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: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Behalf Of Gary Schafer > Sent: Monday, November 22, 2004 9:23 PM > To: [EMAIL PROTECTED]; 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, [EMAIL PROTECTED] 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 > > > > > > > > ______________________________________________________________ > > AMRadio mailing list > > Home: http://mailman.qth.net/mailman/listinfo/amradio > > Help: http://mailman.qth.net/mmfaq.html > > Post: mailto:[email protected] > > > > > > > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected] > --- > Incoming mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.799 / Virus Database: 543 - Release Date: 11/19/2004 > > --- > Outgoing mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.799 / Virus Database: 543 - Release Date: 11/19/2004 > > ______________________________________________________________ > AMRadio mailing list > Home: http://mailman.qth.net/mailman/listinfo/amradio > Help: http://mailman.qth.net/mmfaq.html > Post: mailto:[email protected]
