[AMRadio] AM Usage with Linear AMPS


Brian Carling bcarling at cfl.rr.com
Tue Nov 23 07:24:30 EST 2004


VERY fascinating... yes, this does support the descriptions of 811A 
Pdiss in AM linear mode.
 I wonder if you could use ceramic insulators instead of lucite and 
keep things cooler?

On 22 Nov 2004 at 22:04, Jim candela wrote:

> 
> 
> 	Gary,
> 
>     I like your analysis on the topic. Brian may be looking for hard data to
> back up the theoretical. I recently finished a single EL-34 (6CA7) Class AB2
> RF linear amplifier in a Central electronics 20a. In my case I got only 20%
> efficiency on AM! I am going to present some data achieved on 80 meters
> here:
> 
> Condition Maximum CW output:
> 
> Power Output 40 watts
> Plate Voltage 615 v
> Screen voltage 300 v
> Screen current 30ma
> Cathode current 178 ma
> DC plate input power 91 watts
> Plate circuit efficiency 44%
> Plate dissipation 51 watts*
> 
> Condition ready for AM modulation:
> 
> Power Output 8 watts
> Plate Voltage 636 v
> Screen voltage 309 v
> Screen current 11 ma
> Cathode current 74 ma
> DC plate input power 40 watts
> Plate circuit efficiency 20%
> Plate dissipation 32 watts*
> 
> Keep in mind that the CCS EL-34 plate dissipation rating is 25 watts. With
> forced air cooling, and brief full power tune ups, the Russian tube behaves
> pretty well considering the abuse I am giving it. The tube plate does show
> some red however on AM, and even more at maximum CW output. This example of
> an overloaded EL-34 is somewhat similar to the overloaded 811 in earlier
> examples in this email thread.
> 
> I had circuit variations that offered increased power and efficiency at the
> expense of linearity. With this tube, getting good linearity on AM (seen on
> trapezoid pattern on scope) resulted in a pretty hot tube. I even added
> feedback where I rectified the RF output, obtained audio and sent that back
> to the speech amp for audio inverse feedback. That technique works extremely
> well.
> 
> * This includes losses in the output coupling circuit; the Lucite strips
> securing the tank coil tend to soften and droop at 50 watts RF output when
> on 160 meters. Maybe I actually am getting closer to 30% PLATE efficiency on
> AM.
> 
> Regards,
> Jim Candela
> WD5JKO
> 
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net
> [mailto:amradio-bounces at mailman.qth.net]On Behalf Of Gary Schafer
> Sent: Monday, November 22, 2004 9:23 PM
> To: bcarling at cfl.rr.com; Discussion of AM Radio
> Subject: Re: [AMRadio] AM Usage with Linear AMPS
> 
> 
> Brian,
> 
> What he is saying is: you need to have the 811A tuned up at 200 watts
> output on cw or pep for a 50 watt carrier out. (pep is 4 times carrier)
> 
> When you tune up you tune it for max output at the 200 watt level with
> full drive. At that tune setting the tube will be around 60% efficient.
> Normal ssb or cw operation tune-up. input power will be 333 watts. The
> tube will be dissipating 133 watts in heat at full carrier.
> 
> You then reduce the drive until you have 50 watts out (1/4 the carrier
> of full output). You can not retune at this point or it will not handle
> the peak envelope power of 4 times carrier. At 50 watts out the
> efficiency drops to around 30%. So that means to get the 50 watts out
> the tube has to be running around 150 watts input. 150 watts minus the
> 50 watts output leaves 100 watts to dissipate in heat in the plate as
> long as the carrier is on.
> 
> It doesn't get any better either. With modulation the dissipation does
> not decrease even though the efficiency increases at the peak power
> levels. The carrier power is still there 100% of the time at 30%
> efficiency. The audio is in the form of separate side bands that is
> additional power that the tube has to handle.
> 
> I guarantee you that an 811A will run with a red plate under these
> conditions. If it is not red then it is not tuned up properly for this
> mode of operation at the 50 watt carrier out level.
> 
> 73
> Gary  K4FMX
> 
> 
> 
> 
> Brian Carling wrote:
> > On 22 Nov 2004 at 20:23, W7QHO at aol.com wrote:
> >
> >
> >>Going to shorten it's life considerably.
> >
> >
> > Theoretically maybe. Have you tried it. How can you know
> > for sure? Theory says a bumble bee can't fly. My 811A tubes
> > never burned up.
> >
> > Where did you get your 30% number from? Who transmits
> > their AM with carrier only!? I can't see how a 50 watt carrier input
> > will cause an 811A to DISSIPATE 100 watts!
> > Where are you getting the additional 50?
> >
> >
> >>To properly adjust a linear amp for AM it is necessary to tune the thing
> up for the peak power
> >>level, i.e., 200 watts for a 50W carrier. The linear will be about 60%
> efficient at this output level.
> >>Then WITHOUT CHANGING THE TUNING adjust the output from the AM exciter to
> provide for
> >>50W out of the linear with no modulation. The problem here is that the
> linear will now only be
> >>running about 30% efficiency under carrier-only conditions and the 811
> would be dissipating
> >>something like 100+ watts. The 811A, of course, is only rated at 65 watts.
> >>
> >>Dennis D. W7QHO
> >>Glendale, CA
> >
> >
> >
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> 
> 
> 
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