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Author Topic: Do 3-500's glow  (Read 880 times)
KD2E
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Posts: 305




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« on: July 12, 2019, 05:38:55 PM »

Playing with an L75 amp that I had troubles with in the past, but it may have been related to the setup.
So, differant rig, differant antenna, outlet, table, coax...etc.
Tunes nice, 500 watts or so. Correct current, dips nicely at peak output...all good! But...In just the period of a fairly lengthy CQ, I start to see the plates glow (with lights dimmed in shack) and
the amp switches the fan into high gear.  I know if you see a 6146 tube glowing....it's being pushed!!   But is a 3-500 supposed to start glowing a bit in normal use?   I'm going to put this amp on the bench anyway, blow it out, and change the filter caps...but just wondering if I should be looking at anything else?
Thanks for your thoughts.  I suppose if I got my output, I could just use it 'till something pops. Then I know it wasn't happy!!!
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K6AER
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« Reply #1 on: July 12, 2019, 05:50:17 PM »

Yes, the 3-500Z tubes glow over 300 watts dissipation and that is not a bad thing. When the anodes glow the tube will getter any oxygen molecules and the tube will less likely arc. You don't want the anodes to get white hot.
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KD2E
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« Reply #2 on: July 12, 2019, 08:14:40 PM »

Tonight it went bonkers again. It had been working, then a little while later I was just receiving. I heard the high speed fan kick on, and the tube was glowing a bit, was drawing about 100 mils of current, and no output. What's more, when I keyed down on CW....nothing changed....no output, no change from the 100 mils of current, and no change on the medium red of the plate.  So, something went south. Not sure how the plates could be red, no output, and plate current reading on the meter when on receive!!   Perhaps that tube in there is bad.
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K6AER
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« Reply #3 on: July 12, 2019, 08:23:48 PM »

Possible bad tube or the bias was lost.

Did you have receive signals at the receiver?

Are you sure the ampliifiier was not keyed?

Does the operate light come on?
« Last Edit: July 12, 2019, 08:31:05 PM by K6AER » Logged
WA4JQS
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Posts: 278




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« Reply #4 on: July 12, 2019, 09:43:38 PM »

could be a grid short  in the tube. or the bias in the power supply. i would check the bias voltage first. the Drake is over 25 yrs old now so if you have not upgraded the power supply it is way past due. also check the tube socket clamps if they are blue looking you have a problem with the contact tension,and they will need to be replaced. pull the tube and check for a grid short. these are the first things i would do. good luck
73 Tony WA4JQS
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KD2E
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« Reply #5 on: July 13, 2019, 04:43:25 AM »

My replies to some of the above.
The receiver was still receiving fine, the T/R relay was still clicking fine.
I think here is what I'm gonna do...

I'm gonna throw it on the bench, and change out the filter caps...they are the originals in there.
I am also going to change the tube.
I have 3 or 4 more 3-500's but I don't know the history of ANY of them...I have not purchased a new 3-500 since I've
been licensed in the '70s, so NONE of them are even that new.
So, tube, power supply, and while in there I'll closely inspect everything for signs of shorts, sparks, burnt.
Comment in if there is some more sniffing or investigating I should do while it is gutted open!!
Thanks folks!
....Dave
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W1NK
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« Reply #6 on: July 13, 2019, 05:57:52 AM »

Hopefully, Lou, W1QJ will weigh in with his sage advice...

I had a similar situation with brightly glowing 3-500z (think yellow-orange) & drawing grid current,  (IIRC) while the amp was sitting idle.  If that wasn't troubling enough, the filament would go dark randomly whether sitting idle or when transmitting.  It turned out that I had a grid to filament short that only showed itself while the amp (an AL-80B) was in operation.  Testing the tube "cold" showed no short..
Swapping out the bad tube for a new one from RF Parts was the solution.

Frank, W1NK
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KM1H
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Posts: 5087




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« Reply #7 on: July 13, 2019, 08:38:11 AM »

Here is some sage advice from someone who has more than just a passing experience with that amp and most others going back to 1964.

Any old Eimac 3-500Z is prone to outgassing from the metal anode and will show as a blue glow with just HV applied or being keyed without drive, depending upon the amount of gas. These can usually be regettered but is another step down the road.

Since the series .82 Ohm resistor hasnt blown the tube hasnt arced yet but do check the physical appearance of the grid choke.

That PS is rather prone to problems from age, even 20-30 years ago, and should be overhauled before proceeding and harming things. It is a PITA to work on compared to some others and those pushbutton switches are fragile.

The parasitic suppressor resistors may be well out of tolerance and should be replaced as the amp can go into oscillation without drive.

The Eimac 3-500Z anode will show a small amount of red when keyed for just idle current. The Graphite anode Amperex & Chinese versions will not unless held that way for a long time.
At 500W output the Eimac will get to a mid orange key down which is normal and the graphites a bright red along the lower half. Both tubes require regular use at color to remain gettered, especially Chinese where an internal gas arc is a primary reason for failure. I have a couple of ~ 30 year old Chinese ones plus even older Eimacs used for customer amp bench testing that perform as new. My own primary amp is a 1986 Amp Supply LK-500ZC Ive had since new and is still an easy 1200W even on 10M with ~ 80W drive. Another used primarily as an AM linear is a Hunter 2000C with 24 year old Chinese 3-500G's that get a nice glow on voice peaks Grin

Carl

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KD2E
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Posts: 305




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« Reply #8 on: July 13, 2019, 01:59:33 PM »

Hi Carl!   If I reply to an email from you in 2012, will it still find its' way to you?
Funny, it mentions the 6M SB230 I purchased from you as still chugging along in 2012.
I used it this morning!  Still the same 8873 that you mentioned was weak back when I bought it...perhaps late '90s??
....Dave
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KM1H
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« Reply #9 on: July 13, 2019, 03:29:34 PM »

Check your PM Dave.

Cant say I remember the call but that SB-236 sure is very familiar as I worked you with it a few times and you always sounded real happy Grin. It was one of those amps that others could not stabilize so I took it as a personal challenge. Ive done several others and also have had one for a couple of decades using an 8874 pull I took a hacksaw to, its a rock solid 500-600W amp for roving and portable, and even up into Canada to some rare grids.

Carl
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VR2AX
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« Reply #10 on: July 14, 2019, 06:27:03 AM »

It seems like something heating up. Have you checked the output circuit?
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