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High input SWR PAL200MDX

Toltec

Toltec

New Member
Jul 2, 2017
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Hey guys,

I've been working on this little PAL 200MDX amplifier. I really like it but it's got some issues which I'm slowly resolving. The first is the inverter circuit was fried. I updated to a modern style circuit using an IC and some mosfets and it works like a charm now with more stability than original. I also replaced the main power relay as the contacts on that were spent and it would refuse to key up sometimes.

Right now my main concern is with the input SWR. When the amplifier is off or in standby mode the input SWR is acceptable, about 1.5. With the amplifier in transmit mode the input SWR skyrockets to above 3:1. I see no input adjustment on this thing. The only internal adjustment is a small coil which is between the drivers and the output but this doesn't help the SWR. It's mentioned in the manual as a way of helping make the input drive wattage higher. Despite this the amp does put out about 40 watts in low and about 80 in high but I don't like what it's doing to my radio and I imagine it can do a bit better than that.

The high SWR seems to be due to the connection that takes the input RF to the cathode of the driver tube. This connection is made using some of that really tiny coax. Could this be the issue? I'm thinking of replacing it with regular RG-8 or something. There's an inductor 27uh and a .01 capacitor to ground. Could changing the values of these help? Should I have to though or is this an indication of another issue?

Here is the schematic,

http://www.cbtricks.com/Amp/pal/200_mdx/graphics/pal_200mdx_sch.pdf
 

There is a quick-and-dirty fix to improve the input match for this sort of amplifier.

Find the relay contact terminal that feeds the radio's drive into the tube's blocking capacitor, the one feeding the driver tube cathode. Shows as C4 in the diagram on CB Tricks. Unhook C4, or the wire that leads to it from the relay.

Wind a coil with 6 or 7 turns on a half-inch diameter form. Plain insulated hookup wire is fine, so long as it's solid copper wire. Stranded wire won't retain the coil's shape.

Insert the coil between the relay lug and the wire to C4. Try to leave room to stretch the coil a bit. This is how you adjust it to the optimum inductance, by stretching it.

The idea is to make the coil's closely-spaced inductance just slightly more than optimum, so that spreading the spacing between turns will tune it for minimum input-side SWR. Might need 8 turns, might need 5.

Might or might not get it below 2 to 1. If it won't adding a trimmer capacitor from the relay end of the coil to ground is the next step. Seldom requires more than 150 pf. Might work best with as little is 30 or 40 pf. This is the reason to use a trimmer. You'll find the coil's turns count will need to be reduced to make it work best with the added capacitor. Going back-and-forth trimming the coil, and tweaking the capacitor will probably get you down below 1.5.

Don't have a hard-and-fast inductance value to shoot for. This is a totally cut-and-try trick that's determined by all the other unknown factors in the driver tube's input circuit.

The coil-only trick works best with a four-tube stage, okay with a 2-tube driver stage and is so-so with a single tube. The one-tube situation will most often require the input-side capacitor to ground in addition to the coil.

The tube-type radios that were common when this amplifier was designed didn't care so much about SWR. You may find that once you improve the input matching of this amplifier that you have also improved the drive power reaching the driver tube.

The risk is that this will prove to be too much of a good thing. An intentional mismatch like this also serves to increase the radio drive-power level that it can tolerate. A typical single-final legal 40-channel CB may very well overdrive it.

You may find that just eliminating the driver tube and driving the two final tubes directly works better with radios you can get today. Might not have a radio small enough to work well with that single driver tube once the input match is improved.

73
 
Thanks, great info!

If I removed the driver tube, would that reduce my overall output power? Or would the the radio itself, perhaps turned up a bit, be enough to drive this thing to its full output power? If I had to I think I would prefer lowering my radio power vs losing access to my amps full potential.

Should my radios be too much to drive this thing, is there an easy way to vary the input? In other words trim down the total input power WITHOUT screwing up my input SWR again. Not sure if I want to do a variable power mod on any of my radios.
 
So, how do you run a linear on AM without a carrier control on the radio?

Short answer is that you turn the amplifier down. Way down.

If that's the case, eliminating the driver tube is probably your only hope.

With the driver tube in place, you would probably need to turn the carrier down to a Watt, probably less.

When that amplifier was designed, nobody worried about sounding "tight" from excessive carrier, or wearing out the tubes quickly. They were cheap. Nobody cared.

I don't have a procedure on file for that model, but the gist of it is to remove the plate choke L4 from the driver stage. Remove it altogether. Hook the driver tube's plate cap to the cathode-pin lug of the now-empty driver-tube socket. This re-routes the radio's drive power directly to the two final tubes.

