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Author Topic: Overhauling the AL-811 3 tube amp  (Read 1291 times)
W5DQ
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« on: October 15, 2009, 04:09:12 PM »

I have a the little AL-811 3 tube version amp that needs to have a bit of work done on it. I can do the work easy enough but have never done too much in the area of determining specs on power tubes. I currently have 811 tubes in the amp and have noticed that they are starting to drop in power out. Of course the repairs I need to do are minor. Somehow, the input on the 12/10M circuit failed so it now has a infinite SWR with amp in the circuit. The design of having shared tuned input circuits for some of the bands really stinks since you have to choose which band to favor or find a happy medium and that isn't so easy. I tend to drive the little amp pretty hard and want to get the max out of it. I use it alot on RTTY and CW contesting and turn the loading up a little and drive down after tuning to keep the output at 450W to 500W peak. I don't mind paying a little more for quality parts but I just need to know which ones seem to do the best form others experience.

I plan on purchasing a full set of three 572B tubes to repopulate the amp. My questions are listed below:

1) Which brand is considered better by this forum readers? Having looked at the reviews, I'm still confused. Some like the Chinese no-brands, some like Taylors, some hate Svetlana, some love them while others seem to like anything while others hate the fact that Eimac no longer makes tubes.

2) Do the 3 tubes need to be matched? Seems like they should but then if one tube fails for some reason, there goes the matching premium.

3) Do new 572B in an AL-811 need to be gettered or can you drive'em right off the showroom floor? I see it written both ways?

I still get around 500W max output on 80-20M if I tweak the tuning so the old 811 tubes probably have some life left in them. Used to get around 600-650W max so I figured while I was in there fixing the 10M problem and tweaking the tuning on the 17/15M inputs so to balance it better, I would put in a set of 572B's.

Thanks in advance for any advice.

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
N3JBH
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« Reply #1 on: October 15, 2009, 08:06:06 PM »

800 watts on 3 - 811 tubes is mighty impressive. You think you might be hammering the gal kind hard?Huh
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N3JBH
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« Reply #2 on: October 15, 2009, 08:08:48 PM »

Sorry my eyes  was going bad 6 looked like a 8 my humble apoligee. Dont waste many on matched tubes it is a sales gimmick most of the time....
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N3LCW
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« Reply #3 on: October 15, 2009, 08:58:00 PM »

I replaced my 811A's in my AL-811H amp with Svetlana 572B's from RFParts.  Ameritron customer service told me it is the Svetlana's they sell themselves.

I also replaced the stock fan with a model having nearly twice the CFM rating.  This helps keep the amp cooler especially with the digital modes.

I would not drive the amp any harder than recommended.  While the plate dissipation is greater with the 572B's the grid is not and you can still do damage.

I don't know that I'd run the amp at peak CW power for digital modes even with the new 572B tubes. Ameritron says their 811HD amp with 572B's already installed will run digital at full CW power but I haven't seen the schematic for that amp and don't know if the power supply was upgraded also.

While your at it check the bleeder resistors on the supply caps as well as the plate resistors and replace if they are out of tolerance.

I'm very happy with this amplifier and with the 572B's will probably never have to replace them again.

Andrew
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W5DQ
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« Reply #4 on: October 15, 2009, 10:14:45 PM »

Good idea on the fan. Probably could use it since I have recently gotten hooked on RTTY contests. I try to remember and not drive it too hard but sometimes I do get carried away. The AL-811 seems to handle it pretty well but the 811 tubes don't.

I disassembled the AL-811 this evening and found the center tube with what can be best described as a melt spot on the glass envelope and the plate a weird shape nothing resembling the other two. Guess that tube gaveup the ghost. I didn't find any burnt components on the input tuned circuit area as I first suspected given the very high SWR on 10M. Everything seems to be ok except the dead tube.

My next question is "would it be safe to fire off the amp with only 2 tubes to check the output to see if 10M is now working, after checking all the bleeder and plate resistors as recommended" providing the drive is keep with reason and power out is limited to a couple hundred watts.

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
WB2WIK
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« Reply #5 on: October 17, 2009, 09:26:04 PM »

Burning up the tuned input circuit is highly unusual.

What, exactly, failed??

The 811A is a 70+ year-old design, hasn't been American made in many years.  The 572B can withstand more abuse and is a bit more modern, although no longer American made, either.

