Palomar 300a High SWR on key

[Draxx2755]

Hi All,
I have a Palomar 300a I picked up with the 6KD6 tubes. Thay all test good it powers up and the amplification is working. My problem is that when it's keyed (11 meters) on the 10 meter selector, the SWR goes up to about 2.5. If the unit is on standby and swr is checked it's about 1.2:1. Also when keyed, I'm getting a very audible "hum" from the unit when in the "operate" mode. When on standby, it's there but not so pronounced. So, I'm wondering if the coil needs to be tapped in a different position for 11 meters, or I did notice on the schematic, there is an adjustment (VR) for SWR. Wondering if the SWR adjustment on the amp would work or help. I fear it must have some bad caps somewhere or maybe the hum increases due to higher SWR. In either case, I'm not using it untill I get all this squared away. Any help or insight would be greatly appreciated.... thx.

TRC 303
The Rock Collector (Draxx)

[Night Crawler]

Did you follow the tuning procedure in the owners manual?

[Draxx2755]

Yes I did.... To the tee. I have noticed that after loading and tuning for max, the knob in the back that you're supposed to tune for max seemingly has no effect. I see no difference on the meter at all. I'm concerned about the hum also. Not sure whether a cap or some caps might be bad.

[Nagant]

Hum heard on the air during TX means your power supply capacitors are going bad. Hum heard just with it running might just be the transformer and is not uncommon. If the unit has the original capacitors then I'd replace them regardless.

Are you recalibrating your meter when operating the amp? Are you checking input SWR or antenna SWR on the end of the system?

I haven't messed with a Palomar 300a in many years and I haven't looked at the schematic but if it has an SWR tune my guess is it's an input SWR matcher. If so it won't effect antenna system SWR reading. It would be used to adjust the SWR reading between the radio and the amp.

I'm not sure how you are tuning for max but I would tune for max peak power and not max carrier. Those sweep tubes that amps uses don't care much for max carrier on AM even though many manuals tell you to do that as part of the process. Back when those manuals were written those tubes were a few dollars each and available at the local hardware store but that isn't the case anymore. Injecting a tone during TX and tuning for max peak power I feel is the best method but if you don't have a tone generator then a generic audio in the mic will get you at least in the ballpark. Not everyone will likely agree with my tuning suggestion, or suggestions, but it's worked for me for over 20 years now and fortunately I still haven't replaced any tubes in an amp I've used. Some of those amps I owned and used for quite a number of years. I was the definition of long winded when I was younger too.

[Draxx2755]

Thanks so much for your reply. Actually, I believe I had a stupid attack...lol. The hum I was hearing was from the amplified speakers for my computer which is fairly close. When I turned off the speakers completely... no hum... voila! Magic...lol. As for the tuning I've always used an am dead carrier, but will most certainly try your method and inject a tone into the audio for peak reading. I have noticed my swr goes up quite a bit with the dead carrier but I believe I read on another post that checking swr while injecting a tone, the swr should come down. I'm going to try all of that this next weekend. I'll be gone most of this week, but am quite anxious to give it a try. If it is still out quite a bit, I'll try the SWR adjust underneath the unit. Thanks again for the reply and numbers to ya.

[Night Crawler]

The swr adjustment is used to match the impedance between the output of the exciter and the input of the amplifier and only works in the high power position.

If your swr problem is between the amplifiers output and the antenna that adjustment will have no effect in lowering the swr.

[Ohio_359]

I have noticed my swr goes up quite a bit with the dead carrier but I believe I read on another post that checking swr while injecting a tone, the swr should come down.

You want to check SWR with a dead carrier and recalibrate the meter at each power level. The only way I know of to check SWR under modulation is to monitor the forward/reflect an calculate the SWR.

[Draxx2755]

The mismatch I'm encountering is between the radio and the amp. With the amp off or in the "standby" position, SWR is fine. As soon as the amp is switched to the "operate" position on "low" power, the swr shows about 2.5:1. Now this is using only the SWR meter on my Cobra 2000. I have a SWR/Power meter between the amp and my antenna, and that seems to be fine. I have noticed that when I key it to tune and load the amp, that my SWR is out of whack on the radio and the power output of the radio drops to about 2 watts. This is using only the meters on the Cobra 2000 itself. Next I am going to move the other SWR meter between the radio and the amp and see if I get the same readings. I'm wondering if the 10 Meter setting on the amp needs ro be tweeked for 11 meters. Perhaps tapping the coil in a different spot would actually tune it for 11 meters. Just a thought. Thanks for all the replies. They really do help.

[Night Crawler]

The mismatch I'm encountering is between the radio and the amp. With the amp off or in the "standby" position, SWR is fine. As soon as the amp is switched to the "operate" position on "low" power, the swr shows about 2.5:1.

Why the swr is 2.5:1 in the low position is because there isn't a matching network used in the low position only the high.

Most likely the reason why is at the time that amplifier was manufactured it was sold as an amatuer type amplifier and the ham rigs during that period if it used tube finals used a pi-network for the output tuning which would easily match the input impedance of those tubes.

Refer to the schematic [Please login or register to view this link]
If you trace the wiring from the transceiver input through the high/low switch you'll see in the high position there's a pi-network using a variable capacitor,inductor and a fixed capacitor going to the cathodes of the predriver.

For all intents and purposes the predriver stage was installed so the amplifier could be used with only a few watts of drive for use with a cb which doesn't have the ability to be tuned to match the input impedance of the tubes in that amplifier that's the reason for the matching network on the prediver only.

Perhaps tapping the coil in a different spot would actually tune it for 11 meters.

The only reason that would need to be done is if the tune or load capacitors stator and rotor were either fully meshed or fully open when the amplifier is showing maximum output.


If either the tune or load capacitors are fully meshed when tuned for max that means that there isn't enough inductance on the coil and more turns are needed.

If either are fully unmeshed then it needs less inductance or less turns on the coil.

[Draxx2755]

I have a TenTec antenna tuner I haven't used in many years. Would it be prudent to insert that between the radio and the amplifier? And if so, would it still deliver full power to the amplifier? Just an idea. I do realize that the antenna tuner is only kind of fooling the radio into thinking the SWR is good and no reflecting power. I really like the 300a and would like to be able to use it. Any ideas of how I could do that and maintain a decent SWR on low would be appreciated. I could use the "high" side but I do not like to overdrive an amp. Technically this shouldn't happen anyway as my 2000 is only producing about 6 watts. Thanks again for the knowledgeable reply. For those of us who know enough to be dangerous your replies genuinely help us and I am grateful.

[Night Crawler]

I have a TenTec antenna tuner I haven't used in many years. Would it be prudent to insert that between the radio and the amplifier? And if so, would it still deliver full power to the amplifier?

If you have the tuner use it install it between the radio and the amplifier and adjust it for minimum swr.
The only other alternative would be to add an additional tuned input when the amplifier is switched to the low position.

You should notice an increase in output from the amplifier when the tuner is adjusted for minimum swr because your providing a tuned input for the amp.

Simply put what the tuner will do is act like an adjustable impedance transformer between the radio and amplifier when the radio sees the 50 ohm impedance it's looking for it's able to produce it's maximum designed output into the amplifier.

[Draxx2755]

Thanks so much for all the help. Those of us out here who are not as advanced as others are truly lucky to have such knowledgeable and generous people out there. Thanks again!!!