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PostPosted: 16 Dec 2017, 10:14 
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On rectifier diode in series will provide about 0.7V drop; that's a common trick.
Be sure it is rated for at least 5A or so.

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PostPosted: 18 Dec 2017, 18:11 
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Be sure it can dissipate heat. At 2 amps it will dissipate about 1.5 watts. A long time ago (funny story) in one of my earliest projects I used a number of 10 amp rectifiers to drop the heater voltage in a power amp. It worked great....for about 2 hours. Then quit cold. When I picked it up there was a rattle inside. One of the rectifiers got hot enough to un-solder itself and fell to the bottom of the case. Big power resistors can do the same BTW.

Good listening
Bruce

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PostPosted: 19 Dec 2017, 02:35 
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gofar99 wrote:
One of the rectifiers got hot enough to un-solder itself and fell to the bottom of the case. Big power resistors can do the same BTW.


I actually see that a lot in commercial design HiFi amps with the power resistors and driver transistors :(

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PostPosted: 19 Dec 2017, 17:09 
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I placed a 5amp rectifier and got the voltage down to 13.1 volts. Is this Ok or do I need to add another? I think the target voltage was 12.6 VDC. I also see regulators used in some of the projects on heater supply, I have a couple different types from prior builds and could use if advised.


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PostPosted: 19 Dec 2017, 19:15 
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I guess it's acceptable, but you can still place another diode in series if you wish.


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PostPosted: 20 Dec 2017, 03:20 
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Is the 13.1 volts measured after the filter capacitor, and is the measurement taken with the tube heaters connected so there is load? 13.6 volts sounds kind of low for full wave rectification of 12.6 VAC


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PostPosted: 20 Dec 2017, 03:24 
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coaltrain wrote:
I also see regulators used in some of the projects on heater supply, I have a couple different types from prior builds and could use if advised.
In this case I don't recommend this way. Yes, it is possible, but you should consider several things before use. Let me begin with your measurements: the 13.6V you measured is an RMS value and you did not say anything about the mains voltage at this point.
The mains voltage may vary considerable (depends on your country, usually +/-5%) which changes the output voltage you measured in the same margin (for 5%, if you measured with nominal mains voltage, it would be 12.9-14.3V).
Any regulator has an input-output minimal voltage to work. Even the best LDO regulators should have 0.5-1V for that. Taking the value of 12.9V from the above sentence we'll have only 0.3V across input-output of the regulator. But it is not the only problem. The measured value is RMS. It means that there is some AC voltage (ripple) in the input of the regulator and counting the minimal input-output minimal voltage for a regulator you should consider the negative ripple amplitude too. The ripple can be mitigated by increasing the smoothing (filter) capacitor before the regulator, but this also will increase the current through the transformer secondary.


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PostPosted: 20 Dec 2017, 23:33 
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Questions for Stereo Oddwatt builds with a single power transformer.

1. For the power supply, I read a post with the recommendation to "split" the B+ power supply after the rectifier, basically two strings of the Oddwatt mono block PSU circuit. Is this needed, or is there a simpler approach that can be taken with similar results?

2. Assuming a XPWR117 is used, I would use 3 - LR8 voltage regulators, 1 for each channel, and the heater bias. Would I run 2 LR8s on one power supply string and 1 on the other string? Any issues doing this, or is it such a small current it doesn't matter?


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PostPosted: 21 Dec 2017, 06:47 
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I didn't bother with that. I just filter each channel's B+ separately with a 10uf cap.
AC heaters, non-regulated.. I'm happy with the performance.
Here's my power supply schematic:

http://ohm.bu.edu/~hazen/TubeAmp/Octal_ps.pdf

More details and some photos on my page:

http://ohm.bu.edu/~hazen/TubeAmp/

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PostPosted: 21 Dec 2017, 17:52 
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Hi, In regard to the split and regulated B+ in a stereo build....it is all a matter of how good you want the end product to be. I do this in the Poddwatt Dual Mono Block version (and in the commercial version) as well in any other stereo builds I do. Can you hear the difference...hard to tell, but I can measure an increase in channel to channel separation. Still I am in some ways a perfectionist and do things that are (by some folks) considered over and above necessary. I figure this is a diy project and other than the essential parts of the design it is up to the builder to figure out how various things like the power supply are to be done. IMO a rather good, but not superior amp can be built with AC heaters and only modestly filtered B+ (no regulators, single filter string). Just not my preference.

Good listening
Bruce

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