>> Building a stable amplifier that has high gain becomes increasingly
>> difficult the higher ones goes in frequency. Things such as layout
>> issues, bypassing and genuine parasitics become critical. When I say
>> "genuine parasitics" I don't mean the kind Rich talks about. I am talking
>> about the inherent, real world, stray inductances and capacitances in
>The real thing that separates amps are the tubes. Tubes used at
>VHF and UHF typically have almost zero length grid leads because
>they are brought to a ring or flange.
/\ A 1-inch [25.4mm] long bar of silver has c. 11nH of inductance.
Above 100MHz, 11nH is nothing to sneeze at.
>If you look at tubes that are
>problematic for VHF stability, they are tubes with long skinny grid
>(and sometimes anode) leads and long grid structures inside the
/\ this is generally true.
>Tubes that are problems include 572B's, 811A's, 4-1000A's, etc.
/\ So why do g-g 813s virtually never oscillate?
>Tubes that are almost unconditionally stable when installed
>correctly include 8877's,
/\ "almost" is right. See Figure 24 on my Web site.
/\ both of the 3CX800A7s that I tested had gold-sputtering.
/\ Very low feedback C.
>Some tubes are in the middle, like 3-500Z's and 3CX1200A and D7.
/\ The 3CX1200A7 has a relatively large amount of feedback-C. (? 0.8 pF)
>If you want an easy to stabilize HF amp, use a compact tube with
>a grid flange. If you want a tube that is a problem, use a 4-1000A or
>The problem is virtually always, for the size of tubes we deal with,
>the lead from the anode...out of the envelope...and to the plate
/\ true enough
> and the length of the grid lead from the grid to the
>grounding point and then through the sheet metal back to the
>tuning capacitor ground. And no, you can not evaluate that path
>with a grid dip meter. That is total nonsense, because it is an
>impedance problem not a resonance or Q problem.
/\ "total nonsense" ? According to G.E. engineer, G. W. Fyler, in
''Parasites In Transmitters'', Institute of Radio Engineers journal.
Sept. 1935, resonances support parasites. Are dipmeters useful for
>VHF amplifiers live happily and are totally stable with anode Q's in
>the hundreds, because the grid is simply resonant far above the
>frequency where anode impedance increases.
/\ A UHF resonant grid - such as the 8877's, 8874's, and 3CX800A7's -
can undoubtedly support oscillations above the grid's resonant frequency.
> grid Q isn't even the major issue.
/\ Indeed, Tom. The major issue is the resonant freq. of the grid
>The shorter we make both those leads, the easier the amp is to
>stabilize. The smaller the impedance of the suppressor can be, and
>in many cases a suppressor isn't even needed.
/\ The AL-1500 is the only HF amplifier that has ever made which has no
VHF suppressor whatsoever. I have spoken with a number of owners who
have gone through three 8877s during the 12-month warranty.
> That's why no one
>can offer a formula to calculate the proper size of a suppressor,
>despite people who "claim" they can.
/\ ... an outstanding non-sequitur.
>It's a shame this entire subject, and the topic of why tubes fail, has
>been taken to the low level it is at, with failures caused by tube
>manufacturing or operational problems blamed on parasitics.
/\ The subject of gold-sputtering seems to leave Tom sputtering.
For a 14-month period in the late 1980s, Eimac had a heat dam problem
with the 8877. Mr. Rauch is seemingly still using this as an excuse.
>But then I guess the reason so many people keep swallowing the bait
>is it's easier to tell people it's almost always one single problem
>and to give one magic cure for everything than to give an honest but
>more complex answer.
/\ I have never claimed such. However, the approx. 1400 letters and
telephone calls I received in the 10-months following the article on
anode parasites in the October, 1988 issue of *QST* told me that the
parasitic problem was more pervasive than I thought.
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
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