|[AMRadio] AM Amps|
garyschafer at comcast.net
Mon Jan 10 23:31:54 EST 2005
Here is a ruff rule of thumb for amps on AM:
If the amp takes 100 watts to drive it on ssb then it will take 100
watts pep of AM to drive it. 100 watts pep of AM means 25 watts of
carrier with 100% modulation.
You will also get 25% of the carrier out of the amp from it's pep output
level. If it puts out 600 watts pep (collins 30L1) then you will get 150
watts carrier out when properly set up. If it took 100 watts pep to
drive it on ssb it will take 25 watts carrier on AM.
To see if the tubes will handle it figure the dissipation backwards.
At 150 watts output that amp should be at around 33% efficiency (if
properly tuned). 150 watts divided by 33% = 454 watts plate input power.
Now subtract the 150 watts of output power you are getting from the 454
watts input power and you have around 304 watts going to heat in the
plates. In the case of the 30L1 divide that by 4 (the number of tubes)
and that leave about 76 watts per tube in dissipation.
811A's have a 65 watt plate dissipation ratting. So you are a little
over. For short transmissions you can get by with it but no long buzzard
The power supply in the 30L1 is marginal also. It will get mighty hot.
The other Gary's suggestion of limiting to 125 watts carrier out (500
watts pep)gets you right in the ball park on plate dissipation. 125
divided by 33% = 378 watts input. Subtract the 125 watts and that leaves
253 watts dissipated. Divide by 4 and that is about 63 watts per tube.
For proper loading on the amplifier operating at 125 watts out with
carrier it should be tuned for 500 watts output pep. The drive required
can be figured pretty close by first calculating the gain of the amp at
600 watts output, its rated output. If it takes 100 watts to drive it to
that output power that is a gain of 6.
At 500 watts out the amp will still have a gain of 6 so divide 500 by 6
which gives about 83 watts pep of drive. divide that by 4 for the
carrier needed. That should be around 20 watts.
Keep in mind that the amp needs to be tuned at the pep level. One way to
do that is to modulate the 20 watt carrier 100% with a tone or a long
heeeelooo and tune the amp watching a scope.
Another way is to use the ssb rig with an 80 watt carrier driving the
amp and tune it for max output. Then hook the AM rig up with the 20
You can roughly check to see if you tuned the amp right by looking at
the efficiency that it is running when you think you have it tuned.
Calculate the input power, plate current times plate volts. Divide the
output power you are seeing on the watt meter by the input power. If the
efficiency is around 30 to 35% you should be in the ballpark. Any
greater efficiency and it tells you that amp is not loaded heavy enough.
Or you have too much drive.
More than you wanted to know.
Gary Blau wrote:
> I partially disagree, but with a -big- proviso.
> You'll have to find a way to reduce the Ranger output to the 10-15 watt
> level. Maybe the nicest way to do that is a variable screen voltage
> control, similar to what you'll find here:
> but I'm sure there are other methods.
> Just don't run the stock Ranger straight into the amp without dealing
> with this in some way.
> Don't ask the 30L1 to do more than ~125 watts carrier. The 811's can't
> handle much dissipation.
> Same is true for the SB200 and its pair of 572B's. But they both will
> work fine like this. I ran an SB200 like this for a long time.
> Bigger amps with more plate dissipation, like the Henry or SB-220 are a
> safer bet, but you must be very careful nonetheless.
> Chris wrote:
>>The 30L1 would far too over stressed but the Henry would be perfect, by
>>the way thanks for buying my Ranger, 73 Chris
>>RICHARD W GILLESPIE wrote:
>>>I just bought a Johnson Ranger and wonder if my 30L1 or Henry 2KD
>>>would work okay. 811's in the 30L1 and a pair of 3-500's in the Henry.
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