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
Joe Subich, W4TV wrote:
> Why waste the money ... four 572Bs are half again to twice the
> price of four 811s. The 572B will not provide any more gain or
> power output than the 811 unless you modify the power supply for
> about 40% more voltage (2 KV vs. 1400 V).
>
> The 572B is taller and wider which might represent a problem
> (moving the plate caps too close to the metal case or overly
> restricting air flow between the tubes).
>
> If you are really concerned about reliability pick one of the
> better 811 suppliers - perhaps Marshall (made in the USA) -
> but I would not waste the money and effort on imported 572Bs.
>
> 73,
>
> ... Joe, W4TV
>
>
>
>>
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