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Author Topic: AL-811 with 572B tubes  (Read 15325 times)
WX7G
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« on: April 06, 2010, 06:08:50 PM »

The question has come up as to what happens when the Ameritron AL-811 is retubed with 572B tubes.

I tried this today. With Taylor 811A tubes the output begins to saturate above 600 watts. With Svetlana 572B tubes the output begins to saturate at 700 watts. Both sets of tubes have less than 200 hours on them. The LOAD and PLATE control settings are just about the same with either tube.

The plate voltage under load is 1400 volts.
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KX5JT
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« Reply #1 on: April 06, 2010, 10:13:01 PM »

There shouldn't be much difference in your output.  The 572B is basically an 811A with a large graphite anode that can dissipate 160 watts.  Given the same parameters, i.e. plate voltage, you'll see the same performance.  The 572B's should last a lot longer since you are basically "babying" them with much less plate voltage than they can take.  If you want to pay more for the 572B's, go ahead.  Doesn't make much sense to me however.

john
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WX7G
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« Reply #2 on: April 07, 2010, 12:49:00 PM »

But will a 572B outlast an 811A in an Ameritron AL-811 amplifier?

Let's examine three common failure modes for these tubes:
1) Cathode depletion (low electron emission)
2) Anode damage (over heating)
3) Loss of hard vacuum (out gassing of electrodes and failure of the getter to perform its function)

Cathode depletion is the same assuming the cathodes are the same.

Anode damage is much less likely with the 572B but if not abused there is no damage to either tube.

Loss of hard vacuum. The anode is the primary source of outgassing. It is also the primary getter element. The secondary getter element is the Barium flashed near the tube base in the 811A.

The 572B anode runs cooler, or does not hot spot, because it is thicker and has a greater surface area to radiate heat from. To getter, the anode coating must reach a high enough temperature. The 572B anode my not may reach this temperature. Additionally, the 572B does not have a barium getter. Therefore the 811A is possibly more robust as far as maintaining a hard vacuum.

The glass envelope temperature will be the same for either tube. Adsorbed gas will be the same for either tube.

The end result is that the 811A should provide equal or longer life when used in the AL-811 and not operated beyond an average anode dissipation of 40 watts.

So, install 572B tubes and pay more money for less life.
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VK1OD
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« Reply #3 on: April 07, 2010, 01:55:21 PM »

But will a 572B outlast an 811A in an Ameritron AL-811 amplifier?

Let's examine three common failure modes for these tubes:
1) Cathode depletion (low electron emission)
2) Anode damage (over heating)
3) Loss of hard vacuum (out gassing of electrodes and failure of the getter to perform its function)

Cathode depletion is the same assuming the cathodes are the same.

Anode damage is much less likely with the 572B but if not abused there is no damage to either tube.

Loss of hard vacuum. The anode is the primary source of outgassing. It is also the primary getter element. The secondary getter element is the Barium flashed near the tube base in the 811A.

The 572B anode runs cooler, or does not hot spot, because it is thicker and has a greater surface area to radiate heat from. To getter, the anode coating must reach a high enough temperature. The 572B anode my not may reach this temperature. Additionally, the 572B does not have a barium getter. Therefore the 811A is possibly more robust as far as maintaining a hard vacuum.

The glass envelope temperature will be the same for either tube. Adsorbed gas will be the same for either tube.

The end result is that the 811A should provide equal or longer life when used in the AL-811 and not operated beyond an average anode dissipation of 40 watts.

So, install 572B tubes and pay more money for less life.

I note the progression from "The 572B anode my (sic) not may reach this temperature", "811A is possibly more robust", a conditional "811A should provide equal or longer life" to the more definitive conclusion "install 572B tubes and pay more money for less life".

It seems like you have been convinced by your own qualified workup.

You didn't state the time over which your 40W anode dissipation is averaged. Is it a minute, an hour, a day, a year, something else.

My 811A operated on the lowest output voltage from a 240V mains supply operates at 1750+ DC idle with no signal, idle current is nominally 125mA (but can be higher depending on tubes), so idle dissipation is 55W a tube, well over your unjustified 40W criteria.

Operation of the AL811H under its rated maximum supply voltage typically significantly exceeds the RCA datasheet ICAS ratings for the 811A, up to 400V higher DC supply voltage, and anode dissipation depending on mode / power level.

I am not being defensive, just you make really soft statements, no justification that gettering is inadequate below any particular anode temperature or dissipation level, and wind up with a clear definitive bottom line after pseudo justification.

Owen
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WX7G
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« Reply #4 on: April 07, 2010, 02:30:24 PM »

40 watt is the CCS rating while 65 watts is the the ICAS rating.

Think about it. Tube gettering may be better with the 811A due to a hotter anode and the fact that it has a barium getter. That means longer on time before the gas pressure rises to a damaging level.

