Speaking of that. I belive the Johnson 500 ran about 2kV on the plate of the
4-400A and the same on the 811A modulators. Now that is pushing it.
73
Bill wa4lav
At 10:47 PM 4/9/2008 -0400, you wrote:
>At 11:07 PM 4/9/2008, you wrote:
> >The AL-811H runs about 1600-1800 volts HV, not 1400, which is why it's
> >not a bad idea to run the 572Bs in that amp.
> >
> >While the cost difference seems substantial, you'll probably never have
> >to replace a 572B, imported or otherwise, when run in an AL-811H. RF
> >Parts lists the Marshall 811A at $29.45 each and Chinese 572Bs at $39
> >each, so the price difference at this point is substantial, but not so
> >much so, if you consider the increased life expectancy.
> >
> >The size difference between the two tubes does not present a problem in
> >the AL-811H amp.
> >
> >73,
> >Scott, N9AA
>
> Agreed. If one just considers the initial cost of the tubes,
>the 572B alternative is more expensive. Given that you likely will
>never have to change a 572B in an AL-811H (as opposed to Chinese
>clone 811As that have a half life of 30 days), that alone justifies
>it. We are not talking $1000 Eimac 3CX1500A7s here, but $20-$40
>glass tubes. You absolutely cannot get any NEW glass tubes that are
>made in NA. The best you might hope for are NOS 20+ year old
>ones. All new glass tubes are made overseas, and they are of
>inferior quality, even the 572Bs. The reason they last forever in an
>811A application is because they are running at 2/3 their
>capacity. It's not because they are better built, just taxed far less.
>
> Putting aside the cost difference of four tubes, the real issue
>here is: Do you want to take the 19 screws out of the AL-811H cover
>every month and change a tube or two? That's why I switched to
>572Bs. You are in the middle of a contest, chasing DX, or just
>rag-chewing and "BANG!" Get out the screwdriver and spend 30-45
>minutes figuring out which one (or ones) blew, replace it, and then
>get back to whatever you were doing. Or put 572Bs in and forget them.
>
>73/DX Paul VE1DX
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