[AMRadio] AM Usage with Linear AMPS


Jim candela jcandela at prodigy.net
Mon Nov 22 23:04:34 EST 2004



	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
>
>
>
> ______________________________________________________________
> AMRadio mailing list
> Home: http://mailman.qth.net/mailman/listinfo/amradio
> Help: http://mailman.qth.net/mmfaq.html
> Post: mailto:AMRadio at mailman.qth.net
>
>



______________________________________________________________
AMRadio mailing list
Home: http://mailman.qth.net/mailman/listinfo/amradio
Help: http://mailman.qth.net/mmfaq.html
Post: mailto:AMRadio at mailman.qth.net
---
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




More information about the AMRadio mailing list

This page last updated 18 Oct 2017.