"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,
Martin
W7MJM
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