WA2DTW
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« on: October 08, 2009, 09:06:06 AM » |
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I have been driving an AL84 (sweep tube amplifier) with an Elecraft K2/100. In the past, there has been no problem with the SWR from the tranceiver going into the amplifier. Lately, the SWR has gone up to about 2:1 or even higher, and I am getting "high reflected power" warnings on the K2. (the SWR from the amplifier to the antenna is OK; tuned with a Palstar AT4K antenna tuner). The SWR from the tranceiver to the antenna going through the amplifier (with the amplifier turned off) is also OK. Any ideas as to what might be going on?
Many thanks and 73
Steve WA2DTW
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WB2WIK
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Posts: 19940
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« Reply #1 on: October 08, 2009, 10:27:20 AM » |
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The AL-84 does not have a tuned input circuit.
However it does have an internal "swamping" resistor, R1, in series between the RF input port and the four sweep tube cathodes.
"High input SWR" could be the result of a few things, including:
-Bad T-R relay contact switching the input signal to R1 and the tubes
-One or more bad tubes
-R1 drifted in value and is no longer 22 Ohms (it's only a 50W resistor and if used long enough with 100W drive, it can eventually drift or fry)
-Other stuff.
But I'd "guess" it's one of these three. I'd check the relay and R1 first.
WB2WIK/6
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K4DPK
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« Reply #2 on: October 08, 2009, 06:08:29 PM » |
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I think Steve has it nailed.
Just out of curiosity, though, also check the coax jumper between the radio and the amplifier.
Phil C. Sr.
k4dpk
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K4DPK
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« Reply #3 on: October 08, 2009, 06:10:08 PM » |
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woops!
Forget that. It would show up when you bypass the amp.
Phil C. Sr.
k4dpk
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WA2DTW
Member
Posts: 87
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« Reply #4 on: October 09, 2009, 06:34:55 AM » |
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Many thanks, Steve and Phil.
I think the coax cable going to the amplifier is OK, for the reason that Phil mentioned. The question is- what would have been most susceptible to a power surge. The amplifier is OK, and tunes OK, and seems to have normal output. It's just that SWR. Will start by checking the resistor. I also have some spare tubes, so I can switch them and see what happens. Will check the continuity in the relay contacts.
73
Steve WA2DTW
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NO9E
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« Reply #5 on: October 09, 2009, 07:07:33 AM » |
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My K3 shows SWR anomalies on some bands and sometimes with an amplifier. Partly fixed after a trip under warranty. of the K3 SWR readings. May be one cable touching the wrong one or bad connection.
Ignacy
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W8JI
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« Reply #6 on: October 10, 2009, 06:45:52 AM » |
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An increasing SWR on the input of the AL84 is almost always caused by tubes going bad. As they lose emission the cathode impedance increases.
Odds are nearly 100% if you change the tubes to good emission tubes, the SWR will go back down.
Tom
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WA2DTW
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Posts: 87
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« Reply #7 on: October 12, 2009, 10:25:56 AM » |
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Thanks. I tested R1, and it is 19 ohms, probably close enough to the 22 ohms specified. The TR relay seems OK.
The 6MJ6 sweep tubes are becoming more and more difficult to obtain.
I was wondering if it would be practical to convert the AL84 to use four 6146's instead of sweep tubes?
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WA2DTW
Member
Posts: 87
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« Reply #8 on: October 12, 2009, 10:59:27 AM » |
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And if not the 6146 (since the tube characteristics seem quite different)are there any easily available tubes that can be substituted for the sweep tubes with minimal modification?
Thanks and 73
Steve WA2DTW
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W8JI
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« Reply #9 on: October 12, 2009, 01:44:43 PM » |
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You can't use tubes that have the beam forming plates or suppressor tied to the cathode. This makes the amp unstable.
That rules out 6146's and nearly any beam power tube, and several sweep tubes.
You have to use a tube that has a separate suppressor or beam plate connection, so it can be grounded.
Tom
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WB2WIK
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« Reply #10 on: October 12, 2009, 03:24:57 PM » |
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The EL509 is a good candidate and one of the only sweep tubes still in production (in Russia). http://www.tubezone.net/pdf/el509.pdfWB2WIK/6 |
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WA1RNE
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« Reply #11 on: October 13, 2009, 04:25:50 PM » |
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Have you tried RF Parts? If in stock 6JM6's are $5.95 each.
...WA1RNE
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WB2WIK
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Posts: 19940
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« Reply #12 on: October 14, 2009, 11:12:55 AM » |
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>RE: SWR between K2/100 and AL84 Reply
by WA1RNE on October 13, 2009 Mail this to a friend!
Have you tried RF Parts? If in stock 6JM6's are $5.95 each.
...WA1RNE<
You reversed two letters and came up with a price that's 10x off.
The tubes used in the AL84 are 6MJ6, not 6JM6.
RF Parts does have 6MJ6's, but at $59.95/each.
WB2WIK/6
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WA1RNE
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Posts: 809
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« Reply #13 on: October 15, 2009, 05:36:11 PM » |
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Whoops, that's a big difference.
If the amplifier is really worth the effort, maybe consider swapping the 12 pin novar sockets with 9 pin sockets and convert to 6JB6's.
The tubes are cheaper but the sockets may be tough to find.
...WA1RNE
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WA2DTW
Member
Posts: 87
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« Reply #14 on: October 28, 2009, 10:05:37 AM » |
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Many thanks to everyone. The problem seems to have abated after I switched to a shorter feedline between the K2 and the amplifier. There is still an SWR of about 1:8, but nothing seems to be complaining.
73
Steve WA2DTW
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