[AMPS] 572B and an old FL2000B

2 2@vc.net
Wed, 22 Aug 2001 10:19:20 -0700


>
>Hello Tom
>I'm not convinced that the 572 or the 811 was ever intended
>for reliable operation above 20mhz and would never use
>either of them there.  Didn't you post here a few months ago
>telling us that improper grid grounding was the cause of the
>3cx1200's(d7 & z7)instability and you had found a method to
>ground it correctly?  What sockets are you using to get the
>tube to oscillate?
>Paul


//  Even with direct  grounding, the 811 grid resonates in the low-VHF 
range.    Regeneration is possible above grid-resonance, except for a 
tetrode or pentode.  

>PAUL HEWITT
>WD7S PRODUCTIONS
>QRO HOMEBREW COMPONENTS
>http://wd7s.home.att.net
>----- Original Message -----
>
>>
>> > and see.  You really have to make several mistakes in layout
>> > to require neutralization in a GG amp.
>> > Paul
>> >
>>
>> Not true in the least Paul.
>>
>> A layout has to be extremely poor to require neutralization, but
>> many tubes  have so much anode/cathode feedthrough they
>> require neutralization no matter what the layout.
>>
>> A GG pair of 572B's, perfectly shielded and laid out, are actually
>> not unconditionally stable above 15-25 MHz because of the tube's
>> internal feedthrough. Four 572's are a real major problem.
>>
>> While you can sometimes get away with three 811's in parallel GG,
>> four 811A's are always a problem at higher frequencies.  The Collins
>> 30L1 is one of the few unneutralized 811 amps, and has stability
>> problems above 15 MHz.
>>
//  Heath's Warrior, 4 - 811A amplifier was "neutralized",  Heath never 
made another "neutralized" amplifier.
-   The Warrior was rather well known as an eager oscillator.  

>> 3CX1200A7 and D7's have feedthrough problems above 25MHz,
>> and require neutralization if you expect the anode dip to be
>> anywhere near maximum grid current, although the Z7 cures that
>> with a modified grid.

//  And yet, the AL-1200 currently uses a 3cx1200A7 instead of a 
3cx1200Z7, even though the price difference is no more than a pittance..
>> .
>> 73, Tom W8JI
>> W8JI@contesting.com


-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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