I have old JAN RCA 811As built in 1966 that are far better than the brand new ones are today.  I'm keeping them for my grandchildren.
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W5DQ
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« Reply #6 on: October 18, 2009, 11:06:36 AM »

Steve,
You're right. I went through the amp pretty through and did find the input circuits were ok. My first thoughts were based on the fact that the SWR on 10M was high but the am appeared to work on 20M and 15M (only two bands I checked at that time). While going through it, I found what I think was the source of the burnt component smell I experienced early on. The center tube's plate resistor was toast, burnt smack in too. The tube also was toast as I said earlier. I suspect that that may be the problem with 10M but I have been through the entire amp and checked the power supply, relays, and all and the bad tube and burnt resistor is all I have found. My plan is to check the amp using 2 tubes and see that it loads properly. If all is ok, I'll repopulate with some Taylor 572B'sand new components as needed.

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
WB2WIK
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« Reply #7 on: October 18, 2009, 02:11:59 PM »

The "plate resistor" is just a parasitic suppressor with a coil of wire wrapped around it.

That the resistor burned up is not a good thing, but it wouldn't keep the amplifier from operating.  No DC current flows through the resistor at all (since it's shorted out by a coil of wire), it's only there to dampen possible VHF parasitics.

Sounds like an "event" occurred.  New tubes would probably be a good idea.

An "event" could be something simple like an intermittent antenna connection, which made the amp attempt to transmit momentarily into no load.
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W5DQ
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« Reply #8 on: October 18, 2009, 09:18:14 PM »

Not sure what 'event' could have been as I was tuning the amp into a dummy load when the 'smell' occurred - no bang, no pop, no sound - only the burnt component smell. However the 811A looks like a catastrophic meltdown happened as the plate is well deformed. If the PS in indeed in tact as it measures and appears like, I guess I should count my lucky stars.

I'll update this thread when I get more info for those interested.

Gene W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
W8JI
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« Reply #9 on: October 19, 2009, 05:03:51 AM »

That "event" stuff is nonsense.

The parasitic suppressor resistor runs pretty hot on ten or 15 meters meters. This is common on ten meters if you use sustained carrier modes.

The way you can fix that is use the suppressor scheme in the AL811H. It is a series C shunt L suppressor that has much less fundamental RF current in the resistors. I certainly would do that if planning on RTTY or steady carrier modes on 21 MHz or higher.

Also be sure the resistors are upgraded from carbon comp to metal comp resistors. They handle the heat a lot better.

Tom
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W8JI
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« Reply #10 on: October 19, 2009, 07:06:22 PM »

Gene,

You don't have to match tubes so long as they are all about the same emission. Most matched tubes you buy really aren't "matched" in a meaningful way anyway.

<<<
1) Which brand is considered better by this forum readers? Having looked at the reviews, I'm still confused. Some like the Chinese no-brands, some like Taylors, some hate Svetlana, some love them while others seem to like anything while others hate the fact that Eimac no longer makes tubes.>>>

Eimac never made a 572 or 811, so that point is moot. Svetlana sold some really crappy 572B's many years ago, but that may have changed now. Svetlana's poor reputation could be from the early tubes that were mostly bad out of the box.

Ameritron uses so many tubes they would have a good CURRENT idea of what is a good tube brand.

The Chinese tubes, which is probably all you will find beside NOS tubes, are always inconsistent. When I was working looking at tubes, I found the Chinese were testing 572B's only at 1250 volts. They should have been tested for arcing at several thousand volts. That may have changed.


<<<
2) Do the 3 tubes need to be matched? Seems like they should but then if one tube fails for some reason, there goes the matching premium. >>

Matching is generally a joke. To match a tube, the operating curves should be matched. Some people who sell matched tubes actually don't do anything except match batches, while others run the tubes and match power. Neither is the same as matching the curves, and it is generally not necessary anyway.

<<
3) Do new 572B in an AL-811 need to be gettered or can you drive'em right off the showroom floor? I see it written both ways? >>>

I wouldn't know why. The gettering is that gray coating on the anode, zirconium. To be activated it has to hit several hundred degrees C. Without reaching operating temp, it doesn't getter.

When tubes are made they are often run in a carousal. The carousal takes them in and out of an oven where they are heated pretty hot, and high voltage dc is applied to further heat the anodes and internal elements. While this is going on air is being pumped out. The tubes cycle round and round from hot to cooling down until the air is all removed from materials inside the tube.

There are a few different ways to do this, but this is generally the procedure. After that the bases and anode caps and lettering are installed.

One of the most expensive parts of manufacturing is the energy and time consumed pumping the tube down, so there is always a push to cut the time in that process to reduce cost. This results in tube arcs when powered up because they don't have full vacuum.

It's pretty silly to think running a filament alone for a few hours can correct for poor processing. The anode does not get hot enough to activate the gettering. People are dreaming if they think it does. I suppose one in a few hundred might get better, but I've never seen it happen out of the thousands of tubes I've looked at.

We tried everything to getter the bad Svetlana's we bought many years ago, and not a single tube out of several hundred healed. They may have better processing now.

Tom
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