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Old 9th October 2007, 10:27 AM   #1
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Question Another Power Supply Question

right so to deisgn a power supply i need to know the voltage desired and the current that will be drawn,

but from the curves for the kt88 (my output tube of choice for my amp) i have noticed that when in triode mode (as i want a switch to allow mode changing) the kt88 idles at 60mA as opsoed to ultralinear mode at 80mA.
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Old 9th October 2007, 10:46 AM   #2
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Anode voltage will be somewhat higher in triode mode, depending on regulation.

If choke input you must size the inductance for triode mode. Triodes like a stiff psu.

Hotswitching modes may be problematic.
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Old 9th October 2007, 10:58 AM   #3
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i did i simulation of an idea in duncun psu designer and when i go from 120ma draw (both tubes in triode mode) to 160ma draw (both tubes in UL) i get a voltage drop of about 15V,,,

is there anything i can do about this?

oh and i wouldn't ever hot switch anyway to many expensive components to potentialy damage..
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Old 9th October 2007, 11:07 AM   #4
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just to let you know the amp is kt88 based single ended design with an HT / B+ voltage of 500v....
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Old 9th October 2007, 11:43 AM   #5
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About the only thing you can do to reduce or eliminate it is build a beefier supply with less losses, or active regulation.

A somewhat higher anode voltage in triode mode isnt a disaster (assuming it doesnt take parts past ratings), check for the current it will draw at this different operating point.

Alternate bias for triode mode is another possibility.
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Old 9th October 2007, 11:57 AM   #6
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i think ill just work out what voltage it will be getting in in triode mode, draw an appropriate load line and have a look. as like you say the 15v increase shouldnt affect its behavior too much.
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Old 9th October 2007, 12:22 PM   #7
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heres the schematic
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File Type: jpg 1.jpg (15.8 KB, 202 views)
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Old 9th October 2007, 12:25 PM   #8
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and heres a plot of the voltages over the 2 current dains (the 400v line is for the preamp) at 5 seconds the main drain switches from 120mA to 160mA.
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Old 9th October 2007, 02:13 PM   #9
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That 31 ohm resistance for the transformer seems a little low. That would be a fairly large transformer. You sure about that?
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Old 9th October 2007, 03:31 PM   #10
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no not sure at all, neither am i sure of the cap's resistance ,,

they are just the figures that were on the parts in the psu designer,,, i should adjust them accordingly but i dont know what to ,,, a bit of reading might be order...
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