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Old 8th August 2011, 05:59 PM   #1
ryuji is offline ryuji  United States
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Default Fixed bias on small signal pentode

Heres what i've put together for a schematic for my new amplifier build.

First ill explain myself a bit, i suppose. Im going to try out zero feedback and then see how it sounds with each type of feedback applied using alligator clips to iterate through things until i find what i like best out of every possibility.

I want to use fixed bias on my small signal pentode so i can have a precise, and steady bias for them regardless of signal conditions. My plans are to use a lm337, 1.5A is way more then five signal pentodes are going to draw from the bias.

ill be operating in pure class A all the way through

My question is, i have not seen anyone put together a fixed bias on a input. im wondering if i laid it out right? cant have DC flowing through the input so the capacitor is needed, cant have signal input to a floating lead, so the 1M resistor was needed. Anyone have actual documentation on what should be used for the resistor coming from the bias supply itself? my 807 amplifier as it stands has listed values

Any other comments/suggestions are welcome

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Old 8th August 2011, 09:14 PM   #2
SY is offline SY  United States
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You probably want to bypass the screen to the cathode.
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Old 8th August 2011, 09:43 PM   #3
ryuji is offline ryuji  United States
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You probably want to bypass the screen to the cathode.
I was thinking about doing just that with a .1~1 uF capacitor to prevent voltage from floating around too much on the screens?

No thoughts on value resistor to use in the bias connection? seems like datasheet only says no more then 100k on the 807 is that something that you simply just experiment with? Still dont have a clue what to do with the preamp, 1 meg would be way too large i think. guess ill have to fool around with it and see what gets my intended behavior?

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Old 9th August 2011, 02:25 AM   #4
ryuji is offline ryuji  United States
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heres my new design. i remember from a different thread people suggesting at least 30k and datasheets say 100k is max for the 807, and i saw someones schematic showing 100k for 6au6 power amplification, so i did same for both until people can possibly give me some insight on how the math/it works
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Old 9th August 2011, 02:40 AM   #5
SY is offline SY  United States
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What does the 1R in the cathode of the 6AU6 do?
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Old 9th August 2011, 02:44 AM   #6
ryuji is offline ryuji  United States
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i configured the preamp like you would the fixed bias in an power tube, so i would need the 1 ohm resistor to measure idle current
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Old 9th August 2011, 02:52 AM   #7
ryuji is offline ryuji  United States
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What does the 1R in the cathode of the 6AU6 do?
i do seem to be having difficulty wrapping my head around how it will work out tho, i think i might have to do cathode bias? willing to give it a shot to see how it will work out tho with a junker 6au6 i got sitting around?

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this has modifications for the cathode bias, which i know would function. I would still like to have a fixed bias however

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Old 9th August 2011, 03:02 AM   #8
SY is offline SY  United States
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You could also run an LED in the cathode, which effectively is fixed bias. Also, if you do keep the cathode resistor, the screen should be bypassed to the cathode, not to ground. If the screen rail to the 807 is regulated, you don't need the bypass there.
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Old 9th August 2011, 03:14 AM   #9
ryuji is offline ryuji  United States
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yea, the 300v supply is regulated, so ill omit that part. im correct tho that due to need for high impedance input fixed bias from the point of view of how you would run a power tube is improbable due to needing a 1 meg on the bias, i think?

updated the images on my server side to reflect the change to screens

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