The driver pi-network coil gets the turns count reduced, probably to five or so turns. Simply shorting across the unused turns works fine. C9 gets removed and a variable (or trimmer) cap goes in place of C8. This capacitor and the coil get tweaked for best input match.

At least the final tubes have some grid bias voltage on them. The driver tube does not. This is part of what causes it to exaggerate the carrier power so severely. Adding bias to the driver tube will still be like putting a band-aid on a broken arm.

This box might make a usable 2-tuber. With the driver tube, a 2-Watt walkie-talkie would probably overdrive it.

73
 
Last edited:
Thanks Nomad.

Here's what I did, I added the coil as you said. However I didn't add it directly off the relay because the space there is very limited. Instead I added it directly on the tube socket where the signal comes in. SWR is down to 1.4 which is pretty decent. I used 10 turns. Perhaps I could have gotten it down even lower with a bigger coil but I kinda didn't bother, spreading this one didn't help. In fact I didn't see any difference when I spread any of the coils I tried. I also tried the capacitor to ground with no success, I had to set the capacitor all the way open to make it get back to the lower SWR. By this time I was a ready to call it finished. Not to mention the lower SWR did indeed bring up the overall power. My power supply at the moment has a problem where it protects early in the current draw at around 18 amps even though its a 30 amp supply so I can't have the amp too hot headed for now until I fix that power supply anyway. For now I'll use whatever tiny loss the input SWR gives me to my advantage. I just couldn't have it as it was as it was far too much.

I turned down the carrier on my 148 using the internal pot. It's down to about 2 watts. I also turned the overall power down by adjusting the variable coil between the driver tubes and the output tubes, L2. Like this the power output on the low position is about 40-45 watts. Before the amplifier would swing backward when I talked into the radio. Now it actually swings forward. Locals tell me the audio sounds nice and clean. Do you think that will keep the amp happy or should I go down all the way to 1 watt? I'll have to put variable power in my radio I guess. I hadn't had an amp installed for a while so I didn't need it. Can't work the high position for the time being due to my power supply issue. However I have run the amp before on high and it does around 100 watts.


I think I'll leave the amp as is. I don't wanna gut it. It's a fairly clean example of a vintage amp and it feels a bit sinful to me to do that. I'll just go for variable power on the radio. At the same time turning it into a two tuber would help make it just a little less power hungry. Any idea how much less current draw I could see from removing that stage? I kinda like this thing for the relic that it is. Practically speaking these things are kinda of a hassle.
 
Last edited:
Sounds as if you have found a happy medium with this combination of radio and amplifier. Really looks like a "If if ain't broke don't fix it" setup now.

Yes, bypassing the driver will reduce the DC power demand. Not sure how to predict how much, though.

The driver tube has no fixed bias to control the current it draws. The risk here is that a low carrier drive from the radio may cause the driver tube to overheat. Just keep an eye on the driver tube. Any dim red or orange glow from the tube's gray plate structure means that you're about to drop a valve. Could just stock up on driver tubes.

And if the driver behaves itself with this setup and does not "cherry" with the low drive-carrier level, you're good to go.

A working amplifier is a working amplifier. Brings to mind the motto of the shade-tree mechanic. "If it ain't broke, keep fixing it until it breaks".

73
 
Toltec said:
Hey guys,

I've been working on this little PAL 200MDX amplifier. I really like it but it's got some issues which I'm slowly resolving. The first is the inverter circuit was fried. I updated to a modern style circuit using an IC and some mosfets and it works like a charm now with more stability than original. I also replaced the main power relay as the contacts on that were spent and it would refuse to key up sometimes.

Right now my main concern is with the input SWR. When the amplifier is off or in standby mode the input SWR is acceptable, about 1.5. With the amplifier in transmit mode the input SWR skyrockets to above 3:1. I see no input adjustment on this thing. The only internal adjustment is a small coil which is between the drivers and the output but this doesn't help the SWR. It's mentioned in the manual as a way of helping make the input drive wattage higher. Despite this the amp does put out about 40 watts in low and about 80 in high but I don't like what it's doing to my radio and I imagine it can do a bit better than that.

The high SWR seems to be due to the connection that takes the input RF to the cathode of the driver tube. This connection is made using some of that really tiny coax. Could this be the issue? I'm thinking of replacing it with regular RG-8 or something. There's an inductor 27uh and a .01 capacitor to ground. Could changing the values of these help? Should I have to though or is this an indication of another issue?

Here is the schematic,

http://www.cbtricks.com/Amp/pal/200_mdx/graphics/pal_200mdx_sch.pdf
Click to expand...
The CBTricks site is not working anymore do you have or know another site to get those schematics?? I'm workin' on one of the 200mdx amps. By the way what is the bulb behind the meter, this one is gone. tnx Bill kc4ubc
 

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