power output question about the sd1446

[Robb]

Since the 2290 & 2879 are also "rated" the same I don't see it as realistic or necessary to turn the voltage down on a power supply to 12.6 volts. If it was then we all would have to do the same with say out Texas Star 250 & 350's. I don't see running 13.8-14.5 volts as part of this guys problem.

Not saying that you have to turn the voltage down either. However, if you read the spec sheets for all of those devices (including the 1446), they are most linear at 12.5v. Amp will run fine there too, slightly less output of course. Just think its funny that CB linears are anything but linear. Over-driven, over-modulated, over-volted, and over-heated is the norm - lol.

Think his wattmeter is inaccurate . . . over . . .

 

[Shockwave]

The 3 top reasons that indicate this amp is oscillating are as follows.

1) The single stage exhibits gains over 100 times the drive level.

2) The SWR is high and drops with audio as the amp is driven harder

3) It's a Palomar or similar. These amps have notorious oscillation problems.

 

[Robb]

2watts is as low as she goes. The radio does swing a lot. The meter shows it swinging up in the 300's on higher power. I have been running it for quite a whil now with good reports all over. I guess im cooking those pills? But they keep on working.

I agree that oscillations would account for the wattage discrepancy. But if he is getting "...good reports..." as mentioned above; then it has to be the meter. Otherwise, those 1446's would have cooked LONG ago if it got anywhere near 300w. A 'good report' would not include an oscillation.
Agreed; or am I wrong?

 

[Shockwave]

Since an oscillation is essentially a second carrier on an undesired frequency, the majority of cases do not produce noticeable distortion in the audio. This does however take away power from the desired frequency, makes extra heat in the amp and lots more interference. It is often very noticeable on SSB because in the absence of modulation they can still show lots of output. Perhaps the watt meter is out of calibration but what its showing in the SWR area makes it nearly certain the amp is oscillating. The reason the high SWR becomes low with modulation is because the extra drive power is swamping out the oscillation.

 

[9C1Driver]

Not saying that you have to turn the voltage down either. However, if you read the spec sheets for all of those devices (including the 1446), they are most linear at 12.5v. Amp will run fine there too, slightly less output of course. Just think its funny that CB linears are anything but linear. Over-driven, over-modulated, over-volted, and over-heated is the norm - lol.

Think his wattmeter is inaccurate . . . over . . .

Yes, the watt meter seems to be what I keep returning to as well. Might even be a combo of poor watt meter and too much dead key in.

 

[Shockwave]

Yes, the watt meter seems to be what I keep returning to as well. Might even be a combo of poor watt meter and too much dead key in.

Since the watt meter can only read accurately when the VSWR is 1:1, you can pretty much ignore the watt meter anyhow. Your VSWR is nearly 2:1 and the load probably has an impedance close to 100 ohms which would make the meter read twice the power. The watt meter is an RF volt meter. Put a frequency through it that makes the load appear higher than 50 ohms and you exaggerate the readings. All the signs are NOT pointing to a bad meter. They are pointing to a bad Palomar.

I'll try and make this point clear with this simple rule of thumb that has NEVER failed me in 25 years. ANYTIME that increasing drive to an amp (carrier or modulation) causes a decrease in reflected power (or VSWR), you have a faulty amplifier. Especially since he probably did not reduce the calibration on the meter when he modulated the carrier. I know it feels better to blame the meter but you can't. The meter is just measuring the ratio between forward and reflected power. If forward goes up, reflected or VSWR cannot come down.

 

[9C1Driver]

Yea it's a crap shoot. Bad meter, bad amp, or too much input carrier.

 

[Crusher]

If its a palomar knockoff, it probably doesn't have a feedback network from base to collector. You might want to check and see. This is usually a 100ohm resistor in parallel with a .1 or .01uF cap going from base to collector (input to output). What was the part that you said was rolling around inside amp? I have found several of these amps, where they installed transistors, but amp was not tuned right or that run was actually made for 2290's or something. Cap across output was way off from what should have been installed.

 

[Shockwave]

Yea it's a crap shoot. Bad meter, bad amp, or too much input carrier.

I have to disagree, it's not a crapshoot to those familiar with things like lack of negative feedback, poor RF grounding and oscillations with the typical Palomar circuit. There is nothing that could burn out in your meter or be misaligned that could cause reflected power to decrease when you apply more drive with audio. One watt drive per device is not the problem either. Less than that is actually making it to the transistor bases anyhow. Notice the VSWR symptoms associated with the problem actually improve with modulation and more drive? They will get worse will less drive.

 

[9C1Driver]

I have to disagree, it's not a crapshoot to those familiar with things like lack of negative feedback, poor RF grounding and oscillations with the typical Palomar circuit. There is nothing that could burn out in your meter or be misaligned that could cause reflected power to decrease when you apply more drive with audio. One watt drive per device is not the problem either. Less than that is actually making it to the transistor bases anyhow. Notice the VSWR symptoms associated with the problem actually improve with modulation and more drive? They will get worse will less drive.

Given the limited information from the get go it's a crap shoot. Could be a simple problem of, oh sorry guys I guess my 2950 is dead keying 7 watts, I thought it was 2. Or it could be a crap watt meter. Keep in mind the guy was giving us DEAD KEY WATTAGE numbers. It could end up being a problem with the amp or heck maybe he has a $hit antenna system. Hmmm starting to sound like a crap shoot now don't ya think? LOL whatever just saying. Help him out already then if you have it all figured out then.

 

[Crusher]

Just out of curiousity, what meter are you using? Probably isn't the meter, but I have seen really cheap ones not be able to handle any watts and internal components heat up. Usually it is an undersized coupler or something that is whacky. But this usually only happens running bigger power. Not a 2 pill I mean 2 transistor amp.

