{"id":941,"date":"2010-06-03T07:28:05","date_gmt":"2010-06-03T11:28:05","guid":{"rendered":"http:\/\/www.engineeringradio.us\/blog\/?p=3928"},"modified":"2023-07-09T15:31:43","modified_gmt":"2023-07-09T19:31:43","slug":"filament-voltage-management","status":"publish","type":"post","link":"https:\/\/www.engineeringradio.us\/blog\/2010\/06\/filament-voltage-management\/","title":{"rendered":"Filament Voltage Management"},"content":{"rendered":"
\n
\"4CX35,000C
4CX35,000C ceramic vacuum tube<\/figcaption><\/figure>\n<\/div>\n\n\n

There are still many hollow state (AKA tube type) transmitters floating around out there in the broadcast world. \u00a0High power, especially high power FM transmitters are often tube types and there are many good attributes to a tube transmitter. \u00a0They are rugged, efficient and many of the well-designed tube units can last 20-25 years if well maintained.<\/p>\n\n\n\n

The downside of a tube transmitter is tube replacement. \u00a0Ceramic tubes, like a 4CX20,000 or 4CX35,000C cost $6-9K depending on manufacture. \u00a0A well-maintained tube and last 3-4 years, I have had some lasting 8 years or more. \u00a0My personal record was for a 4CX35,000C that was a final PA tube in a Harris MW50A transmitter. \u00a0The tube was made by EEV (English Electrical Valve<\/a>, now known as E2V<\/a>) \u00a0and lasted approximately 84,000 hours, which is 9.58 years. \u00a0When it finally came out of service it looked like it had been through a fire, the entire metal plate body was dark blue. \u00a0I took it out because the power was beginning to drop a little and it was making me nervous.<\/p>\n\n\n\n

This was not an accident, I did it by maintaining the filament voltage and keeping the tube and transmitter clean. \u00a0The tube filament supplies the raw material for signal amplification. \u00a0Basically, the filament boils off electrons, which are then accelerated at various rates and intensities toward the plate by various control grids. \u00a0The plate then collects the amplified signal and couples it to the rest of the transmitter. \u00a0When a tube goes “soft,” it has used up its filament.<\/p>\n\n\n\n

I had a long conversation about this one day with Fred Riley, from Continental Electronics, likely the best transmitter engineer I have ever known. \u00a0At the time, the consensus was to lower the tube filament voltage by no more than 10%. \u00a0On the 4CX35,000C, the specified filament voltage is 10 volts, therefore, making it 9 volts was the standard procedure. \u00a0What Fred recommended was to find the performance “knee,” in other words, where the power began to drop off as the filament voltage is lowered. \u00a0Once that was determined, set the voltage 1\/10 of a volt higher. \u00a0I ended up running that EEV tube at 8.6 volts, which was as low as the MW50’s filament rheostat would go.<\/p>\n\n\n\n

The other important thing about tubes is the break-in period. \u00a0When installing a new tube, it is important to run only the filament voltage for an hour or two before turning on the plate voltage. \u00a0This will allow the getter to degas the tube. \u00a0New tubes should be run at full filament voltage for about 100 hours or so before the voltage is reduced.<\/p>\n\n\n\n

Tube changing procedure:<\/p>\n\n\n\n

    \n
  1. Remove power from transmitter, discharge all power supply caps to ground, hang the ground stick on the HV power supply.<\/li>\n\n\n\n
  2. Remove the tube, and follow manufacturer’s procedures. \u00a0Most ceramic tubes come straight up out of their sockets (no twisting).<\/li>\n\n\n\n
  3. Inspect socket for dirt and broken finger stock.  Clean as needed.  Finger stock, particularly in the grid section, is important for transferring RF.  Broken fingers can lead to spurs and other bad things<\/li>\n\n\n\n
  4. Insert new tube, follow manufacturer’s recommendations. \u00a0Ceramic tubes usually go straight down, no twisting.<\/li>\n\n\n\n
  5. Make all connections, remove grounding stick, half tap plate voltage supply if possible, close up transmitter<\/li>\n\n\n\n
  6. Turn on filaments and set voltage for manufacturers’ recommended setting. \u00a0Wait at least 90 minutes, preferably longer.<\/li>\n\n\n\n
  7. Turn on plate voltage and tune transmitter.  Tune grid for maximum current and or minimum reflected power in the IPA.  PA tuning should see a marked dip in the PA current.  Tune for dip, then load for maximum power.<\/li>\n\n\n\n
  8. Turn off transmitter, retap plate supply for full voltage<\/li>\n\n\n\n
  9. Turn on transmitter and plate supply, retune for best forward power\/efficiency ratio.<\/li>\n\n\n\n
  10. After the 100-hour mark, reduce filament voltage to 1\/10 volt above performance knee.<\/li>\n<\/ol>\n\n\n\n

    Of course, every transmitter is slightly different. \u00a0There may not be a dip in the plate current if the transmitter is running near its name plate rating, in which case one would tune for maximum forward power.<\/p>\n\n\n\n

    This system works well, currently one of the radio stations we contract for has a BE FM20T with a 4CX15,000A that has 9 years on it, still going strong.<\/p>\n","protected":false},"excerpt":{"rendered":"

    There are still many hollow state (AKA tube type) transmitters floating around out there in the broadcast world. \u00a0High power, especially high power FM transmitters are often tube types and there are many good attributes to a tube transmitter. \u00a0They are rugged, efficient and many of the well-designed tube units can last 20-25 years if … Continue reading Filament Voltage Management<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[32,15,106],"class_list":["post-941","post","type-post","status-publish","format-standard","hentry","category-tech-stuff","tag-am-transmitters","tag-fm-transmitters","tag-tube"],"_links":{"self":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/941","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/comments?post=941"}],"version-history":[{"count":5,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/941\/revisions"}],"predecessor-version":[{"id":12140,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/941\/revisions\/12140"}],"wp:attachment":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/media?parent=941"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/categories?post=941"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/tags?post=941"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}