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Old 25th August 2011, 03:33 PM   #1
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Default reccommend a voltage doubler for bias?

Can anyone reccommend a good circuit for a voltage doubler to rectify a 6.3v filament winding for negative bias for my EL84's. Can you make a voltage doubler for a negative bias supply? Thanks in advance! -Mac
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Old 25th August 2011, 04:06 PM   #2
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A voltage doubler is a voltage doubler. If you connect the positive output to common ground, you have a supply which produces negative voltage.
How much negative bias voltage do you need for your project?
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Old 25th August 2011, 04:27 PM   #3
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Great, thanks! I need around -10volts for this application at practically no current, just shy of what i could get with just a bridge. I happen to have an extra 6.3v @ 600ma. winding which is perfect as i didnt want another trans in the chassis. So the story is, just build a normal doubler and take the neg. voltage from the bottom(usually grounded) and ground the usual pos. output?
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Old 25th August 2011, 04:39 PM   #4
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I am using a doubler like in the old Fisher amps, winding with no c.t., one end of the winding connected in the middle of two stacked electrolytics and the other end with a diode from ground up and a diode foward to the the top of the electrolytics which is B+. This sound right? Thanks!
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Old 25th August 2011, 05:16 PM   #5
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Personally I wouldnt do that....

Mains voltage variations will be exaggerated (Doubled) for the bias and cause bias drift depending on the mains voltage at the time...

Regulating it would have a similar effect, as then the +B is varying and not the bias,....
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Old 25th August 2011, 06:21 PM   #6
DF96 is offline DF96  England
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HT and bias should either both be regulated or both unregulated. Mains voltage variations will not be doubled: 5% is still 5%. The HT will have changed by 5% too and the two effects will partly cancel each other.
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Old 25th August 2011, 08:06 PM   #7
TheGimp is offline TheGimp  United States
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EL84s have a low sensitivity to variation of B+ as do most if not all pentode/triodes. Second most sensitive is the G2 or screen. Finally the G1 grid is the most sensitive.

Regulating the bias supply makes the most sense. A simple zener regulator would suffice as the grid pulls no current to speak of.
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Old 25th August 2011, 08:16 PM   #8
DF96 is offline DF96  England
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The control and screen grids will have approximately the same sensitivity when expressed in percentage terms. The ratio will be the inner mu, often given in pentode data sheets.
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Old 25th August 2011, 08:20 PM   #9
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Regulating the bias supply is not necessary. Any minor voltage change in the mains will result in a fraction of a volt change in the bias.
Certainly nothing to worry about.
Go for the voltage doubler. Use big filter capacitors on the supply.
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Old 25th August 2011, 09:06 PM   #10
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Just wired the doubler up and it works perfectly! Dead quiet too. The amp sounds great, its similar to the schematic by Revintage in the "Pentode driver for RH84" thread, with a CCS loaded 6DJ8 and plate to grid feedback on the EL84. Thanks guys!
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