> Does any man have the experience to mods AL811,AL811H or
> AL572 to 6m?My friend want to purchase one of them(dup to
> the low price) and mods it to 6m band. but I can not
> confiem this,because 811A and 572B are not good at 6m
> band.
> Thanks in advance!
The 811A and 572 have two problems that can affect this.
First, they have high feedthrough capacitance. This requires
neutralization even at upper HF if you use two or more 572
and four or more 811A. Otherwise you will be able to make
the amp oscillate at upper HF. This will certainly be a
bigger issue on six meters.
Second, the 811 and 572 have the lowest frequency parasitic
frequency of any of the popular tubes. The control grid goes
parallel resonant at 100MHz or less. This is one of the very
few tubes that will go unstable so close to 30MHz (3-500Z's
are up around 180-200MHz in contrast). The closer the
control grid parallel resonance to the operating frequency
the more difficult a suppressor design becomes.
I'm sure a 811 or 572 can be made to have gain at 50MHz, but
it is definitely not a preferred tube! I'd stay away from
using them above 30MHz. I'd never try to use the 811H or
AL572 on six meters.
73 Tom
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