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Audio and Video Talk
Valves / Vacuum Tubes
Valve DIY build inspired by Mr Kent Kassler
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<blockquote data-quote="Mars" data-source="post: 225716" data-attributes="member: 817"><p>Hi Ampdog</p><p></p><p>If I may chip in <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile :)" loading="lazy" data-shortname=":)" /> It was my economy that benefitted from this project :whistler: the first tube is a 6080 aka 12AT7. So it is a triode dc coupled to the concertina. This is the circuit:</p><p></p><p><img src="http://diyaudioprojects.com/Schematics/images/Dynaco-ST35-Tube-Amp-Schematic.png" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p>Changes were made (hopefully for the better :thinking<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile :)" loading="lazy" data-shortname=":)" /> since the OPT's are not Ultra Linear; so this is a pentode amplifier. The screen supply goes through a 1000 resistor to a 680F cap and then to both screens. Effectively the amp has dual power supply except for the shared anode supply. The shared 6K8 on the schematic was upped to 15K for the separate supplies to keep the voltage on the 12AT7 the same. Oh, and the output tubes have their own cathode resistor of 220R bypassed with a 1000uF 25V (running about 30mA per EL821). 260V on the anodes; 250 on the screens; 200V on anode load resistors on 12AT7.</p><p></p><p>I did the NFB loop like on the schematic, but the amp has more bass and a better top end without the loop attached. Maybe the 27K resistor should be larger. Anyway, the 150K/0.22uF between the 12AT7 cathodes removes most of the distortion in the pre-amp. The amp sounds pretty clean without the NFB.</p></blockquote><p></p>
[QUOTE="Mars, post: 225716, member: 817"] Hi Ampdog If I may chip in :) It was my economy that benefitted from this project :whistler: the first tube is a 6080 aka 12AT7. So it is a triode dc coupled to the concertina. This is the circuit: [IMG]http://diyaudioprojects.com/Schematics/images/Dynaco-ST35-Tube-Amp-Schematic.png[/IMG] Changes were made (hopefully for the better :thinking:) since the OPT's are not Ultra Linear; so this is a pentode amplifier. The screen supply goes through a 1000 resistor to a 680F cap and then to both screens. Effectively the amp has dual power supply except for the shared anode supply. The shared 6K8 on the schematic was upped to 15K for the separate supplies to keep the voltage on the 12AT7 the same. Oh, and the output tubes have their own cathode resistor of 220R bypassed with a 1000uF 25V (running about 30mA per EL821). 260V on the anodes; 250 on the screens; 200V on anode load resistors on 12AT7. I did the NFB loop like on the schematic, but the amp has more bass and a better top end without the loop attached. Maybe the 27K resistor should be larger. Anyway, the 150K/0.22uF between the 12AT7 cathodes removes most of the distortion in the pre-amp. The amp sounds pretty clean without the NFB. [/QUOTE]
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Audio and Video Talk
Valves / Vacuum Tubes
Valve DIY build inspired by Mr Kent Kassler
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