>> >Rich, maybe you can put aside all the innuendo and diversion and explain
>how
>> >a parasitic can mechanically displace a filament, while thermal cycling
>or
>> >material problems are not likely to do so.
>>
>> Both apparently can. Eimac cautions against having more than c, 29A-rms
>> of filament inrush current - presumably to avoid distorting the filament
>> helices.
>
>Eimac states that as a general rule, and it is very conservative.
>
** The Eimac 3-500Z spec sheet says neither.
>The problem arises when the tube has poor materials, or is improperly
>manufactured.
** a.k.a., NIH syndrome.
>The AL80 series has a very long history with good tube life.
>This filament-grid issue really just started when Amperex tubes were used.
** I sell parasitic suppressor retrofit-kits to AL-80 owners who tell me
they had an Eimac 3-500Z short.
>It appears even in amplifers that have a LONG field history of no failures,
>when Amperex tubes are used as a replacement.
>
** Passing the buck.
>For example, I have an AL80A that was used with an Eimac tube since 1983. I
>changed that tube last year to an Amperex, and it failed G-K within a week!
>A replacement lasted about six months, and failed the same way again. A
>change to one more Amperex tube and the amp has run even since.
>
** not all 3-500Zs have the same vhf gain.
>> I have autopsied a number of grid-fil shorted 3-500Zs that were
>> funtioning normally before their grid choke imploded, and/or the grid-I
>> meter and/or shunt exoloded, and/or the vhf parasitic suppressor resistor
>> more than doubled in resistance without showing external signs of
>> heating. (tubes removed from amplifiers that had c. 60% of 29A of
>> inrush).
>
>Carbon resistors age with time and heat. They are notorious for that. But
>then I'm sure you know (and choose to ignore) that fact, so I won't rehash
>the same old facts of life you choose to reject.
>
** I agree that carbon comp resistors age, but I have not seen a change
of more than 30% in either 5% or 10% tolerance units c. 20-years old.
With what seem to be vhf parasitics, I see 400% changes in newly replaced
units.
>> - A friend took his SB-220 to work, coupled the anodes to a spectrum
>> analyzer, and found there was damped-wave ringing at c. 110MHz at the
>> anodes when sending 50wpm dits, even though grid current was normal -
>> thus, no vhf oscillation was present.
>
>I doubt it.
** He told me he put a probe near the anodes and had a look with a
spectrum analyzer.
>In order to have damped wave "ringing", there must be a
>transient with a response slope much more rapid than the frequency of
>ringing,
** So you are suggesting that a spark transmitter could not produce RF
at a higher frequency than that of it's rotary spark ? Give us a break,
Tom.
>
>Even if there were ringing, which I doubt,
** The long excursion on the wide river in the Land of the Pharoahs
continues.
>it goes nowhere towards proving
>anything except the amplifer is stable. If it were not stable, it would
>oscillate.
** On occasion it apparently had - which is why he took the SB-220 to
work and took a look with a spectrum analyzer.
>
>Many or most people don't know how to use spectrum analyzers properly, and
>that could 9or could not) be the root of what he thought he saw. If you
>collect enough data and disgard what you don't like, you will eventually
>have something to support your wild theories Rich!
** Except for those in denial, spark transmitters are an undeniable part
of history.
>
>> was a feedback path between the SB-220's anode output and cathode input.
>> Unfortunately, there is 0.3pF of feedback C. At 110MHz, 0.3pf = c.
>> 4800-ohms of XC. This doesn't seem like much until one discovers that
>> the length of RG-58/U coax used at the cathode input of the SB-220 is
>> resonant c. 110MHz.
>
>First, the .3pF was taken as an equivalent at 30MHz, not 110MHz.
** Not the case. Eimac describes the method used.
>The peak in
>effective feed through occurs around 200MHz, when grids are grounded
>properly.
>
** Tom does not believe inductively-coupled dipmeters.
>Second, the length of cables mean little or nothing by themselves. It is a
>complex circuit, with stray capacitances and inductances as well as the
>input circuit itself part of the system. If you sweep the input of the tube
>you will find the VHF impedance at the cathode is low.
>
** The problem is that the RG-58's resonance isolates the cathode from
the low-pass (110MHz attenuating) tuned input.
>The length of cable between by dummy load and amplifer is 1/4 wl at 21 MHz
|