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Author Topic: AL-80b low grid current/Power output  (Read 30393 times)
W1BR
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« Reply #30 on: November 24, 2014, 10:07:48 PM »

The Omni has internal ALC control, and that can be used to limit the PEP drive power to an external amp. Regardless of the method used it is important to use a monitor scope if the exciter is capable of over driving an external amplifier.

Peter
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NC5T
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Posts: 44




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« Reply #31 on: November 25, 2014, 05:21:29 AM »

Good Morning and thanks again for the replies!  I agree with you 100% on the Omni being a great rig Frank.  It was night and day going from an old Omni D and a Yeasu 857D to the Omni 6+!  I'm just glad I was able to get one that had been well taken care of.

  Well, I tried driving the amp with 100w to see what would happen and the grid and power out came up a bit, maybe an extra 100-150w output, but not proportional to the drive increase.

  I also checked the AC line and it hardly budged at all under load, so I think I am good there as well.

  I had an interesting discussion with a local ham, who is much more knowledgeable on amps than I am, and looking at the numbers, the suggestion that one of the doorknob caps might have gone bad came up.  I am going to check this out today and see where that goes because that would explain the Load setting being so low, not enough capacitance, and might help out with the power issue as well.

  Although the tube might be soft, it doesn't make sense that it would have a lower power out on 80 and 160 than on 20-10.  Everything I have heard or read says the opposite, that the higher bands go first if the tube is going.

73,

Bob
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W3RSW
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« Reply #32 on: November 25, 2014, 05:35:40 AM »

I think you've probably hit on the right solution.
Alpha and others use several smaller value, but still high voltage load padding capacitors in parallel to handle the now distributed current in each cap.  These are lashed up in a bank using strapping on the hot side.  If using more expensive Doorknobs, you still still use two or three in parallel.  Say three at 10 amp rating each for a legal limit amp.
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Rick, W3RSW
K4RVN
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Posts: 261




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« Reply #33 on: November 25, 2014, 07:48:37 AM »

I am about to get beyond my knowledge scope on the AL 80 B, but I think the padder door knobs would only be used for 80 and 160. There are several on here that would know that's why I threw that statement out for correction. The higher bands need less capacitance is my view which most likely could be wrong as my old memory bank is full.

Frank
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WB2WIK
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« Reply #34 on: November 25, 2014, 09:33:59 AM »

The padder caps used by Ameritron are actually very good and unlikely to fail, but anything's possible.

What is "more" likely to fail is the bandswitch itself, as one wafer of the switch is used to switch in those caps on the lower bands and if -- God forbid -- the bandswitch was ever rotated while transmitting, that can do a lot of damage to the switch.

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WA7PRC
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« Reply #35 on: November 25, 2014, 06:08:04 PM »

One thing to consider is temperature stability for capacitors used in tuned circuits. Tom W8JI ripped a bunch of superfluous (and wrong) parts out of a TL922 (link), which included non-stable doorknob capacitors in the output network. Bypass and coupling capacitors are usually not critical but, caps that affect resonance should remain stable with temperature variations.

It must've been interesting for the op to try to keep the amplifier tuned as its internal temperature went up... and then down... etc.  Wink

vy 73,
Bryan WA7PRC
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K4RVN
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« Reply #36 on: November 25, 2014, 08:17:28 PM »

I still vote for the bad tube as the door knob padders are probably only used on 160, maybe 80 meter bands. If that is not the right answer, then Steve, Lou, or some of the better qualified posters should
correct that thought. If 100 watts won't drive the tube grid current up to specs, then it points to the tube as the load control has been turned counter clockwise to increase  the grid current and it did not solve the low grid current when tuning.
I already mentioned to check all grounds and included the caps.

Frank
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NC5T
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« Reply #37 on: November 26, 2014, 05:20:57 AM »

Good Morning and thanks again for the replies and suggestions. 

  Well, I went through it again last night checking all if the connections and looking for problems and, as best as I can see, I didn't find anything visibly wrong.  No scorch marks, no cracks in the switch wafers, and the tube looks brand new.

  I checked to make sure the door knob caps, which also look new, were switching in as they should and no problems there.  I was surprised to see the 500pf cap looked like a chip and not a door knob cap.

  I also checked and there are no open grounds (checked with a DMM) that I could find, so I cleaned the switch contacts with deoxit and pro gold, cleaned the relay contacts on the T/R relay, and closed it back up.

