I have picked up a few radios and a magnum 125 amplifier, not sure about anything with this amp except the 5 pictures ive seen on google. cannot find info. Any would be appreciated, even an old image of a pamphlet would be awesome!
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"magnum" 125 amp question
- Thread starter Ay589
- Start date
Have a look inside and locate the two white-ceramic "pill" shapes, those are the tops of the RF power transistors. Then look outside at the surface of the heat sink immediately below where they are mounted. If I'm right, you'll see a short threaded stud with a machine nut on it protruding under the heat sink, one below each transistor.
This would be an old 1970s/early 80s amplifier. The 'stud mount' transistors were superseded by transistors with a flange and two screw holes around 30 years ago. If it has the 'studs' sticking out the bottom, we know two things.
Its really old.
The drive power that will kick it senseless is less than you probably think.
Our routine strategy back when mobile radios were only made with one final transistor was to add resistors to the input circuit. This would reduce the drive to the transistors below the over drive level. They do sound better that way, after all.
Also reduces the suicide speed. Using one of these with a modern two-final radio will probably assassinate it pretty fast. It will probably also sound terrible from the overdrive. But at least it won't do it for long. And if enough resistors are added to the input circuit, it will tolerate a two-final radio.
If someone has already blown it out, and turns out be dead when you key it, this is why. A 'small' radio from 1978 would overdrive these. A current-day radio will hit it two or three times as hard.
Seems to me that most amplifiers like this one would top out between 110 and 140 Watts PEP. They were sold by what the wattmeter would read, not by how they sounded on the air. The actual peak drive power needed to make the modulation peaks "flat" would measure between 6 and 10 Watts PEP on a lot of them. This would translate to a MODULATED average-power reading between 1.5 and 3 Watts, more or less.
73
This would be an old 1970s/early 80s amplifier. The 'stud mount' transistors were superseded by transistors with a flange and two screw holes around 30 years ago. If it has the 'studs' sticking out the bottom, we know two things.
Its really old.
The drive power that will kick it senseless is less than you probably think.
Our routine strategy back when mobile radios were only made with one final transistor was to add resistors to the input circuit. This would reduce the drive to the transistors below the over drive level. They do sound better that way, after all.
Also reduces the suicide speed. Using one of these with a modern two-final radio will probably assassinate it pretty fast. It will probably also sound terrible from the overdrive. But at least it won't do it for long. And if enough resistors are added to the input circuit, it will tolerate a two-final radio.
If someone has already blown it out, and turns out be dead when you key it, this is why. A 'small' radio from 1978 would overdrive these. A current-day radio will hit it two or three times as hard.
Seems to me that most amplifiers like this one would top out between 110 and 140 Watts PEP. They were sold by what the wattmeter would read, not by how they sounded on the air. The actual peak drive power needed to make the modulation peaks "flat" would measure between 6 and 10 Watts PEP on a lot of them. This would translate to a MODULATED average-power reading between 1.5 and 3 Watts, more or less.
73
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Yup - those old amps with 454 won't need much drive at all. I had some that would key for 1/4 ratio with just .75w or 1w and then I'd swing up to 10-15 watts for AM. On some of those old ones if you even hit 1.5w ot 2w input they would spike and key the maximum output. They always matched up with with my old 148's or other single final radios.
That amplifier does not have any "idle current" bias on the transistors. Will make it sound like dirt on sideband.
73
73
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