Re: [drakelist] reduced bias on T-4XB
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Jason - I think this is your question!... The tank circuit of a transmitter is essentially a matching network to match the high impedance of the tube plate circuit to the low impedance of an antenna feedline.The plate circuit impedance is approximately equal to the plate voltage divided by the plate current, or approximately 2000 ohms. A typical antenna feedline is 50 ohms, so you need a transformation ratio of about 40. Adjusting the TUNE and LOAD controls tunes the network for a match between these two impedances. The network's job is to couple the RF energy from the plate circuit to the feedline. When the network is tuned, (plate current dipped,) the maximum amount of RF generated is being transferred to the antenna. When the match is NOT correct, (plate current NOT dipped,) the network isn't transferring RF efficiently, the tubes are attempting to drive a mismatched load, and have to dissipate the wasted power as heat. Any time the plate current is NOT dipped, indicating the match is not correct, any additional plate current must be dissipated by the tube plates as heat. Hopefully this isn't too convoluted to follow... 73, Garey - K4OAH Chicago [EMAIL PROTECTED] wrote: On Wed, 30 Nov 2005, Garey Barrell wrote: I ran three 4 Lines on autostart RTTY. They ran 24/7, and often made transmissions in excess of 30 minutes, AT FULL POWER. In this service, the finals lasted a little over two years, typically, with end of life defined as 100W output on 20M. This piqued my curiosity again - Let's say you're running your T-4XB at full power for 30 minutes at a time at 300mA and the plate is perfectly dipped. All is good. But if the plate isn't dipped at 300mA and would be at 240mA if dipped properly there would be 60mA of current wasted as heat running at 300mA. But it's still 300mA, whether resonant and dipped at 300mA or splattering at 300mA - the thing i'm wondering is if the tubes are suffering more at 300mA undipped or if the life expectancy the same as 300mA dipped perfectly, being paced at a rate of current (heat) travelling through the metal. And I suppose the other question is what is a plate dip anyway - can I think of it as if the tank circuit is sucking the power out of the tube or is it best to think of the tank circuit as a valve that passes the power when at resonance, and the current increases as the RF power output is backed up and can't get out? Sorry for the rudimentary questions but I can't find this online anywhere and don't have an old ARRL handbook that explains it. thanks -- 73 Jason N1SU http://n1su.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 --
Re: [drakelist] reduced bias on T-4XB
[EMAIL PROTECTED] made an utterance to the drakelist gang -- On Wed, 30 Nov 2005, Garey Barrell wrote: I ran three 4 Lines on autostart RTTY. They ran 24/7, and often made transmissions in excess of 30 minutes, AT FULL POWER. In this service, the finals lasted a little over two years, typically, with end of life defined as 100W output on 20M. This piqued my curiosity again - Let's say you're running your T-4XB at full power for 30 minutes at a time at 300mA and the plate is perfectly dipped. All is good. But if the plate isn't dipped at 300mA and would be at 240mA if dipped properly there would be 60mA of current wasted as heat running at 300mA. But it's still 300mA, whether resonant and dipped at 300mA or splattering at 300mA - the thing i'm wondering is if the tubes are suffering more at 300mA undipped or if the life expectancy the same as 300mA dipped perfectly, being paced at a rate of current (heat) travelling through the metal. And I suppose the other question is what is a plate dip anyway - can I think of it as if the tank circuit is sucking the power out of the tube or is it best to think of the tank circuit as a valve that passes the power when at resonance, and the current increases as the RF power output is backed up and can't get out? Sorry for the rudimentary questions but I can't find this online anywhere and don't have an old ARRL handbook that explains it. thanks -- 73 Jason N1SU http://n1su.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 --
Re: [drakelist] reduced bias on T-4XB
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Jason Buchanan wrote: If you peak the RF Tune and immediately dip the PLATE control, then two-hand the PLATE and LOAD controls for maximum output power, (using an external wattmeter, holding down that meter switch button makes it a LOT more difficult,) and then DIP the PLATE control as the last adjustment, (which shouldn't be required if the transmitter is properly neutralized, but should be done anyway,) finals will last for 5-10 years of normal amateur use. I have a T-4XB and T-4XC that have been running for over 10 years with the same finals. Both are used several times a week, primarily on CW. Sounds like it is absolutely imperative that the 6JB6's be neutralized as best as possible. Mine aren't producing maximum output at the bottom of the dip. When I replace T-4XB R26 I will make sure they are neutralized as good as I can. Right now I have to rotate the plate control a wee bit CW for max output. But the bottom of the dip vs. max output is about a needle's width so I don't feel too bad about it. Is it necessary to neutralize the finals at 28.500 or can I do this on 15 or 20 into a resonant antenna? Neutralization should be done at 28.5, since the setting becomes more critical at higher frequencies. I usually neutralize first on 15M for this reason, the setting is easier to spot, then only a tiny touch-up is necessary on 10M. If you don't have a dummy load (you REALLY should!) you can do it on 15. Just try and center the adjustment as accurately as you can. By the way, a small fan on the receiver certainly won't hurt anything, (as long as it is quiet!,) but as noted above, three units ran 24/7 for 8-9 years with no fans, and NO failures. I turn my receivers on in the morning, and turn them off when I am through for the day. I have known guys who turn their receiver off when they leave the room for lunch!! NOT a recipe for long life of electronic equipment. When my parents bought the B line I have now I used to turn it