Hi Garey, Thanks, good explanation. I'm not going to try it any time soon ;-) 73, Gary
----- Original Message ----- From: Garey Barrell <k4...@mindspring.com> To: Gary Winblad <garywinb...@comcast.net> Cc: drakelist <drakelist@zerobeat.net> Sent: Fri, 20 Jan 2012 23:24:40 -0000 (UTC) Subject: Re: [Drakelist] AC-4 (bias mod) Gary - Obviously the bias would remain in the Class AB1 area for SSB and AM. Applying a keyed signal to a non-linear amplifier is a bad idea. Every time the drive is started or stopped there is the opportunity for additional distortion products to be generated. Some SSB distortion is caused by biasing the amplifier too high or too low, resulting in 'compression' or limiting the waveform to less than it's peak. More distortion. Perhaps we have an amplifier expert on here who can definitively respond to your question. There are no doubt MANY reasons why all transmitters switched to linear operation for CW, in spite of the 'power/efficiency savings. If nothing else than it takes large amounts of drive POWER, more trouble, including greatly increased Grid dissipation. To me, turning an input signal on and off to a Class C amplifier is asking for trouble. A Class C amplifier is essentially a switch, ON or OFF, and relies on the Q of the tank circuits to 'smooth things out'. A lot of potential headaches for a few percent efficiency increase. There are LOTS of 'ideas' on the internet that 'sound' good.... :-) Let us know what you find out!! 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: > 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 > To: Gary Winblad > Cc: drakelist > 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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