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Valves / Vacuum Tubes
6U8 voltage amplifier and phase splitter in one tube
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<blockquote data-quote="Ampdog" data-source="post: 304113" data-attributes="member: 144"><p>Karel,</p><p></p><p>The low distortion of the pentode is no longer dominant at the high peak output signal required for the typical use of the pentode feeding a cathodyne phase inverter for this application. Also, the distortion is predominantly 3rd harmonic as to 2nd harmonic for a triode input. </p><p></p><p>Then something I do not find in data sheets for these valves are anode graphs for the triode part, so that one can determine which has the wider maximum anode swing. (They were intended for r.f. use which did not include such data.) The use as driver for providing up to 160Vpp output makes that important; I am not fond of using that topology for driving output valves directly - still. </p><p></p><p>From the somewhat higher Gm of the 6U8 triode one would guess that it will have the edge over the 6AN8 output swing-wise - but then it might be slightly less linear. One would expect that in power amplifiers the output stage distortion will be dominant, but that might not be so - EL34s in UL configuration can go as low as 2% distortion at about 50W output.</p><p></p><p>Still, as you say, a twin-triode topology for this type of work is just about out; also too low gain. There is also the ECF80 (6BL8) for this application, but it is less obtainable though cheaper when I last looked. So perhaps a quick hook-up to determine the maximum output voltage? (Sounds interesting, which I might examine myself over the weekend.) For now, I would prefer the 6U8, but conditionally so. </p><p></p><p>(This topology is of course ideal for the low grid drive requirement of valves like EL84, and a viable alternative might be using multiple EL84s as in the Manley amplifier, though that does not use a cathodyne phase splitter.)</p></blockquote><p></p>
[QUOTE="Ampdog, post: 304113, member: 144"] Karel, The low distortion of the pentode is no longer dominant at the high peak output signal required for the typical use of the pentode feeding a cathodyne phase inverter for this application. Also, the distortion is predominantly 3rd harmonic as to 2nd harmonic for a triode input. Then something I do not find in data sheets for these valves are anode graphs for the triode part, so that one can determine which has the wider maximum anode swing. (They were intended for r.f. use which did not include such data.) The use as driver for providing up to 160Vpp output makes that important; I am not fond of using that topology for driving output valves directly - still. From the somewhat higher Gm of the 6U8 triode one would guess that it will have the edge over the 6AN8 output swing-wise - but then it might be slightly less linear. One would expect that in power amplifiers the output stage distortion will be dominant, but that might not be so - EL34s in UL configuration can go as low as 2% distortion at about 50W output. Still, as you say, a twin-triode topology for this type of work is just about out; also too low gain. There is also the ECF80 (6BL8) for this application, but it is less obtainable though cheaper when I last looked. So perhaps a quick hook-up to determine the maximum output voltage? (Sounds interesting, which I might examine myself over the weekend.) For now, I would prefer the 6U8, but conditionally so. (This topology is of course ideal for the low grid drive requirement of valves like EL84, and a viable alternative might be using multiple EL84s as in the Manley amplifier, though that does not use a cathodyne phase splitter.) [/QUOTE]
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Audio and Video Talk
Valves / Vacuum Tubes
6U8 voltage amplifier and phase splitter in one tube
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