[Amps] AL-811 & 572's

[W7MJM]

Thanks Bob and all those who responded to my queries. I now have a better 
understanding of the AL-811 vs. AL811H amplifier capabilities when retubed 
with three 572B tubes.

To be sure, I'm not looking to squeeze out more wattage than these amps can 
reliably, and linearly, deliver, nor am I considering a bigger amp. I'm 
happy with 600 watts and have no desire at this time for more RF power. I 
was just curious about W8JI's suggestion some time back about running only 
three 572B tubes in the AL-811H, presumably at 800 watts PEP output, as 
opposed to running a trio of 572B tubes in the AL-811 at 600 watts PEP 
output.

>From the various comments, I now understand that the power supply in the 
AL-811H is what allows three 572B tubes (or four 811A tubes) to generate 800 
watts PEP output, whereas the power supply in the AL-811 is what limits the 
output to 600 watts regardless of whether you're running a trio of 572B's or 
811A's.

When you come right down to it, it's the transformer that delivers the 
power, though I suppose you could take it further up the line to the power 
company's generating plant. The tubes are, well, "valves" as they say on the 
other side of the pond.

In any case, my AL-811 continues to serve me well, with good reports and no 
issues three or four years after swapping out the 811A's for 572B's.

Incidentally, when I removed the 811A's, which I had been using for maybe 
three years at the time I swapped them out, they showed the tell-tale silver 
plate welts, indicating that I had overheated them; sufficient reason for me 
to go to 572B's for their additional anode heat dissipation capabilities.

73,
Martin
W7MJM


----- Original Message ----- 
From: "Robert Kirkland" <alpha76 at triton.net>
To: <w7mjm at arrl.net>
Sent: Wednesday, February 09, 2011 7:17 PM
Subject: [Amps] AL-811 & 572's


> Martin,
> The amount of plate current a triode tube will draw depends on it's plate 
> voltage. The 572B is usually operated from 2200 to 2700 volts. The 811A 
> usually operates around  1500 to 1800 volts, be careful, some brands of 
> foreign 811A's are not rated for as much plate current as is necessary. 
> The full load voltage on the AL-811 is 1500 and the plate current is 550 
> ma. That's about 600 Watts output, with the 572B's there is not enough 
> plate voltage and plate current to produce much more. The only reason to 
> install 572B's is, used at these low ratings the tubes will last forever 
> and you won't over heat them on a long tune up. You could increase the 
> voltage by messing around with the power transformer primary input 
> settings but then the filament voltage would be way out of limits and 
> surely the transformer would over heat. If I had an AL-811 Linear and I 
> wanted more power I'd trade it in on another amp, or try to improve the 
> antenna.
> Bob,
> w5pvr
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