on and leave it on for days (no drift!), plus it looked cool hi. I did that for years until I was out of high school and they were shut off for 15 years until I got married, bought a house and put an antenna up at the new QTH. I remember reading somewhere that a day's worth of useful filament life is consumed by simply turning the rig on, so I leave them on unless I know i'm not going to use them for another 24 hours. Most of the tubes in this B line are the same ones that it came with from the factory in 1971. I replaced the 6EH5 because it started sounding nasty but I suspect that it's actually the filter cap. I noticed the other day that there is a mustard yellow Sprague ATOM electrolytic can tied from the R-4B power supply board to chassis ground, and I noticed there is a guy selling these exact parts on an auction site listed as a fix for faulty electrolytics - does this sound familiar? I was thinking about replacing the R-4B filter can with the kit from WA9TGT but i'm wondering what kind of work I need to un-do to use this cap replacement kit. The electrolytic (the ones I have seen are aluminum colored) that is between the chassis and the PS board is the filter for the calibrator power supply. It's 1000 uF at 15V, positive end to the chassis. I have only seen a couple of these fail, going high in ESR and supplying some interesting modulation to the calibrator signal. I think Donnie's kit is just the four section Twist-Lok cap. There are a couple of other electrolytics, the 8 uF at 150V on the PS board, and another 200 uF at 6V on the Calibrator board itself. -- 73, Garey - K4OAH Atlanta Drake C-Line Service Manual http://www.k4oah.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 --
[drakelist] reduced bias on T-4XB
Jason Buchanan [EMAIL PROTECTED] made an utterance to the drakelist gang -- hi, A long time ago I remember reading somewhere that bias voltage could be reduced so that there was no idle plate current between CW elements - the purpose being to lengthen the life of the 6JB6 tubes. I remember that the person who wrote that email pointed out that this would not work for SSB, but I don't run SSB on my T-4XB so it's not a problem. I'm just wondering if this holds true or not and if it is advisable to run the rig that way, without idle plate current between CW elements instead of the typical 70 mils of plate current. Thanks, Jason -- 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 --
Re: [drakelist] reduced bias on T-4XB
Garey Barrell [EMAIL PROTECTED] made an utterance to the drakelist gang -- Jason - Probably NOT a good idea. The final stage, regardless of mode, is operated as a _linear_ amplifier in Class AB1 (no grid current). Increasing the bias voltage to cut the tube off between elements essentially takes the stage more towards Class C (normally biased WELL past cutoff) and _non-linear_ operation. The result is distortion of the signal, which translates as spurious output signals. I've never tried this, but would expect clicks, extra output signals and just general crud. The degree would vary depending upon just where the bias actually ended up, just barely on, just barely off, etc. Just my opinion, but I'd say NOT a good idea. Oh, I already said that.. The life of 6JB6 finals in linear service is widely misunderestimated. 6JB6's will last for YEARS with care. They will last for MORE years with a little fan _exhausting_ either out the top or out the back of the final cage area. What they do NOT tolerate well is off resonance operation. Unfortunately, the very characteristic that makes them so useful as power amplifiers (very high plate current at relatively low plate voltages) also makes them easy to kill by misuse. If you are one of those who tunes up their transmitter by keying the transmitter, then looking for the manual to see how to tune it up, then looks for their glasses so they can SEE the instructions, THEN dip the final plate current and only then notice that you have the wrong antenna selected, you'll be very unhappy with them. I can change bands with a T-4(any) and be fully tuned in less than 10 seconds. This comes from years of doing it, knowing about where the TUNE and LOAD controls need to be, and how the meter should respond. Just an aside, Drake issued a change order (associated with FCC regulations requiring lower 3rd order distortion) increasing the idle plate current to 100 mA in the T-4(ANY) transmitters. 73, Garey - K4OAH Chicago Jason Buchanan wrote: Jason Buchanan [EMAIL PROTECTED] made an utterance to the drakelist gang -- hi, A long time ago I remember reading somewhere that bias voltage could be reduced so that there was no idle plate current between CW elements - the purpose being to lengthen the life of the 6JB6 tubes. I remember that the person who wrote that email pointed out that this would not work for SSB, but I don't run SSB on my T-4XB so it's not a problem. I'm just wondering if this holds true or not and if it is advisable to run the rig that way, without idle plate current between CW elements instead of the typical 70 mils of plate current. Thanks, Jason -- 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 --
Re: [drakelist] reduced bias on T-4XB
The 6JB6 tubes will probably last longer, under normal operating conditions, than most of us will be around. Rather than playing around with the bias, a small fan mounted on the rear of the PA cage, pulling air out of the cabinet is recommended. There are several Fan installation articles posted on Ron's web page. www.wb4hfn.com Mark At 08:18 AM 11/23/05, Jason Buchanan wrote: Jason Buchanan [EMAIL PROTECTED] made an utterance to the drakelist gang -- hi, A long time ago I remember reading somewhere that bias voltage could be reduced so that there was no idle plate current between CW elements - the purpose being to lengthen the life of the 6JB6 tubes. I remember that the person who wrote that email pointed out that this would not work for SSB, but I don't run SSB on my T-4XB so it's not a problem. I'm just wondering if this holds true or not and if it is advisable to run the rig that way, without idle plate current between CW elements instead of the typical 70 mils of plate current. Thanks, Jason -- 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 --
