{"id":2921,"date":"2013-12-22T10:20:48","date_gmt":"2013-12-22T10:20:48","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=2921"},"modified":"2013-12-22T10:58:53","modified_gmt":"2013-12-22T10:58:53","slug":"input-capacitance","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2013\/12\/22\/input-capacitance\/","title":{"rendered":"Input capacitance"},"content":{"rendered":"

Drive Mr. Miller, drive…<\/h1>\n
\"20131222-102022.jpg\"<\/a>
Input capacitance test rig<\/figcaption><\/figure>\n

Whilst designing triode driver stages for a 300B SE amplifier, I looked into the input capacitance of high-mu triode (or triode-strapped pentodes) that are suitable candidates here. We know Miller capacitance is the big drawback of the triode driver stage, so I kind of liked the idea of testing how much this capacitance could be for the lovely Russian triode-strapped pentodes like 6e5p, 6e6p, 6j11p, 6j5p, 6j52p, etc.<\/p>\n

The test rig<\/h1>\n

I used my valve curve tracer to provide fixed bias to the DUT. The anode was loaded with a classic CCS to maximise gain close to\u00a0\u03bc. I used my waveform generator to drive the grid with a square wave signal from the low \u00a0impedance (Zo=50\u03a9) output. I used a 1k\u03a9 1% series resistor to form the RC circuit with the valve input capacitance. We know that the easier way to measure the\u00a0\u03c4=RC is by looking at the transient from 0% to 100% which is 5\u03c4. Therefore if we measure\u00a05\u03c4 with the oscilloscope we can then derive capacitance value as C=\u03c4 \/ (5R).<\/p>\n

What I firstly measured was the parasitic capacitance of my valve tester which includes internal wiring and valve socket. I found 5\u03c4=1,000\u03bcs. So input capacitance is 200pF. This value is the one I then\u00a0subtracted\u00a0from all my measures when testing triodes in action.<\/p>\n

Testing input capacitance<\/h1>\n

My first victim was the 6C45P<\/a> triode:<\/p>\n

\"6C45P<\/a>
6C45P input capacitance<\/figcaption><\/figure>\n

The yellow trace is the grid voltage and the blue is the anode voltage. The valve was biased at about 150V\/20mA. The gain is approximately\u00a0\u03bc=Va\/Vg=45.2. The\u00a05\u03c4=3,080\u03bcs so input capacitance is 416pF (excluding socket and parasitic capacitance totalling 200pF). This is clearly a hefty load that our friend Mr. Miller is asking us to bear! If we consider that Ci=Cm\/(\u03bc+1) then the input capacitance of this triode is actually Ci=416pF\/(46.2)=9pF. This is right on the minimum value of the datasheet.<\/p>\n

Looking now on a strapped triode like the 6e6p-e<\/a>, the total input capacitance @ Ia=20mA\u00a0\u03bc=33.9 is Cm=236pF. The input capacitance is therefore Ci=6.76pF. This is lower from the Ci value published on the datasheet which is only reflecting tetrode operation, not triode.<\/p>\n

I found this test very insightful and keen to test other triode-strapped pentode valves to see how they perform under Miller’s kingdom. Naturally if we could remove Miller from his kingdom by fixing the anode voltage like in a folded cascode<\/a> or with a gyrator load, then the preamp will have a much easier life. This could determine whether your amp could lose a lot of treble due to the lack of driving capability of your source (or preamp). Worth considering when designing or setting up your system.<\/p>\n","protected":false},"excerpt":{"rendered":"

Drive Mr. Miller, drive… Whilst designing triode driver stages for a 300B SE amplifier, I looked into the input capacitance of high-mu triode (or triode-strapped pentodes) that are suitable candidates here. We know Miller capacitance is the big drawback of the triode driver stage, so I kind of liked the idea of testing how much … Continue reading “Input capacitance”<\/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":[502,44,17,500,501,345,503,499],"class_list":["post-2921","post","type-post","status-publish","format-standard","hentry","category-valves","tag-folded-cascode","tag-gyrator","tag-input-capacitance","tag-measure-tube-capacitance","tag-miller-capacitance","tag-shunt-cascode-driver","tag-triode-driver","tag-valve-input-capacitance"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-L7","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/2921","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=2921"}],"version-history":[{"count":4,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/2921\/revisions"}],"predecessor-version":[{"id":2926,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/2921\/revisions\/2926"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=2921"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=2921"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=2921"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}