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Author Topic: Should I be concerned about this?  (Read 2173 times)
K3NRX
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« on: November 03, 2017, 06:33:30 AM »

My guess is yes....While transmitting/tuning my AL-84, a loud POP occurred....and nothing came of it.....it still works....no burns, no smoke....no damage...just a good fright....the amp is still transmitting at the full 400 watts it and the tubes are capable of....the SWRs are flat or near flat, and the amp is supposed to take 100 watts drive from the exciter.....any idea what that POP could have been?....was it the result of over driving from the rig?...are my tubes starting to go bad again?...how can I prevent it from happening again?.....any thoughts.....thanks...

V
K3NRX

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K6AER
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« Reply #1 on: November 03, 2017, 06:52:36 AM »

Yes.

Unplug the amplifier and take the top off and look for any visual indication of a component failure. Bypass cap, bleeder resistor, small fried bug. Something happened.
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N7EKU
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« Reply #2 on: November 03, 2017, 07:24:55 AM »

Hi,

Are you tube sets lasting shorter than normal?  If so it may indicate a problem with the amp, antenna system, or tuning procedure.

What was the driving power being used when the pop occured?

What is your antenna and what band were you on when the pop occured?

73,


Mark.
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Mark -- N7EKU/VE3
K3NRX
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« Reply #3 on: November 03, 2017, 07:49:10 AM »

Hi,

Are you tube sets lasting shorter than normal?  If so it may indicate a problem with the amp, antenna system, or tuning procedure.

What was the driving power being used when the pop occured?

What is your antenna and what band were you on when the pop occured?

73,


Mark.

Hi guys....I shall take heed and explore the amps guts...as soon as I get home.....tubes are new....the last ones I had lasted over a year.....I was probably driving 100 watts, which is what the AL-84's tubes can handle....and I was on the 20 meter band on my G5RV.....tuneup procedure is very easy......Thanks for the feedback.....like I said, I will crack open the hood asap...

V
K3NRX

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K6AER
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« Reply #4 on: November 03, 2017, 11:31:36 AM »

Also check the plate cap for arc over spot. A dust bunny might have been between the plates.
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WB8VLC
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Posts: 420




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« Reply #5 on: November 03, 2017, 11:33:02 AM »

Your last tubes only lasted one year?

 I have a late 1990's tube amp with  3-500Z's that's still on the original tubes, another with 3 X 8950's in parallel that's over 20 years old and yes they get used.

My SS amps are original Motorola TMOS  designs from the 1990's at 300 watts for 6 meters and some newer freescale LDMOS 600 watt HF/6meter amps, and a Freescale 1.2 Kw for HF/6 meters  and a 3 year old NXP/freescale 1.5 KW HF/6 meter am that all get used and I have never changed a xsistor in them.

Between 2 different QTHs I have at least one tube and 1 SS amp at each dedicated to 6 meters and HF and I haven't changed any tubes or TMOS or LDMOS fets yet.

Your doing something seriously wrong if your tubes only last just over a year.
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K1ZJH
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« Reply #6 on: November 03, 2017, 12:07:39 PM »

Were you tuning at full power with full drive?  It is possible to create a condition that allows the plates in a tuning cap to arc if the amp is very underloaded.

I keep a log sheet that shows previously settings for various bands and conditions.  That greatly reduces the time needed to get the amp tuned and ready use.  Less stress on the tubes and tank network components as well. 
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VE3TMT
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« Reply #7 on: November 03, 2017, 01:29:17 PM »

Your last tubes only lasted one year?

 I have a late 1990's tube amp with  3-500Z's that's still on the original tubes, another with 3 X 8950's in parallel that's over 20 years old and yes they get used.

My SS amps are original Motorola TMOS  designs from the 1990's at 300 watts for 6 meters and some newer freescale LDMOS 600 watt HF/6meter amps, and a Freescale 1.2 Kw for HF/6 meters  and a 3 year old NXP/freescale 1.5 KW HF/6 meter am that all get used and I have never changed a xsistor in them.

