[Amps] AL811, AL811H or AL572 mods to 6m

[Tom W8JI]

> 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