Gas pressue, 811A possibly superior.
Cathode life, no difference.
Grid life, no difference.
Anode damage, no problem unless the 811A is abused.

The ICAS anode rating of the 811A is never violated when operated per Ameritron's instructions. For the AL-811 it is 800W SSB, 600 W CW, 300 W RTTY.

So, while I have not proved conclusively that the 572B will outlast an 811A in the AL-811, my argument leads to that inferrence while casting doubt on the oft-heard statement that the 572B will last longer because it is 'loafing.'

I have designed much high voltage circuitry and the one thing I have found that can be violated with impunity is the plate voltage rating.

I do wonder about the higher plate voltage rating of the 572B. We know that these two tubes have the same characteristic curves and the same cathode. So the grid spacing must be the same. And the plate spacing must be the same or nearly so. Where then does the higher plate voltage rating come from except that the 572B plate can dissipate more and so the plate voltage can be higher for the same plate current. We might conclude that the tube manufacturers maximum operating numbers are based on plate dissipation, grid dissipation, and cathode current and not on plate-to-grid breakdown voltage.

Either that or being a high-mu triode the plate spacing can be greater without a significant change in the characteristic curves. If so, then the plate could be futher from the grid. I'll take a look at some tubes I have. I should break two open and measure the distance but I'll see if I can eyeball it.

My 811A vs. 572B life analysis is not meant to be conclusive. It is meant to foster thought and debate.
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VK1OD
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« Reply #5 on: April 07, 2010, 03:13:40 PM »

There shouldn't be much difference in your output. ...

There isn't in SSB telephony where the lower anode dissipation of the 811A is not too limiting.

But, the 572B can sustain higher duty cycle modes at higher power, eg AM. For a more detailed analysis, see Ameritron AL811H tube selection.

Even if you only intend using SSB telephony, the 572B will endure prolonged tuning sessions better.

Owen
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WX7G
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« Reply #6 on: April 07, 2010, 08:05:24 PM »

Tuning sessions should be conducted at reduced duty. I use a keyer to tune at 50% duty.

Here is a fascinating paper on tube gettering. The zirconium on the 811A and 572B plates does getter at only 400 deg C. This is lower than I had been led to believe. This means the plates do not have to be heated to a dull red for gettering to occur. The paper on gettering points out advantages to zirconium on the anode aided by a barium flash. The 811A tubes I have use barium flash getters near the base.

So, which tube will last longer? I think that theoretcal arguments can be made for each. Probably the best data will come from hams who have worn out 811A's and then switched to 572B's. Do the 572B's last longer?

http://www.thevalvepage.com/valvetek/getter/getter.htm
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VK1OD
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« Reply #7 on: April 07, 2010, 10:53:27 PM »

Tuning sessions should be conducted at reduced duty. I use a keyer to tune at 50% duty.
...

I did some experiments to try to find a pulsed sine wave that doesn't sag the power supply in the AL811H any more than voice SSB so that when tuning, you are actually replicating the PA operating conditions under voice.

I wound up with a 10% duty cycle, phase switched 1kHz sine wave, with 10ms ON, 90ms OFF. It is integrated into a beep device used for weak signal VHF/UHF working, and under the covers of the TS2000. The low duty cycle was necessary to avoid power supply sag, and allows the AL811H to operate into an 80W continuous rated load indefinitely.

Experiments such as this are more for understanding the technology than QSOing.

Owen
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KX5JT
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« Reply #8 on: April 08, 2010, 12:41:41 AM »

This is a fascinating thread...  everyone believes the 572B's will last longer because they are being "babied" when run as an 811A replacement.  Now I really don't know but I would love to hear Tom W8JI chime in or Carl K1MH.  

I have heard someone post (and I believe this fella) they had run 811A's with a hole in the plate and it still had full output!!

If I had an 811A amplifier, I don't think I would spend double for 572B's.  I really doubt 572B's will last twice as long... in fact, the 572B's nowadays seem to be much more flaky than 811A's, at least I read about more problems.  

John
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K2TY
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« Reply #9 on: April 08, 2010, 06:48:23 AM »

"Additionally, the 572B does not have a barium getter. Therefore the 811A is possibly more robust as far as maintaining a hard vacuum."

I'm not sure so I'm asking. Isn't a flash getter a one shot deal? When the manufacturer fires the getter it deposits the coating on tube adsorbing the stray O2 & N2 molecules and it's done. I thought that a flash getter was just part of the manufacturing process and that the ongoing gettering (in 811s, 572s, 3-500s, etc) was carried out by the getter material on the plate. In fact the mirror like coating on flashed tubes will turn white if exposed to atmosphere. Perhaps this only happens with severe leaks?

