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

[R L Measures]

On Aug 2, 2006, at 2:06 AM, Tom W8JI wrote:

>> 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.

Mr. Rauch:  What is the feedback-C of an 811A?, of a 572B?  Tnx

> 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
>

R L MEASURES, AG6K. 805-386-3734
r at somis.org