Well yeahh Garey.... I am not suggesting we change the bias for SSB (or AM), just for CW (and real FSK).
So, you would need a switch or relay added and USE it to change modes. Yes, bias it past cut-off, will result in no emission between code elements, but also over part of the cycle. Key clicks could be delt with... The reason they had key clicks was primarily because they were cathode keying high voltages with no shaping as I recall. no? Bias is only on the finals, we don't key those or do we? Junk? Isn't our driver pretty clean? If not, wouldn't we have the same spurs and junk with a linear final? We DO have a tuned output tank circuit for the flywheel effect... no? BUT, if we don't have enough (voltage) drive, that might be a problem, but we don't really need full power... Don't ya think? Changing bias for CW wasn't my idea, but it did sound good to me. TIA, I am trying to learn here... There probably IS some reason nobody did this originally! 73, Gary WB6OGD ----- Original Message ----- From: Garey Barrell <k4...@mindspring.com> To: Gary Winblad <garywinb...@comcast.net> Cc: drakelist <drakelist@zerobeat.net> Sent: Fri, 20 Jan 2012 15:31:44 -0000 (UTC) Subject: Re: [Drakelist] AC-4 (bias mod) Gary - In a word.... NO. The PA stage is operated as a LINEAR amplifier, Class AB1, meaning the output signal is a reasonable facsimile of the input signal. The BIAS voltage is carefully chosen to 'center' the operation of the amplifier in the linear portion of the tube's operating region. Increasing the BIAS voltage WILL reduce the IDLE current, (between code elements,) but will shift the PA to a non-linear region. This will result in distortion of the signal, with the potential of spurious signals, clicks, and other 'junk'. This is one reason clicks were so prevalent in the older transmitters. A true Class C amplifier requires considerably more driving power, and relies on the 'flywheel effect' of it's output network to 'smooth' out the 'pulses' of drive that exceed the bias potential. 73, Garey - K4OAH Glen Allen, VA Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line and TR-4/C Service Supplement CDs Gary Winblad wrote: > All this talk of rebuilding AC-4 supplies had me thinking... > > I have seen a mod(?) that said to turn up(down?) the bias when running CW > (and FSK?) to set the finals more to Class C operation to save power > dissipation. > > Is this a good idea? Wouldn't it be easy to add a switch or relay and > another potentiometer while you are in there rebuilding? > > 73, > Gary > WB6OGD > http://mailman.zerobeat.net/mailman/listinfo/drakelist
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