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Author Topic: AL-811 center tube plate glowing...  (Read 25717 times)
1HAMWANNAB
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Posts: 7




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« on: February 05, 2017, 03:18:08 PM »

well the AL-811 is acting up... center tube is glowing red hot...

I do a quick tune up... set it up for about 400-450 pep out on my radio shack, center tubes plate starts glowing orange almost red

then hit it with the minimum the radio can produce which causes about 50 watts out on the Radio Shack watt meter

in a matter of a few seconds of carrier the plate starts glowing

I have repeated this on 2 dummy loads and several antennas across several bands...   what should I be looking for?

I figure a simple thing it to mark the tubes and switch them around, allowing me to see if it follows the tube, or stays on the same socket(center)

ideas?

Eric Groce - KD0SRD - W0BZT ARC  Vice President
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1HAMWANNAB
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« Reply #1 on: February 05, 2017, 04:06:02 PM »

Also I might mention I tried this on several bands as well... 

it doesn't matter how much drive I put into it same result, weather I am just barely hitting it (5-10 watts CW) or 50-60 watts, same result
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KM3F
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« Reply #2 on: February 05, 2017, 05:01:26 PM »

Swap the tube positions.  If the same happens to the same tube in another position, it's the tube.
If not the center tube position has a bias issue.
Over heated plate as compared to the other tubes is excessive plate current or loss of control from the grid.
Good luck.
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VK3BL
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« Reply #3 on: February 05, 2017, 05:35:11 PM »

Lots of Chinese / Russian 811 variants will exhibit this in the AL-811 series of amp.

The fact of the matter is the bias alone consumes a good deal of the 811A's rated dissipation.  When you're using multiple tubes, this isn't a good thing as they will never share perfectly, and one of the tubes may end up using all its dissipation just idling.

Funnily enough, when I owned an AL-811, the plates tended to get less hot when running it balls to the wall. 

I doubt there is anything wrong with the amp (that isn't by design); you probably just have a tube that likes to do more than its fair share.  As it burns itself out prematurely, the load sharing will be bought back in line and it will probably stop glowing as much.

I always tuned my AL-811 by watching power output and the colour of the plates.  I'd find out which of my 3 tubes glowed brightest, and then put that tube on the end next to the air vent so I could easily see it.

Don't run any mode other than CW, and you'll have long tube life Wink

Order of tube Life:
1. CW
2. SSB
3. Digital
4. AM

Honestly, just buy a set of 572Bs whilst you can
« Last Edit: February 05, 2017, 05:38:28 PM by VK3BL » Logged

J.D. Mitchell BA  - VK3BL / XU7AGA - https://www.youtube.com/ratemyradio - Honesty & Integrity
1HAMWANNAB
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« Reply #4 on: February 07, 2017, 12:28:24 PM »

I swapped the tubes around...first thing saw was the one that used to have glowing plates took forever to the heater to fire up...

Then I noticed that both the left and center tubes both glowed on the plate... just barely though... almost not noticeable
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WB2WIK
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« Reply #5 on: February 07, 2017, 01:42:49 PM »

811As are "instant on" tubes, the "warmup time" is a second or two.

Sounds like you have either a bad connection at the socket or that tube is a goner.
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VK3BL
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« Reply #6 on: February 07, 2017, 03:54:35 PM »

811As are "instant on" tubes, the "warmup time" is a second or two.

Sounds like you have either a bad connection at the socket or that tube is a goner.

Agreed.  If a filament is taking a while to light up you've got resistance creeping in somewhere.  Check that the pins on the tube are not overly oxidised, and that the socket is clamping nicely to the tube pins.

I have also seen tubes with dry joints inside the pins - often a 'technical tap' with a VDE / High Voltage screwdriver would do the trick, but this wasn't a long term solution.

At any rate, don't worry so much about the plates glowing.  They can develop a cherry red glow without visible damage (i.e., melting the glass or warping the plate), but tube life won't be great.  That said, it won't be great regardless.

The AL-811 is a great amp for chasing the odd DXPedition or checking into a net once a week on SSB.  Beyond that, installing a set of 572B tubes will move you up to a nice 'Medium Duty' amplifier, capable of running 600-800 Watts PEP & 500 watts carrier.

Beyond that, an AL-80B or AL-572 would make a nice medium duty amplifier.  The AL-80B is a nice 800 Watt carrier amplifier, and the AL-572 will do 1Kw carrier reasonably easily.

If you want "proper duty", its hard to beat an AL-1500.  No frills, but like the Heathkit SB-200 it will be round for years.  Honest 2500 Watt PEP amplifier with a fantastic 8877 triode that is currently being made in both the USA and China.
« Last Edit: February 07, 2017, 03:57:24 PM by VK3BL » Logged

J.D. Mitchell BA  - VK3BL / XU7AGA - https://www.youtube.com/ratemyradio - Honesty & Integrity
KM3F
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Posts: 910




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« Reply #7 on: February 07, 2017, 10:18:41 PM »

To evaluate each tube separate for DC operation, plug one tube in each socket, one at a time.
Turn mike gain off (no RF drive to the amp) and key up in SSB mode.
Note the plate current in each socket position for each tube.
This way if one socket has an issue you should see it.
If one tube is low on emission you should see that.
That's nine tests.
It should tell you something about the tubes and the socket positions.
Good luck.
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K1VCT
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« Reply #8 on: February 08, 2017, 03:18:53 PM »


That's nine tests.


