[Amps] AL-811 & 572's

[W7MJM]

Oops. That quote was from NZ40, not NZ4Q. Got to clean my eyeglasses! Sorry 
for getting your call wrong, Thomas. -MM

----- Original Message ----- 
From: "W7MJM" <w7mjm at arrl.net>
To: <nz4o at tampabay.rr.com>; "aMPS" <amps at contesting.com>
Sent: Sunday, February 13, 2011 10:49 AM
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,
> Martin
> W7MJM