SWR between K2/100 and AL84 - Amplifiers Forum at eHam.net

$\'\'$

$\'\'$

$\'\'$

eHam.net Forum : Amplifiers : SWR between K2/100 and AL84

Forum Help

1-10 of 15 messages

Page 1 of 2 Next

SWR between K2/100 and AL84	Reply
by WA2DTW on October 8, 2009	Mail this to a friend!
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

RE: SWR between K2/100 and AL84	Reply
by WB2WIK on October 8, 2009	Mail this to a friend!
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

RE: SWR between K2/100 and AL84	Reply
by K4DPK on October 8, 2009	Mail this to a friend!
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

RE: SWR between K2/100 and AL84	Reply
by K4DPK on October 8, 2009	Mail this to a friend!
woops! Forget that. It would show up when you bypass the amp. Phil C. Sr. k4dpk

RE: SWR between K2/100 and AL84	Reply
by WA2DTW on October 9, 2009	Mail this to a friend!
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

RE: SWR between K2/100 and AL84	Reply
by NO9E on October 9, 2009	Mail this to a friend!
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

RE: SWR between K2/100 and AL84	Reply
by W8JI on October 10, 2009	Mail this to a friend!
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

RE: SWR between K2/100 and AL84	Reply
by WA2DTW on October 12, 2009	Mail this to a friend!
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?

RE: SWR between K2/100 and AL84	Reply
by WA2DTW on October 12, 2009	Mail this to a friend!
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

RE: SWR between K2/100 and AL84	Reply
by W8JI on October 12, 2009	Mail this to a friend!
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

Page 1 of 2 Next

Next Topic: kenwood 922 amp production end??
Previous Topic: Heathkit SB-220

Email Subscription
You are not subscribed to this topic.

Check our help page for help using Forum, or send questions, comments, or suggestions to the Forum Manager.

$\'\'$