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Audio and Video Talk
Valves / Vacuum Tubes
Sansui 1000A Transformers on chassis
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<blockquote data-quote="Ampdog" data-source="post: 762984" data-attributes="member: 144"><p><em>HEY??</em></p><p></p><p>What happened to my post previous day regarding fixed bias?? If that went astray to somewhere else kindly return to the fold.</p><p></p><p>Again then: Fixed bias is not mainly to be able to adjust for separate valves; that can also be done with cathode bias (see the Williamson amplifier). Valve data sheets will indicate that for the same conditions two valves will give higher output under fixed bias conditions than with cathode bias. This is because at high output the average plate current increases. With cathode bias this increase raises the voltage over the cathode resistor, biasing the valves back as it were to a condition of lower maximum output. Distortion is generally also somewhat less for fixed bias.</p><p></p><p><span style="font-size: 11px">Note a 'trick' I often mention: A very large cathode bias bypass capacitor (>2000?F) will, under music conditions of low average-to-peak current ratio, tend to keep the cathode voltage constant. Thus an almost fixed bias condition is approached with its sonic advantages. The same goes for large h.t. filter caps. Called "music output", this is then advantageous, rendering some 20% - 30% higher peak output. (Not to confused with the P.M.P.O. nonsense.) </span></p></blockquote><p></p>
[QUOTE="Ampdog, post: 762984, member: 144"] [i]HEY??[/i] What happened to my post previous day regarding fixed bias?? If that went astray to somewhere else kindly return to the fold. Again then: Fixed bias is not mainly to be able to adjust for separate valves; that can also be done with cathode bias (see the Williamson amplifier). Valve data sheets will indicate that for the same conditions two valves will give higher output under fixed bias conditions than with cathode bias. This is because at high output the average plate current increases. With cathode bias this increase raises the voltage over the cathode resistor, biasing the valves back as it were to a condition of lower maximum output. Distortion is generally also somewhat less for fixed bias. [SIZE=11px]Note a 'trick' I often mention: A very large cathode bias bypass capacitor (>2000?F) will, under music conditions of low average-to-peak current ratio, tend to keep the cathode voltage constant. Thus an almost fixed bias condition is approached with its sonic advantages. The same goes for large h.t. filter caps. Called "music output", this is then advantageous, rendering some 20% - 30% higher peak output. (Not to confused with the P.M.P.O. nonsense.) [/SIZE] [/QUOTE]
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Audio and Video Talk
Valves / Vacuum Tubes
Sansui 1000A Transformers on chassis
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