  I took everything else out of line and hooked the amp straight to the radio and then to the dummy load and got some curious results.  According to the amp's meters, the power out came up a bit more on the lower bands and even though the settings for the plate and load are different than they were, I am still having the low Load setting and grid current issues I was having before.   

  I also noticed that the radio was showing close to 2:1 SWR while tuning the amp on 40M so I am not sure what is going on there as it was not that high before, or I didn't notice it as I was watching the external meters and not checking the one on the radio.

  I will make a chart for all of the bands again today with just the meters hooked up and see what I get, but it looks like I am still getting more output on the higher bands than the lower which, from my research, seems to point to something other than the tube, I just am not sure what.

73,

Bob
NC5T
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N3DT
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« Reply #38 on: November 26, 2014, 06:06:26 AM »

The input SWR on the AL80B should be very good. My TS2000 shows very small reflected power (certainly much less than 1.5:1) on any band except 30 but I don't use it there. There are input slugs to tune if it's off, or maybe somethings wrong with the input circuit?

When I got mine, I finally found one of those orange caps in the plate circuit had a broken lead, didn't seem to make much difference putting it back on. I think the tank circuit is different on 160 and 80 than the rest of the bands, but I'm just repeating what I've heard, that switched circuit is hard to read without re-drawing it without the switch.
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K4RVN
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« Reply #39 on: November 26, 2014, 07:42:52 AM »

Bob have been following this thread because I have the AL 80B and anxious to see what caused this problem you are having. I have done a lot of searching on the net also and read that a weak tube can cause high SWR on most bands. as well as low grid current.  I tuned my inputs on the AL 80 A I also own, and a previous AL 811 when I could not get rated watts on some bands. My used AL80 B was ok when I bought it at 6 years old. The new tube would eliminate that possible problem, but costs over a hundred bucks. If you find nothing else, I would suggest tuning the inputs.
I did mine by putting about 5 to 10 watts drive into the amp on a dummy load , then tuning the amp for max output on the watt meter. Next I tuned the input slugs for the bands for max output while also checking the SWR seen by the transceiver . That is not the right way probably but worked for me and I am not suggesting you use my procedure. I would suggest that you check with Ameritron or someone who has tuned the inputs the proper method. I would also suggest that you set the load at the preset in the manual, forget the low grid reading, use 70 watts drive and tune the plate for max watts out to compare readings to the 100 watt drive you did with less loading. Once the max watts out possible is reached with 70 watts the drive should not be increased unless more power out is seen. Good luck and hang in there, you will find it and fix it. The Deoxit  5 is wonderful stuff, and that's what I have used to solve a band switch problem on 40 meters.

Frank
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NO2A
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Posts: 1400




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« Reply #40 on: November 26, 2014, 03:18:54 PM »

Bob,when you drive the amp with 100 watts what grid current are you seeing? I know the normal drive level would be less,say 70-80 for full output. If you overdrive the amp,or drive it without being properly loadedyou should be able to get the grid to read at least 150-200ma current. If you are not able to do this,the tube is weak. You stated the grid current didn`t go up proportionetly with the power input as it should. The only other possible issue would be if the amp wasn`t getting enough drive. Regardless of what the amp puts out on 80/160,the fact is you should be able to get the grid to show 150-200ma while tuning for testing purposes. What is the max lg you`re seeing?
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N3DT
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« Reply #41 on: November 26, 2014, 03:20:56 PM »

I called ameritron about tuning the input and they said they use 30W to tune the input. But call them, they will talk to you.
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NO2A
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« Reply #42 on: November 26, 2014, 03:26:10 PM »

I doubt his input is off on every band.
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W1QJ
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« Reply #43 on: November 26, 2014, 04:24:07 PM »

The person who designed the amp prefers that you drive the amp to full rated output and check to see what the input swr is at that point.  If you can't obtain rated output from the amp that indicates another problem that may not be related to  adjusting the input.  Then by adjusting you may bei doing something that really should not be done.  If the amp is new I would doubt they need adjusting unless you are going for  a tweak on the warc bands.
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K4RVN
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« Reply #44 on: November 26, 2014, 09:21:46 PM »

Bob when you make your chart please include 17 meters. The 15 meter and 17 meter band switch are common so that may tell you something. The tuned input could favor 17 meters giving more watts out and  a different grid current reading.  I set mine to favor 17 meters over 15, and 12 meters over 10 meters since they also share the band switch. You may have included 17 meters and I overlooked it, but don't see it. I doubt they are off and suggested to try the tuned inputs if you find nothing else. I still think you have a soft tube so would not do anything myself until I put in another tube and tried it to eliminate that possibility.

Frank
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