{"id":8474,"date":"2020-05-31T09:47:22","date_gmt":"2020-05-31T08:47:22","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=8474"},"modified":"2020-05-31T11:05:26","modified_gmt":"2020-05-31T10:05:26","slug":"300b-se-amplifier-finished","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2020\/05\/31\/300b-se-amplifier-finished\/","title":{"rendered":"300B SE Amplifier Finished!"},"content":{"rendered":"

When everything was going to plan…<\/h2>\n

This build became one of the quickest and eventually the most painful from all, perhaps not really. However, it was very challenging in the end. I will tell you why in more detail. Yet, it has been a fantastic learning experience.<\/p>\n

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The amplifier building experience was going swiftly. For someone who struggles to find some free time to work on the projects, the lockdown gave several nights of work. I rigourosly made progress and everything was working to plan, until the final test was made. I individually tested every stage and part of the amplifier and everything worked just fine.<\/p>\n

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However, when I made my final test and brought up the output HT via a VARIAC, I found that the amplifier driver stage \/ source follower oscillated! Argh, the worst nightmare came true. Very unpredictable behaviour and hard to pin down. Spent few nights without success. After several attempts to find the root cause, I reached out for help to Rod Coleman. Rod has always provided tons of advice and his experience is great. He’s also very helpful and ready to support where he can. That makes him so valuable in these situations.<\/p>\n

The real culprit<\/h2>\n

Although there was a LF oscillation which made the -200V bias raw supply go crazy. It was puzzling to see the circuit to operate well when operated under a bench supply. We suspected on stray inductance and capacitances playing around and turning the SF into an oscillator.<\/p>\n

So ended up adding extra bypass capacitors, changed the Source Follower tail CCS for a resistor load instead. Still after desperate changes and modifications, I was unable to isolate the source of oscillation.<\/p>\n

Looking at the passive -200V supply. I ended up testing leakage currents to earth and found that I had the -200V at +80V from earth! Dissecting the supply, I sadly found the Mains transformer with a leakage short into the copper screen between secondary and primary which is connected to earth. Phew!<\/p>\n

Luckily I had a similar transformer around, which made me compromise a bit the supply and voltage levels, which still deliver what I want, and voila. The amplifier was now working like a charm!<\/p>\n

The amplifier is dead quiet, amazing. Flat response up to 80kHz which is hard to achieve without these OPTs. The hybrid mu-follower plus the source followers have a bandwidth of 3Mhz. At 37dB gain is a fantastic stage, but a devil one if you don’t take care of taming any source of potential wild oscillation due to the high gain and bandwidth.<\/p>\n

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I will post diagrams soon, for now will enjoy the sound of it after so much work.<\/p>\n

I’m delighted with the sound so far. Strong bass, detail and transparency of treble and overall sound image is perfect. Glad to see the 300B back on my system.<\/p>\n

This isn’t a build I’d recommend to get into, unless you’re prepared to live with fixed bias and ton of iron, etc. However, you need to take care of driving a 300B if you want to get the best sound out of it. That doesn’t come cheap and there is no free lunch of course!<\/p>\n","protected":false},"excerpt":{"rendered":"

When everything was going to plan… This build became one of the quickest and eventually the most painful from all, perhaps not really. However, it was very challenging in the end. I will tell you why in more detail. Yet, it has been a fantastic learning experience.<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_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},"jetpack_post_was_ever_published":false},"categories":[673,5],"tags":[610,836,674,1323,1195,294,1338,298,1339,651,741,885,1061],"class_list":["post-8474","post","type-post","status-publish","format-standard","hentry","category-300b-amp","category-valves","tag-300b","tag-300b-amplifier","tag-300b-se","tag-300b-se-amplifier","tag-bartola-hybrid-mu-follower","tag-d3a","tag-d3a-hybrid-mu-follower","tag-d3a-triode","tag-direct-couple-single-ended","tag-mu-follower","tag-sic-bias","tag-single-ended-amplifier","tag-source-follower"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-2cG","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8474","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=8474"}],"version-history":[{"count":3,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8474\/revisions"}],"predecessor-version":[{"id":8482,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8474\/revisions\/8482"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=8474"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=8474"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=8474"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}