{"id":1331,"date":"2013-03-16T17:24:44","date_gmt":"2013-03-16T17:24:44","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=1331"},"modified":"2013-03-16T17:24:44","modified_gmt":"2013-03-16T17:24:44","slug":"307a-dht-in-triode-and-schade-feedback","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2013\/03\/16\/307a-dht-in-triode-and-schade-feedback\/","title":{"rendered":"307a DHT in triode and Schade feedback"},"content":{"rendered":"

\"IMG_0339\"<\/a>Vegard Winge kindly sent me some great DHTs for tracing including the 307a directly heated pentode<\/a>. The sample traced is not an original Western Electric but a lovely Raytheon RK 75 307a NOS. There is limited information of this valve in triode mode and the folks at DIYaudio are looking at potentially using it for a DHT headphone amp.<\/p>\n

This valve has a filament of 5.5V and 1A and an anode dissipation of 21W in class A (including screen dissipation) when triode-connected.<\/p>\n

Let’s see how this valve performs in triode-mode:<\/p>\n

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

 <\/p>\n

 <\/p>\n

How well can we match a triode model for this valve?<\/p>\n

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

Here is the 307a triode composite<\/a>, would be great for someone to test it as haven’t had time to do so yet:<\/p>\n

**** 307A TRIODE DHT Composite ******************************************\r\n* Created on 03\/16\/2013 10:33 using paint_kit.jar \r\n* www.bartola.co.uk\/valves\r\n* Curves image file: 307a triodeSMALL.jpg\r\n* Data source link: 307a triodeSMALL.jpg\r\n* Created by Ale Moglia valves@bartola.co.uk\r\n*----------------------------------------------------------------------------------\r\n.SUBCKT TRIODE_307A-Composite 1 2 3 4 ; Plate Grid K1 K2\r\n+ PARAMS: CCG=15P CGP=0.5P CCP=12P \r\n+ MU=5.08 KG1=2610 KP=29 \r\n+ KVB=186 VCT=-2.26 EX=1.54 RGI=2000\r\n* Vp_MAX=200 Ip_MAX=0.08 \r\n* Vg_step=4 Vg_start=0 Vg_count=10\r\n* END PARAMS -----------------------------------------------------------------------\r\n* cathode resistor is 5.5 ohm, the pins K1 and K2 are 1.375 ohms from the ends of it\r\nRFIL_LEFT 3 31 1.375\r\nRFIL_RIGHT 4 41 1.375\r\nRFIL_MIDDLE 31 41 2.75\r\nE11 32 0 VALUE={V(1,31)\/KP*LOG(1+EXP(KP*(1\/MU+V(2,31)\/SQRT(KVB+V(1,31)*V(1,31)))))}\r\nE12 42 0 VALUE={V(1,41)\/KP*LOG(1+EXP(KP*(1\/MU+V(2,41)\/SQRT(KVB+V(1,41)*V(1,41)))))}\r\nRE11 32 0 1G\r\nRE12 42 0 1G\r\nG11 1 31 VALUE={(PWR(V(32),EX)+PWRS(V(32),EX))\/(2*KG1)}\r\nG12 1 41 VALUE={(PWR(V(42),EX)+PWRS(V(42),EX))\/(2*KG1)}\r\nRCP1 1 3 1G\r\nRCP2 1 4 1G\r\nC1 2 3 {CCG} ; CATHODE-GRID\r\nC2 2 1 {CGP} ; GRID=PLATE\r\nC3 1 3 {CCP} ; CATHODE-PLATE\r\nD3 5 3 DX ; FOR GRID CURRENT\r\nD4 6 4 DX ; FOR GRID CURRENT\r\nRG1 2 5 {RGI} ; FOR GRID CURRENT\r\nRG2 2 6 {RGI} ; FOR GRID CURRENT\r\n.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)\r\n.ENDS\r\n*$<\/pre>\n
Anode resistance is higher than 300B but better than many other DHP triode-strapped which typically are around 1.2K\u03a9 and 2K\u03a9 Let’s see how this valve is expected to perform with a load of about 5K\u03a9:<\/p>\n
\"307a<\/a><\/p>\n
Biased at about 260V\/80mA can achieve 1W@THD=2.1% (mainly H2 as expected) with a driving signal of about 50Vpp. Linearity of this valve in triode is not that impressive. Sure that overall performance with driver H2 cancellation will do better than this.<\/p>\n
So, what about anode to grid feedback a la Schade? Well, tried it with various screen voltages (150V, 200V and 250V) to see what was the impact of a greater pentode performance. Let’s see the first example at screen biased at 250V. All tests were made with 10% feedback:<\/p>\n
\"307a<\/a><\/p>\n
 <\/p>\n
We can see above that anode resistance doubles, mu increases and transconductance decreases with a more pentode-like curves. Using Dmitry’s tool is difficult to match the curves with the triode model as expected:<\/p>\n
\"307a<\/a><\/p>\n
If we decrease the screen voltage to 200V:<\/p>\n
\"307a<\/a>\u00a0The curves are easier to match now with the triode-model with the exception of the knee at low currents which is not critical as we will not operate the valve in this region:
\n\"307a<\/a><\/p>\n
Finally if we look at the screen at 150V:<\/p>\n
\"307a<\/a><\/p>\n
The anode resistance is lowered whilst mu remains the same and transconductance increases. \u00a0The model matches quite well as we can see below:<\/p>\n
\"307a<\/a><\/p>\n
 <\/p>\n
So with a load now of 6K, biasing the Schade-connected 307a at 300V\/70mA we can get 1W @ THD=0.3% (with similar H3 and H2 levels as expected due to the pentode contribution) driven with a 42Vpp grid signal:\"307a<\/a><\/p>\n
Further tests are yet to be done with FB=13% and other operating points and screen voltages, but initially it looks like a better candidate to me with Schade feedback…<\/p>\n
Hope this helps and looking for your comments…<\/p>\n
Ale<\/p>\n","protected":false},"excerpt":{"rendered":"
Vegard Winge kindly sent me some great DHTs for tracing including the 307a directly heated pentode. The sample traced is not an original Western Electric but a lovely Raytheon RK 75 307a NOS. There is limited information of this valve in triode mode and the folks at DIYaudio are looking at potentially using it for … Continue reading “307a DHT in triode and Schade feedback”<\/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":[12,67,24,5],"tags":[348,349,351,353,350,354,352],"class_list":["post-1331","post","type-post","status-publish","format-standard","hentry","category-dht-valves","category-thd","category-valve-curve-tracer","category-valves","tag-307a","tag-307a-curves","tag-307a-schade","tag-307a-schade-feedback","tag-307a-triode-curves","tag-307a-triode-mode","tag-schade-feedback"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-lt","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1331","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=1331"}],"version-history":[{"count":3,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1331\/revisions"}],"predecessor-version":[{"id":1347,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/1331\/revisions\/1347"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=1331"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=1331"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=1331"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}