Tom Rauch wrote:
>> > One factor that apparently
>> >increased the Warrior's VHF instability was so-called
>> "So-called" is correct. According to the schematics, it consisted of a
>> feedback capacitor from the output end of the pi-tank, back to the
>> cathode. This will produce feedback of variable amplitude and phase,
>> depending on the settings of the bandswitch and the tuning and loading
>> controls. It's hardly surprising that the amp was more liable to
>> oscillate at some frequency.
>That is NOT correct Ian. Bad enough that Rich says things without
>being fully aware what is going on!
>The feedback in the Warrior is from the anode, through a capacitor,
>through a phase inverting transformer, to the filaments.
>The tuning of the tank has a very minimal effect on phase shift.
>This is a simple modified bridge system.
>I think you are wrongly confusing it with the system in a Yaesu
>572B amplifier where feedback is taken AFTER the output tank,
>which of course WOULD shift phase with tank tuning and
>destabilize the amplifier at HF with some tank settings.
Sorry, it appears I was confusing the Warrior with another amp... maybe
the Yaesu, though I don't recall ever seen that schematic.
I agree that a phase inverting transformer, coupled as you describe,
produces a bridge neutralizing arrangement which is pretty much as
broadband as the transformer itself.
>> >Gonset also used neutralization in their 4, 811 amplifier.
>> I'm not familiar with that schematic: would that be real
>> neutralization, or more of the bogus kind?
>Before you say "bogus", you should look at the system. It works
>very well in Heath's amp and other amps. It has nothing to do with
>VHF oscilations, it is to stabilize the amp at upper HF.
I applied the term "bogus" to the system I described, not the one that
is actually in the Heath Warrior.
>Let's not feed people trying to learn misinformation Ian!
Sorry, I was misinformed myself!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
FAQ on WWW: http://www.contesting.com/FAQ/amps
Administrative requests: amps-REQUEST@contesting.com