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Re: [Amps] AL-811 & 572's

To: W7MJM <>, "" <>, aMPS <>
Subject: Re: [Amps] AL-811 & 572's
From: "Fuqua, Bill L" <>
Date: Tue, 15 Feb 2011 01:13:11 -0500
List-post: <">>
   If you run 572Bs at say 1600 volts on the plates rather than 2400 as in the 
spec I would expect about 20% decrease in output and gain.
Even going by the specs 3 572Bs would produce 900 watts output with 2400 volts 
plate voltage but the drive power would be around 150W. 
I'd expect if you ran 3 with 1600 volts with 900 W output there would be 
considerable distortion. 
   I am just trying to get the claims to agree with the published 
specifications. Somewhere I have seen some specs for 572B's run at lower 
but can't find the source just now.  
   The nice thing about 572Bs is that they are inexpensive. A rather good value 
value in terms of W/$.
Some day I want to build 5 4CX250Bs in GG ( control grid connected to cathode) 
driven by one in GG for a low distortion
low drive amplifier. Should get a power gain of 20 to 25. 
    I have a 6 tube distributed power amplifier to convert.  Just need spare 
Say, who is going to dayton this year? I plan to bring some stuff. I can't 
bring much heavy stuff.  I have some plasma
generator amplfiers and other interesting stuff. Need to clear out the garage a 
bit if possible. Maybe should have
my own hamfest fleamarket. Hi Hi
   Email direct if you are going and looking for something in particular.

Bill wa4lav

From: [] On Behalf Of 
W7MJM []
Sent: Sunday, February 13, 2011 1:49 PM
To:; aMPS
Subject: Re: [Amps] AL-811 & 572's

"Martin I own an AL-811 with three 572B's and get 800 watts out PEP easily
on SSB but run it at 400 watts PEP. I own an AL-811H with four 572B's and
get 900 watts out PEP easily on SSB but run it at 600..." -NZ4Q

Thanks for the info, Thomas. That makes sense to me. If three 572Bs in an
AL-811H (fourth hole empty) can be safely, reliably and linearly driven to
800 watts PEP output in SSB service, then three 572B's in the 3-hole AL-811
should also be cabable of safe, reliable and linear operation when driven to
800 watts PEP output in SSB service.

The power supplies of the two units seem to be identical, so the AL-811 does
not appear to be any more constrained in that department than the AL-811H.

The neutralization argument would appear to be irrelevant as well, since
3-572B's in parallel, like three 811A's, are not quite at the point where
neutralization is required, assuming I'm correctly interpreting Tom Rauch's
explanation elsewhere in this forum as to why the 4-tube AL-811H (when
actually running 4 tubes) requires neutralization whereas the 3-tube AL-811
does not.

In any case, it's often a good idea to run an amp conservatively. Since I
use a bit of RF speech compression (at most 5db), I figure my SSB duty cycle
is slightly higher than unprocessed SSB, so I back off on the drive a bit
and don't run my AL-811, with 3 572B tubes installed, to the absolute limit
of its capabilities. Though the tubes might be able to take it in stride,
other components in the amp would likely be right at their limit.

I understand the AL-811H may have additional refinements over the AL-811
(aside from neutralization). So even if the AL-811, retubed with 572B's, can
be driven to 800 watts output, there may well be reasons other than maximum
power output to buy an AL-811H, as opposed to an AL-811 plus 3 572B tubes.
(The main reason I went for the 3-tube AL-811, even before I began to
consider the implications of a 572B substitution, was that a good one came
up on eBay at a very nice price from a very reliable seller. I didn't really
care whether it was limited to 600 watts as opposed to 800 watts, since the
difference between those power outputs is less than 2 decibels.)

I am of course still interested in hearing what W8JI has to say, should he
care to comment on this discussion. As the designer of the AL-811 series,
Tom Rauch probably knows this amplifier better than anyone.

Once again, thanks to everyone who contributed their thoughts, opinions and
analyses. You've all been helpful.

73 and see you on the air,

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