From a similar thread:

"The 811A tubes in my amp have barium getters near the base. One wasn't even flashed. " -http://www.eham.net/forums/Amplifiers/17462

I assume that that tube performs correctly. I'd be interested to hear how it lasts compared to the others.

I replaced the 811s in my 811 with 572s. My experience was much the same as the first poster, ever so slightly more output and similar tuning. Where I did see a difference was noticed by observing the plates. At the time, I was playing with SSTV. While operating the amp within the limits specified in the Ameritron manual, the plates of the 811s glowed noticeably. But while operating the same mode, plate and grid currents with 572s, the plates do not appear to glow. When I swapped the tubes, one of the 811s had the classic oval damage to the plate. Yes, the 572s cost more than twice as much and 811s can if treated carefully be made to last a very long time. In my opinion, the 572s are more forgiving and tolerant of high duty cycles. My opinion and 75 cents will get you a cup of cheap coffee. However an extra $70 will get you 3 572s.
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WX7G
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« Reply #10 on: April 08, 2010, 07:30:23 AM »

The barium getter continues to getter until all of the barium has combined with oxygen and other gasses.

When we think of component reliability we usually think of wear-out mechanisms: Thermal cycling of resistors, gate oxide damage for MOSFETs, that sort of thing. But tubes not only have wear-out mechanisms they have 'healing' mechanisms.

Run them for a long time with no HV and no cathode current and the cathode can become poisoned. Run a zirconimum gettered tube too cool and it can become gassy. For a healthy and long lived 811A or 572B tube it needs some cathode current and it needs a high enough plate temperature. It needs a bit of 'stress.'

What I'm getting at is that it is not as simple as saying that a 572B will last longer in the AL-811 simply because the plate runs cooler.
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WI7B
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« Reply #11 on: April 08, 2010, 10:55:42 AM »

OK, let's get "our" facts straight. The Svetlana 572B has an extremely effective getting system using titanium, not barium.

The Svetlana 572B has a superior getter system based on titanium adhered to the external surface of the graphite anode. The titanium coating covers the entire anode area extended by the inherent surface roughness of graphite. Further, the titanium operates over the ENTIRE anode temperature gradient thus assuring absorption of a wide spectrum of gases.

Titanium has an index of gas absorption 6-10 times greater than barium. This getter system is far more effective in comparison to cost saving barium getters silvered on the inside surface of glass envelopes commonly used in 811A tubes. The Svetlana 572B envelope is fabricated from hard glass intended specifically for the high-temperature operation of transmitting tubes.

73,

---* Ken
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WX7G
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« Reply #12 on: April 08, 2010, 01:50:48 PM »

WI7B, thanks for the info.

You are correct and as I pointed out earlier the Svetlana 572B uses a zirconium getter on the anode. Zirconium getters are generally used above 800 deg C but do some gettering temperature down to 400 deg C. Barium getters are used up to 200 deg C.

The quote you have about the zirconium getter operating over the entire anode temperature GRADIENT: What does this mean? It says gradient and not range. Gradient in this case may refer to the anode temperature at any spot on the anode when operating at rated anode dissipation.

And yes the Svetlana 572B uses hard glass. The Svetlana 811A also uses hard glass.

I don't have a Svetlana 811A in my hand, but I do have a Taylor. This tube does have barium flashed near the base. Assuming it also has zirconium on the anode it has two getters. From what I gather the primary purpose of the barium getter is to reduce vacuum pump-down time. A secondary benefit is that the tube has a getter that operates from room temperature to 200 deg C. It will getter when the tube is stored. During storage a glass tube leaks gas thru the metal-glass seals. The 572B does not have this two-getter feature and the zirconium getter will not become fully active until the anode reaches a dull red glow.
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WX7G
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« Reply #13 on: April 08, 2010, 07:44:07 PM »

I think we beat this one to death.

Contrary to my theoretical anaysis, if I had to bet money on it I would bet on the 572B lasting longer in the AL-811.

In my AL-811 I'm running 600 watts on CW and the plates show no color. I'll see how they hold up and when they die I will install 572Bs.
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KX5JT
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« Reply #14 on: April 08, 2010, 08:08:49 PM »

So, I have an SB-200 and a Henry 2K4 floor console model.  I run a lot of continuous duty modes, mostly AM.  The 572B's in the SB-200 start blushing and showing some color after about 45 seconds of 100 watt carrier.  All of the brands did, whether Sylvania (made in the USA) Shuguang or Svetlanas.  The Eimac 3-500Z's in the Henry also blush a bit, but the manual has stated that this is normal.  As long as the cooling system is in place they will show some color at continuous duty.

I've noticed a lot of people thinking that any plates that show color must be bad tubes or they are being pushed too hard.  

Opinions? Facts? What say ye!?
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