Tube Amp causing high SWR at Radio when "on"

[Brian G]

So I have an old 9 tube base amp (PAL 750b). The radio has an SWR meter built into it and I have an external watt/SWR meter at the antenna side . When I key the radio with the amp off or on stand by, the SWR at radio and meter are nice and low.
But when I put the amp on, the SWR in the radio jumps to 3:1 how ever the SWR at the external meter to the antenna remains low. In fact the radio cut the transmit a couple of times almost immediately when it sensed the high SWR.
Obviously sound like something inside the amp. This thing is quite old but I know it has been recapped in the last few years.
Any idea what causes that high SWR when the amp is fired up?

 

[nomadradio]

The meter in the radio shows the antenna SWR when the amplifier is on standby. Well, if the amplifier can properly imitate a barrel connector when it's not keyed. Some amps do this better than others.

But when the amplifier is keyed, the radio is no longer driving the antenna. It's driving the amplifier's input circuit. The high SWR reading on the radio's meter tells you that the amplifier's input impedance is not close enough to 50 ohms for a low reading.

Don't have a diagram to go by, but there should be some sort of adjustment screw visible on the rear panel of the amplifier. Typically this will be the input-tuning adjustment. Typically set for minimum SWR reading on the radio's meter.


73

 

[BJ radionut]

PAL750B_man.pdf (firestik.com)

PAL750B_sch.pdf (firestik.com)

TUBES: 9/ GE 8950....$$$$$$$

Very LOW DRIVE Amp...made when radio's had max output of 3 watts carrier to total 5 watts w/"swing" .
Common SSB was 12 watts PEP MAX

 

[freecell]

"The meter in the radio shows the antenna SWR."

No, it doesn't.

 

[Brian G]

"The meter in the radio shows the antenna SWR."

No, it doesn't.

Super helpful comment....

 

[freecell]

there is no such thing as antenna swr, however one can measure THE SWR ON THE LINE at the antenna and as implied, that requires the measurement to be taken at the antenna feedpoint. the swr meter in the radio is seeing a complex impedance looking into the transmission line input towards the load that is far removed from the existing antenna load resistance and reactance conditions. the closer the transmission line input impedance is to 50 ohms, the more power that will be delivered to the load regardless of mismatch, from your fixed 50 ohm transmitter.

didn't mean to hijack the thread, back to the amplifier.

 

[Brian G]

there is no such thing as antenna swr. however one can measure THE SWR ON THE LINE at the antenna and as implied, that requires the measurement to be taken at the antenna feedpoint. the swr meter in the radio is seeing a complex impedance looking into the transmission line input towards the load that is far removed from the existing antenna load resistance and reactance conditions. the closer the transmission line input impedance is to 50 ohms, the more power that will be delivered to the load regardless of mismatch, from your fixed 50 ohm transmitter.

didn't mean to hijack the thread, back to the amplifier.

So is what Nomad said correct? Look for an adjustment on the back of the amp to see if the impedance can be adjusted? I have not looked at it yet....

 

[Brian G]

The meter in the radio shows the antenna SWR when the amplifier is on standby. Well, if the amplifier can properly imitate a barrel connector when it's not keyed. Some amps do this better than others.

But when the amplifier is keyed, the radio is no longer driving the antenna. It's driving the amplifier's input circuit. The high SWR reading on the radio's meter tells you that the amplifier's input impedance is not close enough to 50 ohms for a low reading.

Don't have a diagram to go by, but there should be some sort of adjustment screw visible on the rear panel of the amplifier. Typically this will be the input-tuning adjustment. Typically set for minimum SWR reading on the radio's meter.


73

Unfortunately there is nothing on the back of the amp to adjust. Only things back there are the connectors, power cord and fuse.

 

[Dmans]

Brian G,
Your amplifier needs an adjustable/tunable input added. Search the forum for information. I believe I posted a thread about a Palomar Skipper 300 (1-8950 driving 3-8950's) a couple years ago showing the input tuning modification to the Skipper. Yes, It is not your PAL 750B but what the radio "sees" first when keyed is 1 8950 tube-as your amp is configured (1-8950 driving 2-8950's driving 6-8950's) I would think that the dimensions of the coil and capacitor would be very close.
When this amplifier was originally made (Early 1970's I suspect) the vast majority of CB's had tube final output sections. SWR was "not important" to the tube output sections. They would tolerate a 3:1 SWR with no sweat. So a tunable input was not included with all "Factory" amplifiers. (Cost cutting mode even then)

Transistor final output sections, as you have discovered, fail to see the humor in a 3:1 SWR.

I cannot agree more with BJ radionut's statement that

Very LOW DRIVE Amp...made when radio's had max output of 3 watts carrier to total 5 watts w/"swing" .

