> -----Original Message-----
> From: Bill L. Fuqua [mailto:wlfuqu00@uky.edu]
> Sent: Wednesday, November 15, 2006 1:15 AM
> To: tonne@comcast.net
> Cc: garyschafer@comcast.net; ars.ka5mir@gmail.com; amps@contesting.com
> Subject: Re: Re: [Amps] Class A for AM
>
> "We were chatting about a circa 1970 screen-modulated AM rig. Those
> things are also "efficiency-modulated" and so operate at about the same
> efficiency as a pure linear amplifier."
>
> Control grid modulation is efficiency modulation because it changes the
> class of operation of the tube. But is screen grid modulation? The tube is
> still operating class C and but at lower power when not being modulated.
> However, the tube must be able to provide 4 times that power on modulation
> peaks. If it is operating 75% efficiency at no modulation and 75% at 100%
> modulation you are not efficiency modulating the tube.
The efficiency is on the order of 35% at carrier with screen grid modulation
or control grid modulation and the peak envelope power efficiency is 70% to
75% very similar to the efficiency changes that you get with a class B
amplifier amplifying an AM signal.
The tube is tuned up at full power as normal and then screen voltage is
reduced so that carrier power output drops to 1/4 power output just like you
do with a class B amplifier.
Modulation peaks drive the output power up to full power. The tube is
supplying all the power for carrier and audio power.
So yes the efficiency does change.
One of the problems with screen modulation is audio linearity. Modulating
the screen produces more distortion than does plate modulation. In
communication quality audio it is not as big a problem as in commercial
broadcast.
73
Gary K4FMX
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|