In your opinion....

[mackmobile43]

Okay, so if you can get full output of the amp on a low variable setting, is your impedence going to change? Aside from the problems related to the switch itself, is this an acceptable way to run a radio/amp setup? I was under the impression that you had to set up the radio to drive the amp not the other way around.

The input to the input transformer of the amp is routed through the variable pot by adding or decreasing the resistance in that circuit which should not change the amps input swr to the radio.

The variable just adds padding to the input of the amp.

 

[swanny]

quoted from shockwave in another thread:

"Perhaps a better demonstration showing how amplifier output impedance does not impact reflected power would be using the modern solid state HF rig with an automatically calibrated SWR meter. As Bob mentioned the output can only be matched to 50 ohms at one value of output power. Designers choose this value to be at full rated power to get the best efficiency.

If you were to set the drive level on CW to produce a 100 watt carrier, this would place the output impedance of the amplifier very close to the 50 ohm design goal (assuming your radio is rated at 100 watts). Care to guess how much the output impedance of the amplifier rises to when you drop the carrier to 10 watts?

Output impedance must rise with the reduction in collector current caused by the reduced drive. Using simple ohms law, if a reduction in drive causes current to drop by ten times at the same voltage, the output impedance has gone up 10 times. Yet the automatically calibrated SWR meter looking at the load will remain rock stable throughout the drive range. "


So, the part that I'm still chewing on is this...you're adding resistance to the output from the radio to lower it enough so the amplifier could handle the drive. Doesn't this added resistance add ohms to the output impedence of the radio giving a mismatch between the output impedence of the radio versus what the amplifier is designed for as an input impedence from the radio? The amplifier is seeing the correct drive but at the wrong impedence? Is it not worth worrying about or am I totally off here?

 

[Captain Kilowatt]

The input to the input transformer of the amp is routed through the variable pot by adding or decreasing the resistance in that circuit which should not change the amps input swr to the radio.

The variable just adds padding to the input of the amp.

Maybe some amps are better than others but I have a small 2 pill Galaxy amp here that has the variable and it does change the SWR going into the amp. The variable is not a true constant impedance attenuator pad as it has only one variable leg. A constant impedance pad would need at least two variable legs and most likely three variable components to maintain a constant impedance. Both the series leg and the shunt leg need to be changed in order to provide constant impedance. Then again like I said some amps may differ in that regard. I solved part of the problem anyway and ripped out the variable and then I solved the rest of the problem by putting the amp on the shelf and not using it.

 

[mackmobile43]

Maybe some amps are better than others but I have a small 2 pill Galaxy amp here that has the variable and it does change the SWR going into the amp. The variable is not a true constant impedance attenuator pad as it has only one variable leg. A constant impedance pad would need at least two variable legs and most likely three variable components to maintain a constant impedance. Both the series leg and the shunt leg need to be changed in order to provide constant impedance. Then again like I said some amps may differ in that regard. I solved part of the problem anyway and ripped out the variable and then I solved the rest of the problem by putting the amp on the shelf and not using it.

Makes sense that one resistors value may not cover the range required for the range of the potentiometer and the addition of another resistor would be required.

What input swr do you consider to be too high?

 

[mackmobile43]

quoted from shockwave in another thread:

"Perhaps a better demonstration showing how amplifier output impedance does not impact reflected power would be using the modern solid state HF rig with an automatically calibrated SWR meter. As Bob mentioned the output can only be matched to 50 ohms at one value of output power. Designers choose this value to be at full rated power to get the best efficiency.

If you were to set the drive level on CW to produce a 100 watt carrier, this would place the output impedance of the amplifier very close to the 50 ohm design goal (assuming your radio is rated at 100 watts). Care to guess how much the output impedance of the amplifier rises to when you drop the carrier to 10 watts?

Output impedance must rise with the reduction in collector current caused by the reduced drive. Using simple ohms law, if a reduction in drive causes current to drop by ten times at the same voltage, the output impedance has gone up 10 times. Yet the automatically calibrated SWR meter looking at the load will remain rock stable throughout the drive range. "


So, the part that I'm still chewing on is this...you're adding resistance to the output from the radio to lower it enough so the amplifier could handle the drive. Doesn't this added resistance add ohms to the output impedence of the radio giving a mismatch between the output impedence of the radio versus what the amplifier is designed for as an input impedence from the radio? The amplifier is seeing the correct drive but at the wrong impedence? Is it not worth worrying about or am I totally off here?

An amp with variable power such as this may have the best input/output scenario with regard to input swr stability being that each level has a predetermined resistor for that drive level.

 

[Captain Kilowatt]

Makes sense that one resistors value may not cover the range required for the range of the potentiometer and the addition of another resistor would be required.

What input swr do you consider to be too high?

Honestly, I don't like it when my radios start to see an SWR of around 2:1. That's the point where a lot of radios start to fold back their power. Admittedly a 2:1 on a driver is not the end of the world but I just don't like it. that just me however and YMMV.

 

[Captain Kilowatt]

An amp with variable power such as this may have the best input/output scenario with regard to input swr stability being that each level has a predetermined resistor for that drive level.

BINGO! You got it Mack. I'm not 100% sure but I do think that power level switch actually switches a proper pad and not just a simple single resistor that will maintain a reasonably constant input impedance. Either that or the amp I had sen was modified after the fact.

 

[C2]

All of the amps I've seen simply switch in a resistor and bork the input impedance.

 

[Captain Kilowatt]

That could be C2. I admit to having not seen too many with the switched level input. The last one I saw was many years ago and had a pair of MRF-454's in it. All I remember was it was a nice amp and the model LA-1500BL comes to mind.

 

[psycho]

the variable on the amp would be for output from the amp not input from the radio. You can't adjust the dead key or output of the radio from a control on the amplifier.

100% wrong......let's leave it at that.

 

[psycho]

the pots used in those variables are only good for several watts at the most and a lot do in fact get burned up. A lot of guys try and run the radio flat out and expect the variable to handle it and they just won't. If the radio has it power turned down, either by doing it internally on a fixed level basis or by using the RF Power control on the front panel, then the poat should be able to handle the few watts from the radio.

In reference to the question about whether the variable will change the driving impedance of the amp; YES as the variable is changed the input impedance and thus the SWR going into the amp will change somewhat. Just how much depends on the basic construction of the amp and value of components used.

The variables in those amps are rated at 2 watts..........ssb output or continuous 3+ watt AM output would ruin the pot if all you did was roll it back.