 

[Shockwave]

Given the limited information from the get go it's a crap shoot. Could be a simple problem of, oh sorry guys I guess my 2950 is dead keying 7 watts, I thought it was 2. Or it could be a crap watt meter. Keep in mind the guy was giving us DEAD KEY WATTAGE numbers. It could end up being a problem with the amp or heck maybe he has a $hit antenna system. Hmmm starting to sound like a crap shoot now don't ya think? LOL whatever just saying. Help him out already then if you have it all figured out then.

I've already helped him out by identifying the problem while you continue to confuse him into thinking he needs a new meter or antenna. It takes years of experimenting with solid state amplifier design before you gain the experience others like Bob have shared on this topic.

Crusher is right to be skeptical blaming the meter while we only have a two transistor 12 volt amp in front of it. The meter obviously has a scale capable of reading 250 watts from his description. I think we can all agree this pair of transistors would struggle to reach that level in PEP watts much less with a 2 watt carrier as drive.

Yes, the diodes in some cheaper meters can have their junctions saturated from too much RF but once you understand the symptoms here, the meter is no longer suspect. There are very few things that will make the VSWR appear higher under carrier conditions than they are with modulation. Downward swing or an unstable amp make up the short list.

With only 1 watt of carrier drive per transistor and many times that with positive audio peaks, we can check downward swing off the list quick. His VSWR is good with no amp and not so bad when the amp is driven with modulation. It's at its worst someplace in the middle under low drive conditions. Exactly when an unstable amp is most likely to act up.

Furthermore, the Palomar type solid state design has a long history of this specific problem dating back decades. Several threads have already been written on this forum covering this topic extensively. Right down to how to add the missing negative feedback and tuned input circuits.

PS: If it's what I suspect there is a very strong chance this amplifier will perform horrible on SSB where the drive level is continuously transitioning between low and high levels. One of the first things you should check is that the watt meter on SSB is behaving like a modulation meter on AM. If the watt meter is not in step with your audio peaks and fails to decrease quickly between words, you've further confirmed oscillations are present.

 

[Robb]

Sooooo . . . What exactly is causing this instability and what is the remedy? IS it a faulty part or is re-engineering the circuit necessary?

What about the bias circuit in this amp? Is it adequate for this design and would modding it there improve low power input stability?

 

[9C1Driver]

I've already helped him out by identifying the problem while you continue to confuse him into thinking he needs a new meter or antenna. It takes years of experimenting with solid state amplifier design before you gain the experience others like Bob have shared on this topic.

Crusher is right to be skeptical blaming the meter while we only have a two transistor 12 volt amp in front of it. The meter obviously has a scale capable of reading 250 watts from his description. I think we can all agree this pair of transistors would struggle to reach that level in PEP watts much less with a 2 watt carrier as drive.

Yes, the diodes in some cheaper meters can have their junctions saturated from too much RF but once you understand the symptoms here, the meter is no longer suspect. There are very few things that will make the VSWR appear higher under carrier conditions than they are with modulation. Downward swing or an unstable amp make up the short list.

With only 1 watt of carrier drive per transistor and many times that with positive audio peaks, we can check downward swing off the list quick. His VSWR is good with no amp and not so bad when the amp is driven with modulation. It's at its worst someplace in the middle under low drive conditions. Exactly when an unstable amp is most likely to act up.

Furthermore, the Palomar type solid state design has a long history of this specific problem dating back decades. Several threads have already been written on this forum covering this topic extensively. Right down to how to add the missing negative feedback and tuned input circuits.

PS: If it's what I suspect there is a very strong chance this amplifier will perform horrible on SSB where the drive level is continuously transitioning between low and high levels. One of the first things you should check is that the watt meter on SSB is behaving like a modulation meter on AM. If the watt meter is not in step with your audio peaks and fails to decrease quickly between words, you've further confirmed oscillations are present.

Outstanding, I guess I missed what the problem was. Can you pinpoint it for us in your next post in say 25 words or less? (by this I mean lets hear the exact problem, not four paragraphs of vague talk)

 

[Shockwave]

Ah yes, the meat and potatoes of it all. To start with, add some negative feedback. You might get away with the simple .01 cap in series with a 100 ohm 2 watt or more resistor between the base and collector of each transistor. If that does not cure it move onto installing a tuned input circuit. Break the RF fed to the input transformer and place and inductor in series with it. Usually 5 turns of #16 wire around a 1/2 inch form works good. Then place a 464 compression cap between the input of the coil and ground. One more 464 cap between the output and ground. Tune the caps for minimum input VSWR to the amp. Feel free to expand or compress the coil to reach the optimum inductance. Another trick in the arsenal is adding some low value non-inductive resistance in series with the input circuit.

The first paragraph covers what is needed in most cases. There are some very stubborn amps that were just designed with no RF layout in mind. They have high inductance ground returns to the transistor emitters and poor isolation between input and output stages. One easy trick for this is to get a piece of copper roof flashing. Cut it for size so that you can solder it on edge standing upright across the top of the transistors. You only want to make solder contact across the emitter terminals and ground on the PCB. You want the copper to block off as much area between the two sides as possible without grounding any other parts out. By doing this you lower the inductance between the emitters and RF ground while providing RF isolation between the input bases and output collectors of the amp.

Many amps also use long unshielded wires to carry RF from the SO-239 connectors and relay. These wire should be as short as possible or run through coax to preserve the 50 ohm impedance and prevent stray output RF from feeding back to the input. Small wire eye connectors under the transistor mounting screws, bent over and soldered to the emitter terminals also reduces inductance. Sorry I couldn't keep it under 3 paragraphs but not everything is so simple.