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Old 28th January 2009, 03:31 PM   #1
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Default Power Supply Question

My question is regarding the power supply xfrmr secondary side.

Click on the image to enlarge it.

http://img149.imageshack.us/my.php?i...or169aazg7.png

The top most portion of the secondary side of the xfrmr is a 5V@2A Tap. The second/middle section is the 612V ct , and the bottom is the 6.3V@1.2V Tap for the heaters.

My question is regarding the connection between the 612V ct section and the 5V section. The 612V section is rectified. THEN it is routed in series back into the AC 5V supply , after which it is fed into the output xfrmr center tap (B+).

What is the purpose of this ? It doesn't make sense to me. Why would you want to mix a 5V AC signal with the DC B+ voltage ?

Are they using the DC voltage to bias the PS xfrmr ? Am I missing something here ?

I am considering rewiring the PS xfrmr so the 5V@2A section is
seperated from the 612V section , rectified (solid state)which will
bring the voltage up around 6.7V , and use it to provide power for
the heaters, thus reducing hum.

Unless there is some reason the PS secondary is wired how it is , can anyone see anything wrong with this ?

Thanks for any help....................Blake
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Old 28th January 2009, 04:12 PM   #2
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You don't commonly see this since the 5V windings usually aren't center-tapped. But it cancels the 60 Hz ripple that would otherwise occur with a directly heated rectifer (it SHOULDN'T be used with an indirectly heated one like the 5AR4). That 60 Hz ripple may not be significant, especially if there's at least two stages of filtering.
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Old 28th January 2009, 04:13 PM   #3
mjf is offline mjf  Austria
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hello.
the schematic seems to be ok.
classical tube amp power supply.
greetings.............
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File Type: jpg tubesupply2.jpg (21.4 KB, 407 views)
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Old 28th January 2009, 05:35 PM   #4
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Thanks guys !

Tom, the 5v supply isn't the center tapped one. The center tapped one is 6.3v.

I confused myself because I am not running a 5Y3 as a rectifier, but rather a solid state rectifier that has a tube socket base. A "plug-in" solid state rectifier , if you will. I had overlooked the obvious , yet again !

It never ocurred to me that the 5V supply was for the rectifier TUBE !

Since I am not using a rectifier tube, then do you see any issue with me using the 5V supply (with solid state rectification it should be about 6.7V) for a DC heater supply for the rest of the valves ?


Thanks..................Blake
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Old 28th January 2009, 06:16 PM   #5
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I misinterpreted the circuit, since I can't see the picture.

With SS rectifier, the 5V winding can be used elsewhere. Don't expect to get much more than 5VDC, though - even with Schottky diodes.
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Old 28th January 2009, 08:00 PM   #6
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Tom,

Sorry about that. You couldn't see the picture at all, or it was too small and unclear ?

Did you try clicking on the pic, or where the pic was supposed to be ? It will enlarge it to a reasonable size.




I just tested my "plug-in" ss rectifier. There is NO connection between any pins and pin 2 . I tested with my ohm meter and I tried it in both polarities. Pin 2 is nothing.

I've corrected this schematic to show how my PS is really wired.

Same as previous pic , you need to click on the link, then click on the pic to enlarge it.

http://img340.imageshack.us/my.php?i...or169aatk9.png

So my 5V supply IS in series with my B+ , as I incorrectly (but actually correctly ) suspected.

I'm going to assume I should remedy this, and until I purchase the proper diodes to try to use the 5V tap for heaters , I will attempt something like this :

http://img530.imageshack.us/my.php?i...pplyfordr2.png


What say you ?


Thanks again..............................Blake
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Old 29th January 2009, 08:24 AM   #7
mjf is offline mjf  Austria
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hello.
there is an example of a tube power supply: the rectifier tube is substituted by diodes.
be carefully,there is dangerous high voltage!
greetings............
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File Type: jpg tuberect1.jpg (17.9 KB, 285 views)
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Old 29th January 2009, 09:09 AM   #8
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Quote:
My question is regarding the connection between the 612V ct section and the 5V section. The 612V section is rectified. THEN it is routed in series back into the AC 5V supply , after which it is fed into the output xfrmr center tap (B+).

What is the purpose of this ? It doesn't make sense to me. Why would you want to mix a 5V AC signal with the DC B+ voltage ?
The 5v winding is there to heat the tube rectifier, as you seem to have realized. The B+ output from a tube rectifier is taken from its cathode and, in the case of a 5v tube, the heater may also be the cathode (directly heated, e.g. 5U4) or the cathode may be a separate cylinder around the heater but not electrically isolated from it (indirectly heated, e.g. GZ34). Either way, the 5v winding has to be connected directly to B+. This can't be avoided but does not cause hum, because the 5v winding isn't connected to anything else but the rectifier tube's heater.

Incidentally, with 6.3v indirectly heated rectifier tubes (e.g. EZ81, 6D22S), the heater and cathode are electrically isolated, enabling the 6.3v winding to be shared by the rectifier and other tubes in the amp.

Since you don't use a tube rectifier, you don't need to use the 5v winding. It cna be left disconnected or you could use it for some other pupose.
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Old 29th January 2009, 04:21 PM   #9
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I would really suggest using tube rectification. In my experience with twiddling with these things tubes sound better. 5U4's are cheap on Ebay and elswhere and if you want more current (can't see why you would need more than 300mA's) you could parallel two 5U4GB's (assuming you have 6+ Amps 5.0V supply) and get about 300VDC @ 600mA!
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Old 30th January 2009, 03:55 AM   #10
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Thanks for all the info guys.



I've tried a 5Y3 rectifier with stock 80ufd can cap. -VS-. a SS rectifier and 900ufd capacitance.

I can't see (hear ?) any reason why I would want to go back to a tube rectifier.

With a SS rectifier I have a higher B+ , more power supply filtering, and tighter , more solid bass. Why would I use a tube rectifier ? What is the benefit ?


I am by no means an expert. I have been putzing with tubes for about a year and a half now, electronics and speakers for about 14 years. I have very little exposure to tube equipment , although I have heard a decent variety of SS gear.


In my short time of playing with tubes, I have come to love their sound. While tweaking the circuit , changing OPT's , switching from Pentode to Triode , etc. etc. , have all altered how my amp sounds, it still has something in any of these configurations that a SS amp lacks. I can't tell you exactly what it is, but it just sounds more like music.

That being said, perhaps I should try a different tube rectifier than the 5Y3. Perhaps an indirectly heated rectifier , as I just can't get my head around the AC (filament power) and DC (B+ power) being together but supposedly seperate.


I have a hard time believing that the capacitance used in most tube gear is satisfactory. IME , 4700ufd is really about the minimum needed to get rid of AC ripple . I know this flys in the face of all conventional tube designs , but it is my experience . As I've said, my experience is mainly with SS , but I can't imagine tubes are any less susceptible to AC hum.


Sorry about the novel.............................Blake
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