{"id":8841,"date":"2020-09-19T18:22:57","date_gmt":"2020-09-19T17:22:57","guid":{"rendered":"https:\/\/www.bartola.co.uk\/valves\/?p=8841"},"modified":"2020-09-19T18:22:57","modified_gmt":"2020-09-19T17:22:57","slug":"pp-with-local-feedback-bias-pcbs","status":"publish","type":"post","link":"https:\/\/www.bartola.co.uk\/valves\/2020\/09\/19\/pp-with-local-feedback-bias-pcbs\/","title":{"rendered":"PP with local feedback: bias PCBs"},"content":{"rendered":"

I’ve been doing some push-pull output stage experiments with either cathode feedback (CFB) via the output transformer (Toroidy custom-made OPT) and parallel-parallel feedback (aka plate-to-grid or a la “Schade”) with PMOS driving the cathode. See some funny experiments here<\/a> on the driver side.<\/p>\n

<\/p>\n

I breadboarded multiple auxiliary circuits to supply the Screen, elevate grid DC and drive the cathode with great results. I decided to produce a series of modular prototype PCBs to assist with multiple designs. They all have the same size and mounting holes so can stack up one over the other. I have used the same board size of the HV OSRAM LED Voltage Reference<\/a> I use on my 300B SE Amp<\/a>.<\/p>\n

The boars go like this:<\/p>\n

\"\"<\/a>
PP Bias PCB<\/figcaption><\/figure>\n

The circuit is out of the book standard. A voltage divider with bias and balance trimpots to adjust both valves. They are buffered with a PNP source follower which has the option of either an emitter resistor or a CCS formed by a simple LND150 which should give about 1 to 2mA stable current depending on the device IDSS:<\/p>\n

\"\"<\/a>
Push-Pull Bias Circuit<\/figcaption><\/figure>\n

The second PCB is a bit more complex, albeit it takes 15min to solder it through. This is a flexible screen bias PCB for a pair of valves. Also it has some flexibility to use as screen DC feedback stabilisation from the output. See the example here<\/a>.<\/p>\n

\"\"<\/a>
Screen Bias PCB circuit<\/figcaption><\/figure>\n

The actual circuit is a tad more complex. There is a current limiter circuit added to T1. You can use it as a source follower if you want as well. R6 is needed to avoid oscillations on T1 should it get to cut-off. You want at least 1-2mA dragging out of T1 at all times. In most of the cases you have a jumper on J1. If you want to use the circuit to stabilise screen with feedback from an output stage, then you will use B1 separate from B2:<\/p>\n

\"\"<\/a>
Screen Bias and Decouple PCBs<\/figcaption><\/figure>\n

Also made another PCB (the one in black on the right) which decouple +B with an electrolytic, a PIO cap and has a bleeder resistor plus an INS-1 neon bulb on board. It can connect 4 daughter boards.<\/p>\n

The PMOS driver looks like this:<\/p>\n

\"\"<\/a>
PMOS driver PCB<\/figcaption><\/figure>\n

Similar to the screen circuit. If you’re using to drive the cathode of an output stage, you won’t use the bias voltage divider and cap-couple the input signal to the PMOS gate instead. No C1 of course.<\/p>\n

Output stages<\/h2>\n

So playing with some nice output stages with GU-50, RL12P35<\/a>, 6P36s<\/a> and other lovely TV vertical pentodes here are some of the configurations you may find some use to these circuits:<\/p>\n

\"\"<\/a>
PP output (pentode)<\/figcaption><\/figure>\n

I’m not a fan of gNFB and will never be. However, the PP bias circuit and the screen bias circuit in 2 boards can bias any pentode pair like the above.<\/p>\n

\"\"<\/a>
PP output stage with CFB (1)<\/figcaption><\/figure>\n

I have the OPTs with CFB winding (10%) so the above is a good option to apply the local feedback to the output stage.<\/p>\n

\"\"<\/a>
PP output stage with CFB (2)<\/figcaption><\/figure>\n

If you don’t want to cap-couple the driver (which should be an LTP) you can use an IT configured for push-pull operation. I love this option, however you will have to bias both grids with same regulator. You can balance the anode currents with the screen regulators which are still independent, so this is handy.<\/p>\n

Finally, if you want to implement the parallel-parallel local feedback arrangement with the PMOS driver, here is how you will do so:<\/p>\n

\"\"<\/a>
PP output stage with PMOS driver<\/figcaption><\/figure>\n

The feedback is set by the pair R1-R3 and R2-R4. DC balance can be adjusted by both the screen bias as well as the PMOS drivers. Each PMOS driver (there are 2 in a PCB) will drive the cathode. You will need likely a bigger heatsink as you will have about 10-12W in many cases dissipated at the PMOS.<\/p>\n

Hopefully will get some more time to play with them soon.<\/p>\n","protected":false},"excerpt":{"rendered":"

I’ve been doing some push-pull output stage experiments with either cathode feedback (CFB) via the output transformer (Toroidy custom-made OPT) and parallel-parallel feedback (aka plate-to-grid or a la “Schade”) with PMOS driving the cathode. See some funny experiments here on the driver side.<\/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":[673,157,5],"tags":[1373,1367,1368,1372,1366,682,1365,1371,1369,1370],"class_list":["post-8841","post","type-post","status-publish","format-standard","hentry","category-300b-amp","category-push-pull-designs","category-valves","tag-local-feedback-output-stages","tag-pmos-cathode-driver","tag-pmos-driver","tag-pp-fixed-bias","tag-pp-output-stage","tag-push-pull","tag-push-pull-output-stage","tag-push-pull-grid-bias-pcb","tag-screen-bias","tag-screen-bias-pcb"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p2r2tK-2iB","jetpack_likes_enabled":true,"jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8841","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=8841"}],"version-history":[{"count":1,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8841\/revisions"}],"predecessor-version":[{"id":8851,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/posts\/8841\/revisions\/8851"}],"wp:attachment":[{"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/media?parent=8841"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/categories?post=8841"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.bartola.co.uk\/valves\/wp-json\/wp\/v2\/tags?post=8841"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}