Not to be nit-picky, but why cant he test all three tubes in the non-suspect socket, then take a representative "good" tube and test the remaining two sockets, reducing the number of tests from nine to five?  Am I missing something in the presentation of your otherwise excellent logic?
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VK3BL
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« Reply #9 on: February 08, 2017, 05:26:56 PM »

Its likely just a poor connection between the tube and the socket.  Given it was working perfectly before, I suspect a little wiggle is in order and thats about the extent of it.

As previously mentioned, it is not in any way uncommon for one or two of the plates to glow in the AL-811 series.

Remember: Idle / bias plate consumption is around 30 watts / tube.  Thats 2/3rds of the rated CCS dissipation, and its unlikely the tubes will share it equally...

The way I see it, the amplifier is operating within its design parameters.





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J.D. Mitchell BA  - VK3BL / XU7AGA - https://www.youtube.com/ratemyradio - Honesty & Integrity
K4RVN
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« Reply #10 on: February 08, 2017, 08:22:49 PM »

If none of the previous post solves the problem, Check the screws and nuts on the tube sockets for the ground to chassis  to make sure they are tight. Spray each socket with Deoxit after removing the tubes, then reinstall them and try again. I had a three tube AL 811 for three years but no tube problems with the 811As.

Frank
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G3RZP
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« Reply #11 on: February 10, 2017, 03:09:57 AM »

Unfortunately, there is apparently no published data on the spread of plate current for a given bias voltage on the 811A. Unlike the 6146 family which has a published  2:1 spread of plate current for the same bias voltage. I have seen a 3:1 spread on sweep tubes....
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W8JX
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« Reply #12 on: February 10, 2017, 03:22:59 AM »

Unfortunately, there is apparently no published data on the spread of plate current for a given bias voltage on the 811A. Unlike the 6146 family which has a published  2:1 spread of plate current for the same bias voltage. I have seen a 3:1 spread on sweep tubes....

The balance or "spread" of plate current will depend on the health of tubes (emissions of cathode) as current flow is divided. Healthiest will tend to draw the most current and therefore run hottest/have highest dissipation. 
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--------------------------------------
Ham since 1969....  Old School 20wpm REAL Extra Class..
KM3F
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Posts: 910




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« Reply #13 on: February 10, 2017, 06:36:23 PM »

The spread in plate current depends on a number of factors in any given tube.
1. Emission of the cathode or filament.
2. The number one grid condition. Was any turns pushed out of place during assembly, is the grid bowed, is any element out of normal physical spec. at assembly.
3. The bias applied from the control circuit.
4. The plate voltage applied.
5. The Gas content that may be present.
6. Total mis use of the amplifier on tune ups causing tube damage.
.
 If one tube out of 3 has excessive plate current for any of these reasons, there is either a tube fault or the bias is too low causing the plate current to be excessive.
Why does the plate go Orange or Red?
Electrons hitting the plate structure at a rate the structure can not radiate the heat produced away fast through the glass envelope.
Good luck.
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VK3BL
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« Reply #14 on: February 10, 2017, 07:33:28 PM »

If one tube out of 3 has excessive plate current for any of these reasons, there is either a tube fault or the bias is too low causing the plate current to be excessive.

Why does the plate go Orange or Red?

Why do you believe there is a problem?  I have already explained the following:

1) RCA explicitly states that a dull red glow is permissible for ICAS usage.  The tube manufacturer doesn't have a problem with this - why should an Amateur?!

2) The AL-811 series is a 3-4 tube amplifier with NO tube balancing circuitry, biased at 30 watts / tube no signal dissipation.  RCA even STATES that if more than 2 tubes are used, attempts at maintaining balance SHOULD be implemented; they used to be, but Ameritron removed the grid resistors in later models as poor tube quality resulted in them being a failure point.

Given those facts, if *anyone* considers one tube glowing with no drive a fault, they either need to invest in a set of 572B tubes OR a more expensive amplifier.

811As are crap tubes for anything other than radiotelegraphy use.  They were probably alright for Class B modulator service too in a push-pull pair, but they just don't have enough plate dissipation for Class AB radiotelephony usage in a 2+ tube amplifier.  Not if you want them to provide useful power output, last a long time and not glow, anyway.

Please don't treat a glowing plate under zero drive as a fault condition, because it is not.  Its not even like Tom W8JI / Ameritron strayed from RCA's design parameters; if you look at the official datasheets the maximum ratings ARE NOT exceeded.

Repeat after me:

NOT A FAULT CONDITION
NOT A FAULT CONDITION
NOT A FAULT CONDITION
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J.D. Mitchell BA  - VK3BL / XU7AGA - https://www.youtube.com/ratemyradio - Honesty & Integrity
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