{"id":2838,"date":"2013-11-23T09:29:22","date_gmt":"2013-11-23T09:29:22","guid":{"rendered":"http:\/\/www.bartola.co.uk\/valves\/?p=2838"},"modified":"2014-01-06T09:44:38","modified_gmt":"2014-01-06T09:44:38","slug":"814-se-a2-amplifier","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2013\/11\/23\/814-se-a2-amplifier\/","title":{"rendered":"814 SE A2 Amplifier"},"content":{"rendered":"

Goodbye 4-65a SE, at least for now<\/h1>\n

\"IMG_1401\"<\/a>After\u00a0enjoying\u00a0the 4-65a SE amplifier<\/a> for many months, I couldn’t resist myself from upgrading the output stage to the 814s. \u00a0I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very encouraging. The 814 seemed to perform much better than the 4-65a in delivering 10W of class A2 sound at half the distortion levels. This to me, was only worth trying.<\/p>\n

HT supply levels are closely similar to my previous 4-65 design so biasing could be easily adjusted given the DC coupling capability of this amplifier. At least I can take advantage of an additional benefit from this monster amplifier!<\/p>\n

Last week<\/a> I prepared the UX5 sockets with the screen and suppressor grid resistor and protection diodes as a single module. It didn’t take me too long to fit them in the amplifier.<\/p>\n

\"IMG_1384\"<\/a>Adjusting the Rod Coleman filament regulators wasn’t a difficult task either as you would expect. 3.5A regulators could easily deliver the 3.25A needed by these filaments. After 30min of burning them in, I did minor adjustments to set the current back to the right levels: 3.25A @ 10V.<\/p>\n

I found my stash of 814\/VT-154 to be extremely consistent. I looked for a useful operating point to compare valves in triode mode. I used the following as a reference using my uTracer:<\/p>\n

    \n
  1. Va=250V<\/li>\n
  2. Vgk=-5V<\/li>\n
  3. Ia=66mA<\/li>\n
  4. Gm=4.5mA\/V<\/li>\n
  5. Ra=1.8K\u03a9<\/li>\n
  6. \u03bc=8<\/li>\n<\/ol>\n

    Not representative of the normal operating point but good enough to me to compare valves. Ra will normally vary from 1.4K\u03a9 to 2K\u03a9 across the loadline but mainly is around 1.6K\u03a9.<\/p>\n

    My 814s run at about 100mA\/540V and needs about 120Vpp to deliver 10W @ THD=2.3%. Clearly they could run at 600V or higher to deliver greater power in A1 but I’d need a different power supply. So that is for a later upgrade.\"814<\/a><\/p>\n

    THD looks very promising when operating in class A2 providing the driver is capable of handling the low impedance of the grid and delivering the grid current required. Whilst running on class A1, this stage will only deliver 3.3W@1.4%. It’s obvious we are not squeezing the most out of this valve in A1 so a higher HT supply would be needed for more power.\u00a0Interesting to see that if we bias instead higher up at around 650V\/92mA, at this bias point the stage will deliver about 6.7W@2.7% in class A1 or 10W@3.3%\/15W@4%. The effect of curve crunching at higher voltage is more evident here.<\/p>\n

    \"IMG_1398\"<\/a><\/p>\n

    The higher mu of the 814 in triode mode is less demanding in terms of drive. To drive the 814 at full tilt I need 120Vpp, about 40Vpp less than what was needed by the 4-65a. This is not a problem for the 46 driver as it was design to deliver 200Vpp at less than 0.2% THD.<\/p>\n

    My fear was clearly whether the 814 triode strapped could run at +540V or not given the screen voltage limitations. Some people have tested other valves in this mode with success inspired by Morgan Jones in his “Crystal Palace” design (Page 461 of “Valve Amplifiers”<\/a> 3rd Edition). However the proof is in the pudding. It was all yet to be seen when the 600V HT was fired up.<\/p>\n

