Gentlemen:
For what it's worth, my experience on this subject.
A friend has a TL922A which gave him problems. First it was fried contacts
on the bandswitch, then a burned out zener bias diode (twice), and finally a
shorted 3-500Z. He reported hearing a sizzling/arcing sound on several
occassions. This all over a period of probably a year or so. We purchased
and installed AG6K's parasitic supressor kit, without using any of the other
parts in the kit. He has not had any problems with the amp in 2 years since.
And as a caveat to any who might be thinking of doing some work on your amp.
I converted my SB220 over to QSK using AG6K's mods. Part of the mod involves
removal of the zener cathode bias diode and installing a string of diodes on
a board and an optoisolator and switching transistor to replace the original
relay. The bias voltage is adjusted by selecting however many diodes in the
string that produces the correct amount of bias. Since I had mounted the
diodes on a board under the chasis, I had the amp laying on it's side on the
bench so it could be accessed to make the adjustment. When I powered it up,
the fireworks began. Ended up with a burned up grid choke, a couple burned
up grid bypass caps, some fried diodes on the HV rectifier board. I am
pretty sure that having the 3-550Z's in a horizontal position was the
culprit, and the grids probably sagged over and shorted to the cathode.
Moral of story: DON'T run your 3-500's laying on their sides.
73 to all.
Tom K0SN
----- Original Message -----
From: "ad5gb" <ad5gb@myway.com>
To: <w8ji@contesting.com>; <ad5gb@myway.com>; <amps@contesting.com>
Sent: Saturday, March 08, 2003 9:16 PM
Subject: Re: [Amps] AL-80B questions
>
> 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@contesting.com > wrote:
> From: Tom Rauch [mailto: w8ji@contesting.com]
> To: ad5gb@myway.com, amps@contesting.com
To: <amps@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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