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 benecessary . 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 à [EMAIL PROTECTED] a écrit :
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
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