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Old 1st October 2008, 03:20 AM   #1
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Default Bias supply idea

Is there anything wrong with doing something like this for a grid bias supply? In the real world I would probably include something like a comparator looking at each tubes cathode current to cut out the b+ in case of failure.
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Old 1st October 2008, 08:36 AM   #2
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I don't think so.
I've seen similar arrangements in older amps,they just used a resistor to "lift" the ground,since they didn't have zeners.
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Old 1st October 2008, 09:39 AM   #3
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Bias supplies typically require only a few mA, hence why you see them with halfwave rectification instead of fullwave as shown.

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Old 1st October 2008, 10:24 AM   #4
Colt45 is offline Colt45  Serbia
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looks fairly standard to me
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Old 1st October 2008, 10:36 AM   #5
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I think that method is described in detail here:
http://www.aikenamps.com/BackBiasing.html

Scroll down to the middle of the page to see a similar schematic.
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Old 1st October 2008, 12:06 PM   #6
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What's the problem in case of failure?

also, can this be used for a -15v supply for a CCS?

andy
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Old 1st October 2008, 12:33 PM   #7
sajti is offline sajti  Hungary
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It could be OK. The only problem is, that the regulation of the bias voltage can results overdissipation if the HT of the output tubes is not regulated too.

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Old 1st October 2008, 12:42 PM   #8
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Thanks everyone. Ty_Bower I'll have to give that idea a try. andyjevans: If the bias supply fails the output tubes will conduct fully and glow red. I guess you could use this for a CCS supply if you only need a few milliamps.
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Old 1st October 2008, 04:27 PM   #9
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I had a little read of the very good web page referred to. It seems to me that for a CCS you don't have a problem. And for a start it's on the input tube. But if the HT goes up and down, the CCS could have a -15v voltage regulator on it anyway. So it should be stable. No?
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Old 1st October 2008, 05:14 PM   #10
kevinkr is offline kevinkr  United States
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It will work, and I have done similar things in the past, but I wouldn't do it this way now for the following reasons:
  • Lots of power dissipation in that resistor depending on load current.
  • Zener goes open and you end up with more than -50V across that cap.
  • Not a good idea to regulate grid bias in an amplifier with unregulated plate or screen supplies. To some extent properly designed amplifiers with unregulated plate/screen supplies and unregulated bias supplies will compensate for significant variations in mains supply voltage, regulate one without the other and this is no longer the case.

I'd use a small 12V - 15V transformer with a bridge rectifier and CRC filter and the pots instead - more reliable, less heat and mostly addresses last concern listed above.
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