[Amps] AL-80B questions

Sun Mar 9 08:33:58 EST 2003


>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.

I was curious about the no horizontal operation caveat from Eimac.  Being 
a curious guy,  I operated a 3-500Z on the bench horizontally for a month 
24/7 to see how much sag occurred.  I measured sag by measuring the 
change in breakdown V from the grid to the filament.  The change was c. 
1kv in one month with 5.0V on the filament.  Since a new 3-500Z has a 
grid-fil BD of c. 8000V, my guess is that a day of horizontal operation 
with 5.0v on the filament wouldn't be much of a problem unless one 
operates QRO aeromobile in a stunt plane.    My guess is that the tubes 
which produced the fireworks didn't have straight filament helices before 
the were operated horizontally.  Incidently, a SB-220 breathes a fair 
amount of cooling air if is inverted 180º provided the left side is not 
obstructed.
>
cheers, Tom

>----- Original Message -----
>From: "ad5gb" <ad5gb at myway.com>
>To: <w8ji at contesting.com>; <ad5gb at myway.com>; <amps at 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 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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-  R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K, 
www.vcnet.com/measures.  
end