Between 2 different QTHs I have at least one tube and 1 SS amp at each dedicated to 6 meters and HF and I haven't changed any tubes or TMOS or LDMOS fets yet.

Your doing something seriously wrong if your tubes only last just over a year.

The tubes in the AL84 are sweep tubes,  6MJ6's, if I recall correctly. They are notorious for premature failure, especially if tune up is done longer than specified. 3-500Z's they aren't.
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N3QE
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« Reply #8 on: November 03, 2017, 01:52:12 PM »

Your last tubes only lasted one year?
...
Your doing something seriously wrong if your tubes only last just over a year.

Ahem. Sweep tube amplifier.
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N7EKU
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Posts: 706




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« Reply #9 on: November 03, 2017, 02:41:24 PM »

Hi,

The tubes only handle 100W of drive when the amp is tuned up.  Are you keeping the drive low so that the Ip is less than 200mA during tune up?

73,


Mark.

Hi,

Are you tube sets lasting shorter than normal?  If so it may indicate a problem with the amp, antenna system, or tuning procedure.

What was the driving power being used when the pop occured?

What is your antenna and what band were you on when the pop occured?

73,


Mark.

Hi guys....I shall take heed and explore the amps guts...as soon as I get home.....tubes are new....the last ones I had lasted over a year.....I was probably driving 100 watts, which is what the AL-84's tubes can handle....and I was on the 20 meter band on my G5RV.....tuneup procedure is very easy......Thanks for the feedback.....like I said, I will crack open the hood asap...

V
K3NRX


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Mark -- N7EKU/VE3
W1QJ
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Posts: 2605




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« Reply #10 on: November 03, 2017, 03:04:23 PM »

obviously you had a high voltage snap.  It could have been dust or a bug or something like that which will clear itself and never happen again.  Anything serious would persist for sure.  It is prudent however to look for what may explain what happen and see if it is something that needs attention.  You may not find anything and then again you may.  You need to know where to look and what to look for.
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KM1H
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« Reply #11 on: November 03, 2017, 04:46:36 PM »

Give it a good vacuuming then eyeball everything.

The AL-84 is just a CB amp with a bandswitch.
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KOP
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Posts: 234




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« Reply #12 on: November 03, 2017, 06:38:34 PM »

Info on all upgrades can be found on the homepage of SM0O or be derived from the homepage of PA0FRI.
EL519's are a good tube within limits. PA0FRI has some detailed information on much better bias methods as well as some very real expectations of power and longevity .
I've used the EL and PL 509/519 and found them to be not a transmitting tube. Yes they work and work well but they are not transmitting tubes.

I'll just hazard a guess about your loud SNAP. look for the remnants of a moth, fly, roach, bug of some kind. Usually these bits will be found around the tuning caps or around the HV filter caps and diodes. Takes a good eye as in taking the amp apart the evidence will be moved or shifted. What it looks like is a couple of dark spots that "were" bridged by a no longer with us bug. There may or may not be a track left between the two obvious points of contact. I've been told that a black light is also a good diagnostic tool to look for these carbon tracks but I haven't used one.

Nurse this amplifier paperweight along until a better option is found. If not mis-tuned or over driven there should be no problems.


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October 02, 2017, 07:53:41 PM
KOP
Member

Posts: 234




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« Reply #13 on: November 03, 2017, 06:44:11 PM »

Give it a good vacuuming then eyeball everything.

The AL-84 is just a CB amp with a bandswitch.

...come on Carl, :-) "CB" amplifiers were almost never as good as the AL-84 .
THIS is a CB amp


bonus ! who can identify it ?
« Last Edit: November 03, 2017, 06:46:15 PM by KOP » Logged

October 02, 2017, 07:53:41 PM
KM1H
Member

Posts: 2499




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« Reply #14 on: November 04, 2017, 06:00:04 AM »

I wonder if a coupla 4D32's would fit? A pair do a good 250W out on plate modulated AM.
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