Sounds neat!
Mike wy6k
--- Tom Rauch <w8ji@contesting.com> wrote:
>
> > But then why does nearly every current-day HF
> amplifier manufacturer use
> > "1935 technology" with the inclusion of the
> classic coil turned around a
> > carbon composition resistor for the purpose of
> serving as a parasitic
> > suppressor?
>
> 1935 technology was to use distributed losses in the
> system, with
> or without additional lumped components.
>
> There was a valid reason for that in the 30's, tubes
> had long internal
> connections and shielding was poor. Components were
> poor
> quality, frequency performance was limited by stray
> reactances in
> the system. Amplifiers built with 1930's technology
> and
> components tended to oscillate at or near the
> operating frequency,
> builders had to reduce system Q at or near the
> operating frequency.
>
> If you tried to suppress oscillations near the
> operating frequency, or
> even at twice or three times the operating frequency
> with
> conventional suppressors used today, the resistors
> would have
> produced too much heat. A more simple way to get the
> loss was to
> distribute the loss...and the heat.... in the
> wiring.
>
> As tubes have higher and higher upper operating
> frequencies and
> chassis became metal with better wiring techniques,
> and as
> components became better, the tendency to oscillate
> was not only
> reduced...the frequency also moved higher.
>
> If you have a peek at amplifiers, you'll see tubes
> like the 572B or
> 811A (old long-in-tooth designs a few steps removed
> from the 30's)
> have to use large inductors in the suppressors,
> diverting more
> current into the resistors. They have to use more
> resistance in the
> resistors, to affect the impedance of the more
> reactive anode to
> tank path caused by the long skinny leads inside the
> tube. These
> same tubes often need neutralized in grounded grid
> amplifiers, to
> prevent oscillations at HF!!!
>
> You see tubes like the 8877 or 3CX800, or 3CX1200Z7
> often don't
> require any suppression at all....or at the best
> minimum
> suppression.
>
> That's because those tubes grid are anti-resonant up
> around 500
> MHz or higher, where tube gain is suffering. The
> anode systems
> are usually resonant far below the grid's "problem"
> frequency, and
> so the stage is stable or easily stabilized.
>
> > If parasitics are of little concern, who on this
> list (manufacturers
> > included) is perfectly comfortable with running
> (or manufacturing) their
> > multi-band HF amplifier without a parasitic
> suppressor? One manufacturer
> > we know of uses an 8877 without the use of any
> suppressor, but other
> > amplifiers in their product line do. Does this
> infer that some of their
> > amp products are prone to potentially damaging
> parasitics while others are
> > not?
>
> Exactly. Let me tell you what I've been doing the
> past few months.
> I put a 1200Z7 in an RF deck of the AL1200, and with
> multiple
> fingerstock contacts the tube was totally stable
> with no
> suppression at all. That allowed me to add
> inductance between the
> tube and the tank, to reduce Q on ten meters and
> bring efficiency
> way up. The only reason that works is because the Z7
> has virtually
> zero grid impedance to the chassis. A 1200A7 will
> oscillate like
> crazy with no suppression, or with suppression and
> that series coil.
>
> I did the same to a 3CX800A7. I added a grid collet
> instead of a few
> fingers on the grid, and I could remove all
> suppression on a
> 3CX800. That let me use the same technique to put an
> AL-800
> amp on six meters (bandswitched from 160-6 meters)
> and get 1200
> watts out on six meters!
>
> I have a 3CPX800A7 with 3000 volts on the anode, no
> suppression,
> a VHF Q of over 1000 in the tank system, and it is
> completely
> stable. That PA runs 1100 watts out on 2 meters with
> 30 watts of
> drive, and is rock stable. The reason? The grids are
> very well
> grounded both inside and outside the tube. The grid
> "problem" is far
> above the operating frequency so stability isn't an
> issue.
>
> Now ask the engineers on this reflector who
> disagrees with what I
> have just said. Watch how it stacks up.
>
>
> 73, Tom W8JI
> w8ji@contesting.com
>
> --
> FAQ on WWW:
> http://www.contesting.com/FAQ/amps
> Submissions: amps@contesting.com
> Administrative requests:
> amps-REQUEST@contesting.com
> Problems: owner-amps@contesting.com
>
__________________________________________________
Do You Yahoo!?
Yahoo! Mail - Free email you can access from anywhere!
http://mail.yahoo.com/
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|