{"id":3090,"date":"2014-02-01T17:03:48","date_gmt":"2014-02-01T17:03:48","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=3090"},"modified":"2014-02-01T17:03:48","modified_gmt":"2014-02-01T17:03:48","slug":"4p1l-driver-ll7903ll1671-tests","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2014\/02\/01\/4p1l-driver-ll7903ll1671-tests\/","title":{"rendered":"4P1L driver – LL7903+LL1671 tests"},"content":{"rendered":"

Last week <\/a>I did some preliminar tests with the LL2746<\/a> in 1:2 step-up mode. \u00a0Despite having measured good results with it, it will be a challenge to drive grid current given that the output impedance of the 4P1L will be multiplied by 4 so about 5K\u03a9.<\/p>\n

Before looking at the LL1671\/20mA<\/a> which is suitable for multiple driver valves, let’s see how the LL2746 driver performs with the addition of the input step-up microphone transformer LL7903<\/a>. I’m currently using the LL7903 in my 814 SE A2 amplifier<\/a> and sounds really nice. The LL7903 was wired up in 1:4 setup so gain can get about 63:<\/p>\n

\"4P1L-LL2746<\/a> \"4P1L<\/a>The addition of the LL7903 doesn’t impact negatively the performance of the driver. In fact, at full swing of 200Vpp the THD is consistent at about 0.5%. The secondary of the LL7903 doesn’t include any zobel compensation so we can see the 30kHz peak in the frequency response.<\/p>\n

Now, let’s look at the LL1671. I have one gapped at 20mA, so the driver operating point has to change in order to allow full 200Vpp swing without clipping. I changed the filament resistor to 25\u03a9 and the set the variable power supply to the following operating point:\"4P1L<\/a><\/p>\n

The performance is better than the previous circuit:
\n\"4P1L-LL1671<\/a> \"LL2746<\/a>Compared to the LL2746, the gain now has a reduced gain of about 36. Still the gain is as good as an average triode-strapped pentode\/tetrode. \u00a0The overall THD has been reduced to about 0.3% instead of the 0.5% we had before. The frequency response shows a clear peak at about 50kHz now as neither the LL7903 or the LL1671 has a Zobel compensation. The -3dB corner is roughly the same at about 10Hz. The LL1671 gapped at 20mA has a slightly higher primary inductance (5H difference estimated), however the 10\u03a9 increase in the filament resistor increases the anode dynamic impedance by 100\u03a9 approximately. In summary the net effect of change in L and R makes the pole to stay in the same place \ud83d\ude42<\/p>\n

This final circuit seems like a reasonable compromise to use the 4P1L DHT driver in filament bias to deliver 200Vpp drive at low distortion and low impedance. An LL1692a in (2:3.5) could be investigated, but I don’t have the transformer to do the tests \ud83d\ude41<\/p>\n

What I didn’t do was to compare the LL1671 in Alt T mode (1:2) which should be similar to the LL2746.<\/p>\n

 <\/p>\n

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

Last week I did some preliminar tests with the LL2746 in 1:2 step-up mode. \u00a0Despite having measured good results with it, it will be a challenge to drive grid current given that the output impedance of the 4P1L will be multiplied by 4 so about 5K\u03a9. Before looking at the LL1671\/20mA which is suitable for … Continue reading “4P1L driver – LL7903+LL1671 tests”<\/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":[5],"tags":[782,537,286,540,396,539,534,538,542,541],"class_list":["post-3090","post","type-post","status-publish","format-standard","hentry","category-valves","tag-4p1l","tag-4p1l-dht","tag-4p1l-driver","tag-dht-driver","tag-ll1671","tag-ll1692a","tag-ll2746","tag-ll7903","tag-tube-driver","tag-valve-driver"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-NQ","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/3090","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=3090"}],"version-history":[{"count":2,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/3090\/revisions"}],"predecessor-version":[{"id":3098,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/3090\/revisions\/3098"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=3090"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=3090"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=3090"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}