[Amps] AL-80B questions

[ad5gb]

Greetings,

I finally got over to Don's house this evening to take a look at the amplifier.  I didn't realize my dip meter has quit functioning 'till I got there but my DVOM still works.  Filament voltage is 5.1V at the tube socket.  Plate V is 2.9KV.  

The time period Tom, is within the last 6 months or so.  And I found out further that this is the 3rd tube failure and not the 4th.  I misunderstood him during the phone conversation the first night he called me about this.  Further, he's had it around a year or so, and not 6 months like I originally understood, but in an out-of-band email from Ameritron, I found out the amp is still under warranty.

Don was pretty insistent about installing the AG6K Low VHF-Q supressor kit based on the numerous articles written and also based on other amateur's reported success with the kit.  He also told me tonight that he knows a guy in Wyoming with an older AL-80 who apparently had similar problems 'till he installed a different supressor.  We did not install any of the other items in the kit yet.  I left him with a semi-worn Eimac bottle to use 'till his replacement arrives.

For extra luck we used a whole chicken Tom... and not just the leg.  :-)  Since I work for America's largest poultry producer, they're rather abundant.  :-) :-)  <kidding... we liked your joke!>

Power cycling wouldn't seem to be too much of a problem as it's normally on for the duration of operation... be it a 2 hour net or a marathon contest.  He doesn't switch the unit _on when he needs it_ and then _off again 'till he needs it again_.  I do know some people that do this and I wouldn't think it would do anything good at all to tubes.

I know that this has been a hotly debated topic here so I'm a bit reluctant to continue with this thread, but I'm a novice when it comes to RF electronics... but have a huge desire to learn.... scientifically.  Therefore, I'm curious to know if there are others on this reflector who've experienced similar problems (any brand amplifier) that have been successfully solved with the AG6K ( or another design ) VHF supressor. To round out the study, one would also need to know if someone had installed such a thing and found that it *did not* solve the problem it was intended to correct, or if it caused other problems.

I wonder if Amperex is making master cylinders?  I just replaced it on my Volkswagen for the 3rd time this year.


Sooooo much to learn..... Sooooooo little time


Kind regards all

Randy DuCharme (AD5GB)
--
Microsoft is NOT the answer
Microsoft is the question and the answer is NO!

Proud to be 100% Microsoft Free



 --- On Sun 03/02, Tom Rauch < w8ji at contesting.com > wrote:
From: Tom Rauch [mailto: w8ji at contesting.com]
To: ad5gb at myway.com, amps at contesting.com
Date: Sun, 2 Mar 2003 06:47:30 -0500
Subject: Re: [Amps] AL-80B questions


>
> 1: Does this sound like a symptom of parasitic oscillations?
>   I thought the 3-500Z was pretty rugged with respect to faults.

Parasitics, even assuming they would occur, can't cause tube failures unless
you see some solid evidence of excessive current on meters.

I don't know what time period you are talking about, but I've had several
Amperex 3-500Z's fail in my own equipment. Amperex, like all foreign tube
companies, is a bit difficult to work with. My last conversations with
Amperex were only through Richardsons Electronics(who owns Amperex), and
involved 3-500Z grid-to-filament shorts.

I had four 3-500Z tubes in a test fixture cycling only the filament
off-and-on once every minute, and within a day two tubes failed. In a week
they were all bad. All of them failed because the center of the filament
helice moved over and touched the grid.

My conclusion is the repeated thermal cycling of the filament, and a
material or assembly flaw in the tube, caused the helice to move the few
thousands of an inch required to cause a short.

> 2: Does anyone know of any problems with recently built Amperex bottles?

Absolutely there are.

> 3: I'll probably help him install the parasitic supressor kit somethime
this week.  Are there any other things to look for or suggested corrections
to this thing that would make it less prone to eating another tube?

Put a dead-chicken leg over the cabinet, and say a prayer instead. It will
be more effective.

Seriously, ask him how much often he cycles the amp on and off. Also be sure
voltages are set correctly on the power transformer taps, and that there is
no error in filament voltage. Ameritron's transformer vendor, Schumacher,
moved to Mexico about a year or so ago and transformer quality was suddenly
horrible. Bad connections were the least of the problems (it looked like
they soldered transformers with a hot rock), there were also some build
problems in transformers.

I'd assume Ameritron measured filament voltage at the tube, but they might
not have measured it correctly or with a good meter. The quality of
technical people is dropping like a rock as time passes. I'd re-measure the
filament voltage, especially in light of the nature of tube failures.

Make sure your friend isn't needlessly cycling the filament off-and-on,
since the failures are on-and-off cycle related in tests I have done. I knew
a guy who ate 3CX1200D7's in a Henry like pop-corn because he cycled the
filament off and on about ten times a day. He'd get about two months on a
tube.

Remember you are dealing with a white-hot filament that has one common
mechanical support point *at the very bottom*. The filament is about 2-3
inches long when wound in a tight helice, and is thousands of an inch from
the grid. Even if the tube is built properly, it isn't a good idea to
heat-cycle the filament needlessly.

Ask him how often he turns it on and off BEFORE you tell him anything else,
and see if it is abnormal. Also be sure he knows to keep operating grid
current below 125-150mA on carrier.

73 Tom



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