F3WT <[EMAIL PROTECTED]> made an utterence to the drakelist gang ---------------------------------------------------------------------- Hi Garey:
I find this exchanges very very interesting also for a TR4! Now I wonder abt the following: I run only CW and would like to bias for class C operation those finals as I did once for my home - made CW Xmter which has a couple of 807's. Ever tried this on a TR4/T4Xany? I presume the bias just to be set to further below -60V so that at least iddle Anode current become zero. But more precisely what is the exact class C polarization ( bias) of the 6JB6 finals? TU! Vy 73 Pierre. F3WT - le 13/02/05 6:49, Garey Barrell à [EMAIL PROTECTED] a écrit : > > Garey Barrell <[EMAIL PROTECTED]> made an utterence to the drakelist gang > ---------------------------------------------------------------------- > > > Eric Webner wrote: > >> Garey, >> >> Several weeks ago when I discovered the play in one of the bandswitch >> wafers, I set the wafer where it should be and used adhesive on the >> inner wafer and shaft. It appears to work very well, and with no >> binding. I wouldn't recommend that for all the wafers, but just one >> seemed ok. >> >> Regarding my low output, I may be ok. First of all, I was not loading >> it to full power, since one of the errata sheets that Drake put out >> said to load it for only 340 mA in order to avoid any spurious output. >> After reading your latest message, I loaded 80 and 40 meters to full >> power, obtaining 135-140 watts measured on my Bird wattmeter. Loading >> it to full power ran the plate current to 500 mA! > > A plate current reading of 500 mA tells you that your 3.3 ohm meter > shunt has gone up in value. Probably to about 5.6 ohms! :-) > >> >> I also took your suggestion and checked a few components. R44, the 680 >> ohm resistor, is good. The cathode resistors are good and very well >> balanced. The screen resistors checked out as well. However, the >> resistor across the meter measured 5.6 ohms. I disconnected one end >> from the circuit board and the resistor snapped in half. I couldn't >> come up with another one, so I hope to try a 2.2 ohm unit (measures >> around 2.6) so I can get on the air this weekend. > > That will "work", but you really need to replace it with a 3.3 ohm > unit. For "emergency" purposes, you can measure the 2.6 as accurately > as you can. then adjust the bias voltage for 100 mA of current through > that resistor by measuring the voltage drop across said resistor as > accurately as possible. Then note the reading that gives you on the > plate meter. If it reads 50 mA (or whatever) that becomes your "bias > adjust" setting for the finals. Then just Tune and Load for maximum > output, using the plate meter to adjust for minimum plate current. > > Then, when you get a "real" 3.3 ohm resistor in there, again accurately > measure 100 mA through that resistor and pick a "select at test" > resistor to make your meter read 100 mA. I wouldn't worry too much > about the "340 mA limit", since the meter isn't that accurate anyway. > The only way to determine the "cleanest" operating point is with a > spectrum analyzer, and it just isn't that critical. If you're driving a > 5 KW amplifier then you need to worry about it. > >> >> Do you recommend that I vary the other resistor in order to achieve >> accuracy on the meter, or can I play around with values on the shunt >> resistor (the 3.3 ohm resistor)? >> > > See above. > >> One other observation: my low power output could be attributed to this >> meter shunt resistor being off, as that would cause me to back off on >> the loading quite a bit. This is also why I didn't want to load it to >> full power (500 ma???) for fear of melting a couple sweep tubes. It >> also makes me wonder about my bias. Is there a voltage one should >> shoot for, or is getting the bias (idling) current still the >> preferable approach? Do I need to realign everything if I need to >> reset the bias? >> > > This is exactly the case. When you thought you were loading to 340 mA, > you were probably only drawing 150 mA, resulting in low output. THE > MOST IMPORTANT CURRENT READING IS THE BIAS SETTING. This is why you > "calibrate" the metering circuit at 100 mA. These meters only cost > about $1.25 and are NOT highly accurate, linear meters. So we make a > "reference" point for the _important_ meter reading and use it as an > "indicator" only for the far less important absolute plate current > reading at full power. > > 73, Garey - K4OAH > Atlanta > > ---------------------------------------------------------------------- > On Behalf of Garey Barrell <[EMAIL PROTECTED]> > Submissions: drakelist@www.zerobeat.net > Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body > Hopelessly Lost: [EMAIL PROTECTED] - help in body of message > Zerobeat Web Page: http://www.zerobeat.net > Brought to you courtesy of TLCHost.net http://www.tlchost.net/ > ---------------------------------------------------------------------- > ---------------------------------------------------------------------- On Behalf of F3WT <[EMAIL PROTECTED]> Submissions: drakelist@www.zerobeat.net Unsubscribe: [EMAIL PROTECTED] - unsubscribe drakelist in body Hopelessly Lost: [EMAIL PROTECTED] - help in body of message Zerobeat Web Page: http://www.zerobeat.net Brought to you courtesy of TLCHost.net http://www.tlchost.net/ ----------------------------------------------------------------------
