hi, you'll probably get a lot of responses but i'll throw my 2 cents in...
1. the L4/L4B does not have an exhaust system that will keep the tubes cool if you use something other than SSB on the SSB position. The plates should show some color but if they become orange that is too much. The amp was never designed to push enough air to meet today's 1500W output limits. 2. never let the grid current go past 300mA 3. 500V is a lot of sag - get some 10 or 12 gauge wire and bring 240V to the amp. My old L4 will produce around 1100W on the original 8163 tubes (Amperex graphite 3-400Z, made in 1968) but this is on a 240V circuit fed with #10 gauge wire 30 feet from the breaker box. This output was measured against an LP-100 digital wattmeter to a dummy load. New RF Parts tubes will produce maybe 1300 but the Drake cooling system is not up to the task of handling this kind of heat. Take a look at the photo of my L4: http://n1su.com/tubes/3-400Z-idle.jpg - notice the glass chimneys have a fluted top that push the air over the anode heat dissipator (they're curved intentionally). If you have Coleman lantern glass with straight sides your tubes' anode seals won't get enough air across them and are going to be put under a lot of stress... and may either fail or the joint inside the tube where the plate meets the anode pin could deform. The short answer is "use low voltage mode for everything but SSB" - be sure to oil the fan motor bearings once a year. I use Norvey, Inc turbine lubricating oil (never gums up - go to home depot). 3-400Z / 3-500Z tubes are rated at a max of 140mA grid current each. 300mA grid current for both tubes is pushing it but full scale is certain death for the tubes and if you're not careful you will deform the grid cage inside the tube and short the grid to the filaments, which would do even worse damage. The grid current increases as the plate voltage decreases - you're having to push it very hard. Perhaps the tubes are weak but I think your lower output power is more likely due to running the amp on 120V. 400mA grid current is probably splattering all over the band. You can reduce the grid current by rotating the LOAD control clockwise until your power drops around 50-100 watts - this will help the linearity and reduce the grid current but the pressure you take off the tubes puts pressure on the tuning network. 500V is a lot of sag. You're probably putting the power supply through some hard work... 10-3 or 12-3 romex would be a good choice, 14-3 would not be. Bigger is better. 120V ain't cuttin' it. hope this helps! -- 73 Jason N1SU http://n1su.com/ ----- Original Message ----- From: Joe Roth To: drakelist@www.zerobeat.net Sent: Friday, November 24, 2006 10:04 Subject: [drakelist] Bad tubes in L4B "Joe Roth" made an utterance to the drakelist gang ---------------------------------------------------------------------- I suspect I have a week pair of 3-500Z's in my L4B. Tell me if you agree. I restored an L4B and rebuilt the power supply. PS voltages check out fine. I drive it with a T-4XC at 90 watts. On 40m, RF output is 800w. Plate amps shows .5. Plate voltage is at 2000 on TX (2500 at rest). The grid bothers me as the meter pins past full scale. If I drop the RF drive to around 45-50 watts, the grid current drops to 300ma. The amp is running on 120 VAC for now. Thanks for your tips! 73 Joe, WC4R www.wc4r.com www.rothweb.net http://members.cox.net/kg4kaw ------------------------------------------------------------------------------ Lycos Cinema : Catch up with your friends and see free movies online - watch, chat & connect now >> http://cinema.lycos.com ---------------------------------------------------------------------- Submissions: drakelist@www.zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: www.zerobeat.net - sponsored by www.tlchost.net ----------------------------------------------------------------------