    814 SE Amplifier Circuit<\/h1>\n

    \"814<\/a><\/p>\n

    The amplifier circuit is exactly the same as the one I used for the 4-65a. The driver is an onerous implementation of the 46 (triode-strapped) in filament bias with a FET gyrator load in mu-follower mode to deliver sufficient current and low output impedance to handle A2 needs. The great concept implemented by Michael Koster in this design is that the grid current has an independent loop without disturbing the driver and output stage bias as the grid current only goes through V5, M2 and the output valve. The MOSFET provides a high PSRR so a simple supply can be implemented for V5. The price paid for DC coupling and flexibility is the number of power supplies. On this 100% DHT implementation a total of 7 supplies are needed! A simpler design could instead include a nice pentode driver like 6e5P, D3a, 6J52P, C3g or even 4P1L. I’m a big fan of DHT and wanted to experiment with this amp. The LL7903 is a superb step up transformer which in\u00a0conjunction\u00a0with the 46 driver provides sufficient gain to the driver stage. It has been already pointed out that C3 can be as low as 100nF with R4 being 4.7M. I just haven’t had the time to change it \ud83d\ude42<\/p>\n

    To prevent the 46 to be driven at lower current due to the lower bias requirement of the 814 compared to the 4-65a, I lowered the V4 SSHV2 output voltage to 265V. The 46 is now running at about 205V \/ 36mA.<\/p>\n

    After creating a new Spice model for the 814 to improve A2 grid current simulation, I run in LTSpice a THD analysis versus output power:<\/p>\n

    \"814<\/a><\/p>\n

    We can see in the simulation that the THD at 10W is just about the same to the graphical analysis from the curves. \u00a0However, H2 is lower and H3 is higher. Some even cancellation of harmonics is taking place as expected but also the H3 contribution of the 46 driver and the output stage are clearly there.<\/p>\n

    \"IMG_1394\"<\/a><\/p>\n

    I\u00a0normally listen to this amplifier at about 3-5W. The fingerprint is clearly H2 but would be interesting to listen the H3\u00a0contribution\u00a0in this amplifier.<\/p>\n

    Initial tests<\/h1>\n

    I had a bad experience with this valve in the past. I blew my curve tracer when did first tests 18 months ago. Paul LeClerq encouraged me to try the 814 in an amplifier, so I had to do this! When finished all basic individual tests, I placed a pair of US ARMY Signal Corps VT-154 made by General Electric in pristine NOS condition. After reseting the HT levels as needed, I fired up the 600V supply with my VARIAC. Everything worked well but after some playing time one valve started to drop gradually the anode current. I suspected biasing needed adjustment but quickly realised that the valve wasn’t performing correctly. Before anything unwanted happened, I switch the amp off and replaced the valve to check whether it was the valve or not. It was. I played the amplifier for about 2 hours last night.<\/p>\n

    \"IMG_1399\"<\/a><\/p>\n

    After some initial burn in time, and as Morgan wrote in his book, the fact that I poured myself a glass of 10 year old Aberlour, didn’t influence my perception that the sound gradually improved after the initial hour. The solid bass was still there and greater detail in the sound. A less sweeter sound given lower H2 component was noticeable but without lossing the warmth. I just like this valve, but need further listening to seriously review this modification.<\/p>\n

    Glad to see that the 814 performed sturdily at +540V in triode. The case has been proved that yet another transmitting tetrode can deliver outstanding triode performance and sound in an amplifier.<\/p>\n

    Stay tuned…<\/p>\n

    Ale<\/p>\n

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

    Goodbye 4-65a SE, at least for now After\u00a0enjoying\u00a0the 4-65a SE amplifier for many months, I couldn’t resist myself from upgrading the output stage to the 814s. \u00a0I just needed changing sockets and filament raw supply transformers to fit the requirements of this lovely transmitting valve. Needless to say, my recent tests on 814s were very … Continue reading “814 SE A2 Amplifier”<\/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_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":[150,476,12,153,154,158,67,5],"tags":[780,137,182,470,183,178,180,49,414,478,477,78,479,428],"class_list":["post-2838","post","type-post","status-publish","format-standard","hentry","category-4-65a-se-amplifier-2","category-814-se-amplfier","category-dht-valves","category-driver","category-output-stage","category-se-designs","category-thd","category-valves","tag-4-65a","tag-4-65a-se-amplifier","tag-182","tag-814-se-amplifier","tag-814-thd","tag-814-triode","tag-814-triode-curves","tag-dht-amplifier","tag-dht-class-a2","tag-dht-dc-coupled","tag-dht-se-amplfier","tag-rod-coleman-regulator","tag-transmitting-tube-amplfier","tag-utracer"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-JM","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/2838","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=2838"}],"version-history":[{"count":9,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/2838\/revisions"}],"predecessor-version":[{"id":3002,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/2838\/revisions\/3002"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=2838"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=2838"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=2838"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}