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Old 10th June 2003, 05:20 AM   #1
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Default DuncanAmps PSU Designer Question

Quick Question:

When using the Duncan PSU Designer to design a power supply for a tube preamp with a valve rectifier and cap input, should the load be set to "Resistive" or "Constant Current"?

Also, if trying to achieve a B+ of 300V, I should have the PSU putting out about 212V RMS, right?

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Old 10th June 2003, 05:34 AM   #2
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I'm still a beginner at this, but anyway... I prefer using current loads set to the tube's idle current draw, because IMO that models a tube more easily. With a resistive load, the current and voltage both change as you change the PS design, so it's harder to see what's going on. I also use stepped current loads to see the effect on the PS when the demand on it changes. Sometimes two PS designs may look similar (or identical) when using fixed values for the current load, but behave differently when the load current changes (for instance, one may oscillate a bit, or something like that).

Also, if trying to achieve a B+ of 300V, I should have the PSU putting out about 212V RMS, right?
B+ is the DC voltage output. It's not AC, so AFAIK you can't multiple by .707 to get RMS. I think you should be trying to get 300V at the output of your PSU, not 212V. The output of your PSU is practically DC (with some AC ripple in it). The .707 would apply if you had a 0-300V AC sinewave output, but that's not what your PSU is generating.

And of course, I could be wrong.
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Old 10th June 2003, 02:36 PM   #3
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If you want 300v B+ with a cap input you gotta have 300/1.4=212 volts AC hitting the rectifier. But, you need more AC out of the power transformer because you will have a lot of ripple. You will lose some voltage in smoothing this ripple and that depends on how you filter (smooth) the ripple.

So, you will probably need significantly more than 212 VAC assuming you want good, clean DC for your amp. Also depends on the type of amp for allowable ripple.

Good luck, ask questions here for there are lots of folks with lots of experience--I am a beginner, too.

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Old 10th June 2003, 02:53 PM   #4
Joel is offline Joel  United States
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Default Re: DuncanAmps PSU Designer Question

Originally posted by trustalbini
...should the load be set to Resistive or Constant Current?
I would just use constant current. The reason is that a preamp is going to be using class A voltage amplifiers that do not vary much in their current draw, and PSUII is just for roughing out the design. (Notice I said "much". They do vary some, but you can't get that granular when designing a PSU)

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Old 11th June 2003, 01:33 AM   #5
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Thanks Joel. I'm really just hoping that, measuring 212V at the constant current load, I'll get the 300V DC I need- not the 400V or so I get when I use a resistive load...
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