{"id":1629,"date":"2013-06-02T21:33:11","date_gmt":"2013-06-02T20:33:11","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=1629"},"modified":"2013-06-02T21:33:11","modified_gmt":"2013-06-02T20:33:11","slug":"307a-with-schade-feedback-part-ii","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2013\/06\/02\/307a-with-schade-feedback-part-ii\/","title":{"rendered":"307a with schade feedback (Part II)"},"content":{"rendered":"

Some time ago I traced the 307a that kindly Vegard Winge sent me from Norway<\/a>. As suggested by Vegard, I re-traced them with lower screen voltage and increased feedback to improve the linearity of the 307a in this mode.<\/p>\n

\"307a<\/a>The modification to the curve tracer is simple. A simple resistor divider (R1, R2) provides the feedback to the grid. The grid driver provides a constant impedance so works fine. With a potentiometer you can adjust the feedback ratio and look at the impact on the curves. Very handy!<\/p>\n

With 15.5% feedback and 125V screen voltage I got the best curves as shown below:
\n\"307a<\/a><\/p>\n

Nice to see the anode resistance coming down to 1k\u03a9.I tried matching a triode model to these curves with a good result<\/p>\n

<\/p>\n

\"307a<\/a><\/p>\n

Still a 5K-7K will do best, but let’s have a look at the performance with a 5K load first.<\/p>\n

A) 5K Load and OT gapped at 50mA<\/span><\/p>\n

\"307a<\/a><\/p>\n

With 125V screen, an 5K\/50mA OT and biased at 308V you can get about 2W @1.12% with 86Vpp.<\/p>\n

B) 5K Load and OT gapped at 65\/70mA<\/span><\/p>\n

\"307a<\/a><\/p>\n

With 125V screen, an 5K\/65mA OT and biased at 307V you can get about 3.6W @1% with 113 Vpp. About 1.6W more of output power by increasing bias current by 10mA and\u00a0increasing\u00a0the grid drive from 86 to 113 Vpp.<\/p>\n

C) 6.5K Load and OT gapped at 50mA<\/span> \"307a<\/a><\/p>\n

If you consider an 6.5K OT instead, then an LL9202\/50mA (or even a LL1623\/50mA but impedance is 6K instead) would perform nicely to achieve 2.5W@0.85% but with a drive of \u00a0 \u00a0102Vpp. This would be enough for a DHT headphone amp, as some were considering in DYIAudio forum not long ago.<\/p>\n

 <\/p>\n","protected":false},"excerpt":{"rendered":"

Some time ago I traced the 307a that kindly Vegard Winge sent me from Norway. As suggested by Vegard, I re-traced them with lower screen voltage and increased feedback to improve the linearity of the 307a in this mode. The modification to the curve tracer is simple. A simple resistor divider (R1, R2) provides the … Continue reading “307a with schade feedback (Part II)”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[12,33,154,67,24,5],"tags":[348,349,351,353,354,6,405],"class_list":["post-1629","post","type-post","status-publish","format-standard","hentry","category-dht-valves","category-headphone-amp","category-output-stage","category-thd","category-valve-curve-tracer","category-valves","tag-307a","tag-307a-curves","tag-307a-schade","tag-307a-schade-feedback","tag-307a-triode-mode","tag-dht","tag-dht-headphone-amp"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-qh","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1629","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/comments?post=1629"}],"version-history":[{"count":6,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1629\/revisions"}],"predecessor-version":[{"id":1805,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1629\/revisions\/1805"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=1629"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=1629"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=1629"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}