{"id":8688,"date":"2020-07-25T19:29:51","date_gmt":"2020-07-25T18:29:51","guid":{"rendered":"https:\/\/www.bartola.co.uk\/valves\/?p=8688"},"modified":"2020-07-25T19:29:51","modified_gmt":"2020-07-25T18:29:51","slug":"300b-se-amp-47-driver","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2020\/07\/25\/300b-se-amp-47-driver\/","title":{"rendered":"300B SE Amp: 47 Driver"},"content":{"rendered":"

Going DHT end to end<\/h2>\n

As previously mentioned<\/a>, I played around with the 46 driver. \u00a0I love it sound and is a great valve. However, there were 2 reasons that pushed me to switch to the 47. Firstly, I wanted an extra tad of gain. Secondly, I have a nice stash of RCA 247 globe which measure extremely well. I’ve been reserving it for a 47 PP amp with local feedback (a la Pimm) and hopefully will get to in the future. Anyhow, the 47 in triode mode has a mu of about 8 which in combination with the SUT, gives me good gain to drive my 300B. After tweaking on the bench the stage for optimal swing and distortion performance, I ended up with the following circuit:<\/p>\n

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

Ok, there is a catch. I use a low output impedance DAC which I then control via software the volume. If you want to add a volume potentiometer then you have an issue as will impact the frequency response. If you want to implement this, you will have to use a cathode follower to drive the LL7903, something like this<\/a> will work perfectly. An AVC would be an option. I’m using Slagle’s AVC which is superb.<\/p>\n

Taking the 47 (or 46) around 200V and 30mA gets the best out of it. You also need to watch out for some old valves as they may struggle to provide full swing at low distortion and (more importantly) generate too many unwanted odd harmonics at high voltage swings. You can check this with an FFT view at the mu output of the hybrid mu-follower.<\/p>\n

I will not explain again why I personally use the mu-follower, however there is a great advantage here. You can use avoid a bypass \u00a0capacitor at the cathode resistor which is great for this higher Vg valves. As filament bias becomes unpractical here (I’ve been there done that<\/a>), the hybrid mu-follower wins here, and sounds great!<\/p>\n

There are a few compromise adjustments made here as have maximum 200mA DC capacity at the HT, so dialled the 300B down to safe levels. Also I’d reduced about 1V the filament regulator raw voltage to help with the heatsink dissipation. Other than that, the amp is the same as previously shared<\/a>:<\/p>\n

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

 <\/p>\n

If you look at the driver performance, it’s great to see that it can do about 0.12% for 200Vpp output. Mainly H2 and the relative H2 level is -58.1dB. Not many drivers can do this. Only few DHTs. Best I could get with IHT is about 0.2%. Anyhow this is great performance. Same flat distortion measured up to 30kHz, the driver has enough drive current. At LF (below 100Hz), the distortion of the LL7903 dominates and brings it up to 0.3-0.5% at 20Hz. Still very good:<\/p>\n

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

Ignore LF noise as my bench is noisy and get 50Hz plus byproduct creeping in as induced noise into the system.<\/p>\n

Looking at the frequency response you will see a very nice performance up to 26kHz. I tested with a 220nF\/510K output network for coupling out the mu output:<\/p>\n

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

There is a resonance \u00a0above 100kHz which will be tamed by the output stage, so I won’t bother on it.<\/p>\n

Here is a 247 in action:<\/p>\n

\"\"<\/a>
Testing a 247 valve<\/figcaption><\/figure>\n

Ok. The modular build of the 300B allowed me to swap easily the D3a driver board for the 47 one. Only a few changes needed to adjust the filament raw supplies and I was ready to test:<\/p>\n

\"\"<\/a>
Testing the 47 driver board for first time<\/figcaption><\/figure>\n

Sound impressions<\/h2>\n

Ok, I’m a DHT lover and was right to get back to 100% DHT. The D3a is amazing driver, don’t get me wrong. However, the 46 and 47 brings a level of extra detail I was missing slightly. Also, removing the resistive volume control and getting back to Dave Slagle’s AVC was a perfect match. I think having only 2 valves in the signal path pays off big time. The dynamics are there, strong, fast amplifier. the detail is unique, as you can only get with DHTs. Clearly, I’ve tried many option and keep going back to DHTs. \u00a0Bass is deep and powerful. Man, I love this amp and can get loud if you push it!<\/p>\n

\"\"<\/a>
47 driver in action<\/figcaption><\/figure>\n

Going full DHT has its toll. The power supply complexity can be daunting but worth it. A lot of iron for such a simple signal path. Well, that’s the DHT reality. I highly recommend this setup. If you can’t get the 47, there are plenty other DHTs to consider. I’d go for a VT25\/10Y or 801a. Mainly a lot of the mid-mu DHT can work here. The circuit is very easy to adapt. You only need to change the cathode resistor and dial the right anode voltage on the hybrid mu-follower board. That’s it. Nothing more.<\/p>\n

I have some EML20a valves around which I didn’t like much as preamps. they pick up him easily but they are design for this job. Also the EML30a can be a good option. I would also test my ER801a as am loving to get back so thoriated-tungsten filament sound back into my system.<\/p>\n

Anyhow, I’m enjoying thoroughly the sound of this amp. Will keep this for a long while.<\/p>\n","protected":false},"excerpt":{"rendered":"

Going DHT end to end As previously mentioned, I played around with the 46 driver. \u00a0I love it sound and is a great valve. However, there were 2 reasons that pushed me to switch to the 47. Firstly, I wanted an extra tad of gain. Secondly, I have a nice stash of RCA 247 globe … Continue reading “300B SE Amp: 47 Driver”<\/span><\/a><\/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_feature_clip_id":0,"_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":[783,610,1323,778,92,1348,625,864,903,777,982,1357,287,44,875,78,118],"class_list":["post-8688","post","type-post","status-publish","format-standard","hentry","category-300b-amp","category-valves","tag-10y","tag-300b","tag-300b-se-amplifier","tag-778","tag-92","tag-47-driver","tag-bartola","tag-bartola-gyrator","tag-bartola-gyrator-pcb","tag-bf862","tag-bsh111bk","tag-dht-se-amplifier","tag-filament-bias","tag-gyrator","tag-gyrator-pcb","tag-rod-coleman-regulator","tag-vt-25"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-2g8","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8688","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=8688"}],"version-history":[{"count":3,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8688\/revisions"}],"predecessor-version":[{"id":8700,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8688\/revisions\/8700"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=8688"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=8688"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=8688"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}