>Dear Rich,
>
>I have an Alpha 77SX with a low frequency problem (parasitic?). The sympton
>only shows up on 160 meters, where the grid current at maximum output and
>apparently proper loading is about 350 ma. instead of 100 to 150 ma. or
>less.
** this does not sound like a parasitic.
>For 1500 watts output I have about 400 ma plate current. I cannot
>find any fuzzies on a Heath or Yaesu monitor scope. Drive is 45 to 50
>watts. The exciter is an FT-1000MP.
>
>On 80, for 1500 watts out it takes 45 watts drive, 100 ma grid current and
>about 400 ma. plate current.
>
>I have used both the original RFChoke and an Ameritron choke out of an
>AL1500 and there is no change in the high grid current.
>
>Do you have any thoughts?
** I would:
1. measure the f.s. accuracy of the grid-current meter.
2. upgrade the 160m Tank-C padders to capacitors that are rated for
tuned-circuit use.
3. Since two 8877s exhibit 0.2pF of feedback C, lowering the VHF-Q of
the suppressors might be a good idea. Another gimmick to ameliorate the
effect of feedback C at VHF is to install a series R-C VHF load at the
cathode. Typical values are 10 or so ohms and 30 - 200pF in series --
use short leads. The R should have minimal intrinsic L. [note -- The C
added by this network subtracts from C-2 in the tuned inputs.]
-- What is the value of the HV+ glitch resistor?
>
have phun, George K8GG
>----- Original Message -----
>From: "2" <2@vc.net>
>To: "Phil Clements" <philk5pc@tyler.net>; " AMPS" <amps@contesting.com>
>Sent: Thursday, March 27, 2003 8:35 AM
>Subject: Re: [Amps] Parasites (parasitics)
>
>
>>
>>
>> >
>> >
>> >> I have gone back thru some old e-mails and re-read some concerning
>> >> "Parasitics" and "parasites" and ETC.
>> >> what is most interesting to me is not so much how to control and?or
>> >> eliminate them , but the fact that they were recognized "Way back" in
>the
>> >> twenties and such, and yet there are those folks today who seem to
>believe
>> >> that such things don't exist.!!!! I don't have the books from those
>days ,
>> >> so i'm having to assume that the info does in fact exist., but believe
>me,
>> >> I'm following the thread all the time.
>> >> It's very interesting !!!!!
>> >
>> >Any "doubting Thomases" can do an easy experiment. Just breadboard up
>> >a circuit with a glass tube sans suppressors, and observe the results. I
>> >don't
>> >think anyone could possibly deny the existence of VHF parasitics. We have
>> >been living with them since at least the 1920's. The argument is how to
>> >identify them and prevent further oscillations. As ceramic tubes became
>> >popular, with their shorter leads and compact designs, it was found some
>> >could be tamed sans suppressor,
>>
>> However, the tradeoff with ceramic tubes is that they are rated for
>> "Amplifier and Oscillator Service" to higher frequencies, so there could
>> still be surprises lurking about,. For example, the 3-500Z is rated at
>> 110MHz and it has 0.15pF of feedback C. Since the 8877 has only 0.1pF of
>> feedback C, it would seem to be a more stable device. The sticky-wicket
>> is that the 8877 is rated at several times the amp/osc frequency of the
>> 3-500Z, so the 8877 actually has fewer reactive ohms of feedback at its
>> F-max than does the 3-500Z. However, although this may be bad news for
>> amplifier builders, for oscillator builders, it's good news.
>>
>> > or with only an inductor (3CX3000A7, etc.)
>> >or with any old suppressor laying in the junque box. In other words, with
>the
>> >advances in tube technology, suppressor design had room for error.
>> >
>> >There now seem to be four camps out there.
>> >1. Those who believe ALL tubes and layouts will have VHF parasitics.
>> >2. Those who believe that a suppressor of a certain makeup and design
>> > is better than anyone else's design.
>> >3. Those who start out from scratch on a new amp design, and tackle each
>> > problem as it comes up, installing only the preventative devices
necessary
>> > (if any) then quitting when the beast is tamed.
>> >4. Those who will bet with you either way that the Sun will rise in the
East
>> >tomorrow morning.
>>
>> 5) My position: Trying to squeeze out the last watt at 29MHz is
>> probably not going to do much for reducing VHF gain.
>> History tells those who will listen that betting against Mr. Murphy is
>> neither likely to be good engineering practice or likely to be profitable.
>> > ...
>>
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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