Good Luck

73
David

 

[Brian G]

Brian G,
Your amplifier needs an adjustable/tunable input added. Search the forum for information. I believe I posted a thread about a Palomar Skipper 300 (1-8950 driving 3-8950's) a couple years ago showing the input tuning modification to the Skipper. Yes, It is not your PAL 750B but what the radio "sees" first when keyed is 1 8950 tube-as your amp is configured (1-8950 driving 2-8950's driving 6-8950's) I would think that the dimensions of the coil and capacitor would be very close.
When this amplifier was originally made (Early 1970's I suspect) the vast majority of CB's had tube final output sections. SWR was "not important" to the tube output sections. They would tolerate a 3:1 SWR with no sweat. So a tunable input was not included with all "Factory" amplifiers. (Cost cutting mode even then)

Transistor final output sections, as you have discovered, fail to see the humor in a 3:1 SWR.

I cannot agree more with BJ radionut's statement that



Good Luck

73
David

Ok thanks. Can’t seem to find that thread. You think you can throw me a link to it? I’ll check it out.

 

[Brian G]

Brian G,
Your amplifier needs an adjustable/tunable input added. Search the forum for information. I believe I posted a thread about a Palomar Skipper 300 (1-8950 driving 3-8950's) a couple years ago showing the input tuning modification to the Skipper. Yes, It is not your PAL 750B but what the radio "sees" first when keyed is 1 8950 tube-as your amp is configured (1-8950 driving 2-8950's driving 6-8950's) I would think that the dimensions of the coil and capacitor would be very close.
When this amplifier was originally made (Early 1970's I suspect) the vast majority of CB's had tube final output sections. SWR was "not important" to the tube output sections. They would tolerate a 3:1 SWR with no sweat. So a tunable input was not included with all "Factory" amplifiers. (Cost cutting mode even then)

Transistor final output sections, as you have discovered, fail to see the humor in a 3:1 SWR.

I cannot agree more with BJ radionut's statement that



Good Luck

73
David

I think I found the thread but it does t really say how to construct it. You just have a pic of it at the end of the thread. I need a step by step. Sorry, I’m not that advanced to figure that out otherwise.

 

[Brian G]

If not I’ll probably just sell the amp. It works well otherwise and I think I can get good money for it and get something without that issue. Though I would prefer to fix this one of I can.

 

[nomadradio]

I’ll probably just sell the amp

Good call. While it still works.

My take on this problem is that fixing one problem will cause another.

This design has just too much gain. Way too easy to overdrive.

The untuned input the radio is driving into causes the high SWR reading on the radio's meter, but it also reduces the efficiency of the one-tube driver stage.

A tuned input will increase the drive power to the single driver tube.

Probably a bad idea, since it's already on the edge of the overdrive cliff as it's built.

You could add some attenuator resistors between the relay and the tuned input circuit if you installed it. That would knock the drive to the tube back down to where it is now, but with a lower input SWR.

But that's "fixing a fix". More trouble than I would want to go to.

Besides, the tubes would probably sell for more than the working amplifier. More people looking for tubes than are looking for an amplifier.

Get your money out of it now before it pumps out a mushroom cloud.

73

 

[Brian G]

Good call. While it still works.

My take on this problem is that fixing one problem will cause another.

This design has just too much gain. Way too easy to overdrive.

The untuned input the radio is driving into causes the high SWR reading on the radio's meter, but it also reduces the efficiency of the one-tube driver stage.

A tuned input will increase the drive power to the single driver tube.

Probably a bad idea, since it's already on the edge of the overdrive cliff as it's built.

You could add some attenuator resistors between the relay and the tuned input circuit if you installed it. That would knock the drive to the tube back down to where it is now, but with a lower input SWR.

But that's "fixing a fix". More trouble than I would want to go to.

Besides, the tubes would probably sell for more than the working amplifier. More people looking for tubes than are looking for an amplifier.

Get your money out of it now before it pumps out a mushroom cloud.

73

Based on that, can anyone make a recommendation on a newer amp or type I can look for that will not have these issues?

 

[Mike007]

The meter in the radio shows the antenna SWR when the amplifier is on standby. Well, if the amplifier can properly imitate a barrel connector when it's not keyed. Some amps do this better than others.

But when the amplifier is keyed, the radio is no longer driving the antenna. It's driving the amplifier's input circuit. The high SWR reading on the radio's meter tells you that the amplifier's input impedance is not close enough to 50 ohms for a low reading.

Don't have a diagram to go by, but there should be some sort of adjustment screw visible on the rear panel of the amplifier. Typically this will be the input-tuning adjustment. Typically set for minimum SWR reading on the radio's meter.


73

I'm having the same problem with an amp I bought from a guy, but the amp works great loud and clean. What would happen if gone unchecked?