicts the part of the schematic, the lower is from the 6X4 rectifier spec.

The first shows that two tubes are used for the full-wave rectification; the spec has only one tube for the same application. "Audio Classroom" series in Glass Audio says, "When two rectifiers (or two halves of a dual rectifier) are used in full-wave rectification ...] - so I guess both scenarios are possible.

So my questions are:

1. How can one tell whether a tube is a dual rectifier? If it has two plates? Or if the tube spec states "full-wave recitifier" it means it is also a "dual rectifier"?

2. If 6X4 is a dual rectifier why would somebody put two of them in PS instead of one?

3. How many is really needed - one or two?

4. While on the topic of the spec vs schematic: for 6X4 the filter capacitor is indicated max 40uF, but the schematic uses 100uF - what are the real limits? Can, let's say, 470uF be used?

Thanks!
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 13th February 2004, 07:45 AM #2 diyAudio Member     Join Date: Jul 2002 Location: New Zealand The first diagram shows two 6X4 rectifier in parallel, a strange combination, probably done because the current was too high for a single one. Each rectifier is basically a double diode, you can use two silicon diodes to replace it. This topology, with the centre tap of the transformer connected to ground, is called full-wave rectification. The second diagram shows 6X4 in typical operation, with a max ouput current of 70mA. What is the current in your schematic? If it is higher than 70mA you might want to use a single EZ80 or EZ81 rectifier, much the same sort of thing except Ez80 will take 90mA and EZ81 150mA. In answer to your specific questions: 1) Most tube rectifiers contain two diodes joined at the cathode. (two anodes, 1 cathode.) 2) see above. 3) whatever is necessary for your max current. 4) I wouldn't go above 40uF for a 6X4, but for two in parallel, 100uF is probably ok. For EZ80 and EZ81, use 47uF. Keep in mind that in a CLC power supply there is a lot of filtering, and a 40u-10H-40u filter will give very very good filtering in most cases, for currents up to several hundred mA. Adding more capacitance won't do very much and will shorten the tube's life.
diyAudio Member

Join Date: Jan 2003
Location: Sweden
Quote:
 1. How can one tell whether a tube is a dual rectifier? If it has two plates? Or if the tube spec states "full-wave recitifier" it means it is also a "dual rectifier"?
I haven't seen dual rectifier but usually Full wave rectifier, a FW rectifier tube has 2 isolated anode plates

Quote:
 2. If 6X4 is a dual rectifier why would somebody put two of them in PS instead of one?
The only technical reason is to take care of more current then one tube can handle, this has been done before but usually it is recomended to use low value resistors in series with each tube in order to avoid that one tube is overloaded if it would take care of more current.

Quote:
 3. How many is really needed - one or two?
I think it is better to use a larger rectifier tube if one is not enough, if one is enough currentwise it should be OK to use one, I use 6X4 almost on max ratings in my preamp but experience no problems whatsoever

Quote:
 While on the topic of the spec vs schematic: for 6X4 the filter capacitor is indicated max 40uF, but the schematic uses 100uF - what are the real limits? Can, let's say, 470uF be used?
The max value of capacitance is choosen in order to limit peak current, if a series resistor is used then almost any cap value can be used with big enough resistor but it is a very inefficient way to design like that. By using PSUD the peak current can be simulated and appropiate resistor value selected

Regards Hans

 13th February 2004, 02:43 PM #4 diyAudio Member     Join Date: Jan 2003 Location: Milwaukee, WI < What he said. Capacitance factors out so you have to have 40uF MAX no matter how many tubes are in there. Better would be 10uF with 6X4s I think... Yes, series current-limiting resistors can be used to drop voltage and peak current, allowing more C. I did this with my first amp, which used a 35W4 half wave rectifier feeding 100uF. It's not an effcient use of resources. If you need to handle 150mA, use a 5Y3 or 5U4... 2x6X4 could do it in parallel (as shown) if you must. Again, use a CLC or CRC filter rather than using a large first C and limiting resistors. Tim __________________ Seven Transistor Labs, LLC Projects and Resources / Electronic Design and Consultation
 14th February 2004, 08:32 PM #6 diyAudio Member     Join Date: Jul 2002 Location: New Zealand Only problem is, with a 350-0-350 transformer, you will get ~370V out! A bit high for your use. Unless you can find a schematic that uses this kind of voltage (and there are many), this won't be suitable. However, it is a very useful voltage for other tube projects. BTW Tube rectifiers cannot handle large peak currents. If you think of an uncharged capacitor as a dead short, the initial current into it will be huge. The tube WILL arc over internally if you have too large a FIRST capacitor, and will have a short life. However, if you have a C-L-C filter (L is an inductor aka choke) you can use a very large cap after the inductor as it limits any current surge. This gives you very good filtering. If you can still get that 175V transformer that would be perfect for a "hybrid" rectifier bridge. Your voltage will still be a bit high, perhaps ~200V, But you could adjust resistors to suit. There isn't anything wrong with using diodes, there are some very good ones around that are quite quiet. You could try UF4007 or better yet UF5408. Also a 0.1uF 3KV ceramic cap across the power transformer secondary removes a lot of noise. With PSUD, change the resistive load to a constant current load. You can probably just use a ballpark figure.
 15th February 2004, 03:00 AM #7 diyAudio Member   Join Date: Feb 2004 Location: Silicon Valley Thanks a lot for the helpful reply, ShiFtY. I feel it puts me back on the right track.

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