AL-811 with 572B tubes

(1/10) > >>

DAVE CUTHBERT:
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.

John D. Tate:
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

DAVE CUTHBERT:
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.

Owen Duffy:
Quote from: WX7G 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.


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

DAVE CUTHBERT:
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.

Navigation

[0] Message Index

[#] Next page