>Hi Carl,
>
>> Please explain the relationship between the resistor value and the coil
>> Tom.
>
>The rule of thumb in a tube with poor grid grounding is the
>resistance has to nearly equal or slightly exceed the impedance of
>the anode to chassis path at the oscillation frequency. Sufficient
>parallel resistance in the suppressor insures a SYSTEM Q of near
>unity, and sufficient damping to prevent oscillation.
This appears to contain a circular argument. Seem to me that this is
Tom's thumb.
...snip...
>...while tubes like the 8877 often need no suppressor at all.
...snip...
Often? It appears that the ONLY commercial 8877 amplifier ever built
that used no vhf suppressor at all is the AL-1500, which Mr. Rauch
designed.
The 8877 is well-designed without doubt. However, it is not perfect.
At maximum ratings [4kV/250MHz] , 6300 ohms of capacitive reactance
(feedback) exists between the 8877's anode/output and the cathode/input.
The apparent RF current in this feedback path (assuming that the anode is
appropriately loaded) is 2600Vrms/6300 ohms = 0.412A rms. For a tube
with extraordinary VHF amplification ability, all things considered, does
it seem likely that the possibility of regeneration can be safely
ignored?
During the parasitics debate, I received a telephone call from a fellow
who uses an amplifier-design computer application at work. He said he
plugged in the Eimac parameters for an 8877. Potential self-oscillation
above 100MHz was indicated.
>
>Treat the amp like you are building a VHF amp, especially the grid
>and anode, and it will be much easier to stabilize.
True. The anode and C1 should be close together to increase the anode's
VHF resonance. Increasing the grid's VHF resoance also improves VHF
stability. For tubes with much grid L, like the 811A, the VHF grid
resonance can be increased slightly by tuning out some of the X-L with
X-C, shunted by around 30 ohms of R--as is done in the 30L-1. Anyone
with a dipmeter can confirm this.
>> >Series resistive losses (such as winding the coil from a lossy
>> >material) in the coil have greater effect as the frequency is
>> >lowered, while parallel resistances (the kind most people use) have
>> >less effect at low frequencies.
>
>> There I must disagree. Pure resistive losses due to skin effect become
>> more pronounced as frequency is raised, not lowered. ...
Not in the Rauchian World, Carl.
When the parasitics debate started, he proclaimed that a nichrome Ls
destroyed 10m tank performance. When the parasitics debate was pretty
much over, Mr. Rauch proclaimed that nichrome destroyed 160m - 10m tank
performance. According to Wes' measurements, the nichrome Ls exhibited
0.79 ohms of ESR at 30MHz and 0.47 ohms of ESR at 10MHz. The output Z of
the tube in question was 4000 ohms. Does it sound reasonable that less
than 1 ohm of R in a 4000 ohm circuit is going to wreak havoc upon the
ether?
>> So why are there no articles? Even "RF Design" in its heyday didn't
>> broach the subject. Or any tube manufacturer has surfaced with a
>> definitive paper or article.
.....which is why I decided to write the Oct. 1988 *QST* magazine
article.
>
>Eimac has published some tech bulletins, but the subject has
>mostly been ignored.
In 1986, I asked Mr. W. B. Foote if there were any Eimac Technical
Bulletins on VHF/UHF parasitics? He said there were none. Why didn't
Mr. Rauch reference the alleged Technical Bulletins?
>I'm not aware of any detailed comprehensive
>article on the causes and cures of instability in amateur
>publications. I have no idea why there has not been one published,
>but I do know tube design is a dying art.
Three such articles have been published in *QST* magazine. Philips Ltd.
published a book which contained a chapter on curing vhf parasitics. I
referenced this book several times during the vhf parasitics debate. Mr.
Rauch did his unlevel best to ignore it. [see Will, KN6DV's archive].
Rich---
R. L. Measures, 805-386-3734, AG6K
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