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Audio and Video Talk
Valves / Vacuum Tubes
Valve driving screens in Leak TL/25A amp
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<blockquote data-quote="handsome" data-source="post: 941286" data-attributes="member: 772"><p>The cathode in the follower 'sees' the screen current. For maximum efficiency - in this case power - a pentode's screen potential should be constant over the signal cycle. This implies a voltage source which is of course a low impedance hence the cathode follower. For a while it seems Leak favoured using a voltage divider to set screen potential and since screen current in a power stage can be substantial you would have to buffer the screen voltage to avoid interaction between divider output resistance and said voltage. The standard (cheaper? simpler?) way of setting screen potential is of course a resistor from HT to screen to set the potential and a capacitor from earth to screen to remove any AC. A KT61 plus the extra heater current seems a tad extravagant compared. </p><p></p><p>As to the heater connection accepted wisdom is to never let heater supplies float (from personal experience NEVER let the HT supply float :dance<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile :)" loading="lazy" data-shortname=":)" />. Cathode followers invariably need heater elevation and with a DC cathode follower using the cathode as a reference is rather expedient. Some valves were designed to be used as DC cathode followers (in power supplies) and their heater-cathode voltage appropriately high: 6AS7 were rated 300V heater to cathode but only 250V anode to cathode.</p></blockquote><p></p>
[QUOTE="handsome, post: 941286, member: 772"] The cathode in the follower 'sees' the screen current. For maximum efficiency - in this case power - a pentode's screen potential should be constant over the signal cycle. This implies a voltage source which is of course a low impedance hence the cathode follower. For a while it seems Leak favoured using a voltage divider to set screen potential and since screen current in a power stage can be substantial you would have to buffer the screen voltage to avoid interaction between divider output resistance and said voltage. The standard (cheaper? simpler?) way of setting screen potential is of course a resistor from HT to screen to set the potential and a capacitor from earth to screen to remove any AC. A KT61 plus the extra heater current seems a tad extravagant compared. As to the heater connection accepted wisdom is to never let heater supplies float (from personal experience NEVER let the HT supply float :dance:). Cathode followers invariably need heater elevation and with a DC cathode follower using the cathode as a reference is rather expedient. Some valves were designed to be used as DC cathode followers (in power supplies) and their heater-cathode voltage appropriately high: 6AS7 were rated 300V heater to cathode but only 250V anode to cathode. [/QUOTE]
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Audio and Video Talk
Valves / Vacuum Tubes
Valve driving screens in Leak TL/25A amp
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