F3WT <[EMAIL PROTECTED]> made an utterence to the drakelist gang ---------------------------------------------------------------------- Thanks Garey for your advise. That puts an end to my intention I think. 73. Pierre. F3WT.
le 13/02/05 20:08, Garey Barrell � [EMAIL PROTECTED] a �crit�: > > Garey Barrell <[EMAIL PROTECTED]> made an utterence to the drakelist gang > ---------------------------------------------------------------------- > Pierre - > > Even with your situation, I would strongly recommend against trying to > implement Class C operation of the final stage. > > In addition to the bias level, you will also need considerably MORE > drive than is available from the 12BY7 driver stage. Remember, your > drive has to overcome that much higher bias level since the tube has to > be turned ON for at least a small portion of the cycle. This would > probably require, at the very least, a voltage step-up transformation > from the 12BY7 to the 6JB6 grids. The 12BY7 would probably not be able > to provide enough driving _power_, particularly on the higher bands, to > drive the 6JB6's in Class C. > > Keep in mind. Class AB1 linear stages require NO driving power (since > the grid is never driven positive), only voltage into a very high > impedance grid circuit. Class C stages require considerable driving > power, since you need to generated a MUCH higher RF voltage (it has to > exceed the "several times cutoff bias voltage" over at least part of the > cycle) into a much lower impedance grid circuit, and the grid is > actually driven into the positive grid current area. > > To answer your direct question, bias "switching" is generally achieved > by having some "fixed" protective bias to protect the finals against > loss of drive, and then develop the "operating" bias across a grid resistor. > > Sorry, I still say it's a bad idea, especially for negligible (at best!) > "potential" benefits. > > 73, Garey - K4OAH > Atlanta > > Drake C-Line Service Manual > <http://hr99.home.mindspring.com/R-4C_Servicez/> > > > > F3WT wrote: > >> Thanks a lot , gentelmen , for your replies - very much appreciated . >> >> As all my antennas ( wire - based) aren't fed barefoot but with various >> high -Q couplers (silvered coils, symetrical into Lecher feeder always >> tuning to SWR=1), this should add to the plate PI output network filtering >> by cleaning out any further the spectral overtones generated by the >> C-class operation. Shouldn' t it? ( Can in most instances tune the couplers >> while in reception by noise peaking only , suggesting Q is probably rather >> high- at least in low bands.) >> >>> From what you suggested, a seperate biais generation will probably be >> necessary . Now, using a ramp-up from AB2 to C class op biais when key is >> down and reverse when up again all in a few millisec or so should reduce >> clicks , right? >> >> I read abt the many warnings as you, Garey , signaled to ensure that if >> biais is not present, plate voltage has not to be either!! hi! This could >> be ensured with a relais safety circuit arrangement. Any comment and >> reference of past realisations or mods on this as well? >> >> thank you, >> 73. >> Pierre. >> F3WT >> >> >> le 13/02/05 17:20, Ron Wagner � [EMAIL PROTECTED] a �crit : >> >> >> >>> The bias voltage is used in other places on the rig besides just the >>> finals. That said, have a look at the "drake mods" article written by >>> VE3EFJ http://www.dproducts.be/drake_museum/Drakemods.htm select the PDF >>> version and look for bias. The 5 or so hit will be the CW discussion. >>> >>> He discusses backing down the bias provided that you are only running CW. >>> Keep in mind that the 6JB6s are a small linear amp, and adjusting then out >>> of linear will effect you CW note. Might get the FCC looking your way, or >>> at least a few fellow ham unhappy with your key clicks. >>> >>> 73, >>> Ron >>> >>> On Sun, 13 Feb 2005, F3WT wrote: >>> >>> >>> >>>> 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. >>>> >>>> >> >> Vy 73 >> de Pierre. >> F3WT >> - >> >> >> >> >> >> > ---------------------------------------------------------------------- > On Behalf of Garey Barrell <[EMAIL PROTECTED]> > Submissions: [email protected] > 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: [email protected] 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/ ----------------------------------------------------------------------
