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Understanding amp designs: Texas Star
- Thread starter Wire Dawg
- Start date
They are biased, but the design has well know flaws.
As long as you can feed them a solid 14 or so volts, and don't drive the snott out of them they are fine.
Dive them too hard, toast the chokes in the bias circuit and you will have a problem.
There are better examples of biased amps out there, back when the original Palomar Magnum series and TX amps were built they had regulated bias that actually held the voltage to a much better tolerance.
73
Jeff
The bias current in a Texas Star is not regulated. Turn up the power supply voltage and the zero-signal "idle" current goes up with it. How much? They don't provide you with a good way to measure this. But turn the supply voltage high enough and it will run dangerously hot.
For a while.
The bias is set by one resistor for each push-pull pair of transistors. The current gain of a RF transistor determines what value resistor will make it behave properly. And all transistors are not created equal in this respect. Any time RF transistors get changed in a Texas Star you need to see that this "adjustment" has not changed for the worse. You'll need a proper supply of resistor values between 1 and 3 ohms to get it adjusted right.
The bias current has no temperature compensation. As a bipolar junction transistor heats up it draws more current. A temperature sensor (like a diode) is typically used to throttle down the bias current as the temperature rises. But not in a Texas Star.
I remember a customer decades ago who spotted the ad for the Ameritron 600-Watt amplifier and exclaimed he could buy two or more "CB" amplifiers with that power rating for the price listed in that magazine ad.
I pointed out that it had three times as many parts in it. A fancier bias circuit is only part of the difference. Texas Star made the judgment that a more-sophisticated bias circuit wouldn't improve their sales, and declined to spend the additional money.
Can't fault their business judgement there.
73