>
>> 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.
>
813s never oscillated.
>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.
>
[chortle]
>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.
>
Why do so many 8877s reportedly fail in AL-1500s? Why did you stonewall
me six times when I repeatedly asked you if the AL-1500 uses a VHF
parasitic suppressor?.
cheers, Mr. Rauch
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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