[AMRadio] My Amplifer Quest took a strange turn today

John E. Coleman (ARS WA5BXO) wa5bxo2006 at pctechref.com
Mon Apr 23 11:03:31 EDT 2007

	You need to start over if you want to get 600 watts carrier output.
A pair of 813s is just not capable of doing this in linear AM service.  150
to 200 watts is about it.  And you don't have enough power supply either for
large AM service.  Keep in mind that to get a carrier of 600 watts on AM you
must have an amplifier capable of over 2400 watts (4 X carrier).  Even so
there is another draw back in that when a linier amplifier is operated at
full drive and full output (lets say 2500 watts) its efficiency is 60-75
percent and with no input its efficiency is Zero percent.  With this in mind
you can figure that at 25% of the full drive power and output, that the
efficiency will be between 0 and 70 percent and in most cases it is around
25-30 percent.  This means that the amp can produce a lot more heat when
driven at 25% of full output than it does at full output. Hence the
probability of damage to light weight part is great.

There are some rules of thumb to be used for linear amplifiers in AM
service, and these rules are a lot different in requirements for big
equipment than the rules of thumb for SSB service.

This is the reason that many times it is better to use class C plate
modulated service instead.

You have some parts there that could be very usefull and the 813s or capable
of all you want and more, in class C service

I would be happy to help with some graphs and drawing to get you started if
you want to come by for so coffee.

The Computer Fixer
1622 Sawdust Rd. Ste A1
The Woodlands, TX 77380

PS: bring Chocolate Chip cookies


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Bow
Sent: Monday, April 23, 2007 2:39 AM
To: 'Discussion of AM Radio in the Amateur Service'; Glowbugs
Subject: RE: [AMRadio] My Amplifer Quest took a strange turn today

 Well, A bit of an update on my new toy..

I tested it today, after repairing some solder joints that were not up to my
specs and replacing some wire I didn't like that looks of.

The test was:
Icom IC-718 --> MFJ-949E antenna tuner-->Amplifier-->Daiwa 3kw watt
meter-->MFJ legal limit dummy load.

The IC-718 will do 100 Watts on CW/SSB/Rtty and 40 watts on AM

I used the frequency of 3870 kHz

I first used a RTTY signal 
Input from Rig - Output from Amp - Grid Current (amps) - Plate Current
30 - 180 - .02 - .24
50 - 380 - .07 - .34
60 - 400 - 0.1 - .36
75 - 410 - 0.2 - 0.4

Next I tried an unmodulated AM carrier

25 - 200 - .04 - .26
40 - 375 - 0.1 - .34

Final Check was on SSB, modulated to the peak ALC setting with a 1 kHz tone.
30 - 110 - .02 - .16
40 - 200 - .02 - .18
50 - 210 - .02 - .22
60 - 300 - .04 - .22
70 - 375 - .04 - .24
80 - 400 - .06 - .26

I figured 80 watts was about as hard as I want to drive it.
I don't have a High Voltage test lead here to actually check the plate
voltage, but I am going to take this featherweight (NOT!!!) into work with
me and check it out there.

Here are some more photos, including the guts. I may just rewire this thing
for my own piece of mind.










I guess now I need to figure out how to do a few things.... Get an opinion
from someone who knows something about amplifiers and homebrewing....
1. Make it band switch able for 80/40/20/10
2. Figure out a tuned input for the above bands
3. figure out how to get it up to a 600 watt output level (because everyone
tells me the first 600 watts are the best for signal boost)



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