On Wed, 12 Nov 1997 09:58:55 -0700 "richard w. ehrhorn" <w4eto@rmii.com>
writes:
>Hello John and Hi Tom...
>
>According to a conversation I recall several years ago with Eimac's
>(now-retired) former chief engineer for power grid tubes, this
>internal-cavity type parasitic oscillation showed up in a new BC cmtr
>design using the 3CX15,000B7 tube, which it happens ETO uses in its
>current version 20-30 kW MRI amps (principal operating ranges
>21-42-64-85 MHz). At that time Eimac hadn't made any internal
>suppression mods.
Dick..is this the real 3CX15000B7 or the medical pulsed Y-156?
I'm curious since the Y-156 is showing up quite a bit lately as pulls.
Tnx Carl KM1H
>
>I won't try to explain it, but for whatever reason, a big
>Carborundum-type bulk resistor mounted vertically near the anode
>external to the tube with one end to chassis avoided the problem.
>
>73, Dick
>-------
>From:
>
> w8jitom@postoffice.worldnet.att.net[SMTP:w8jitom@postoffice.worldnet.att.net]
>Sent: Wednesday, November 12, 1997 2:25 AM
>To: amps@contesting.com
>Subject: Re: [AMPS] Yet more parasitics
>
>> To: amps@contesting.com
>> From: John Lyles <jtml@lanl.gov>
>> Subject: [AMPS] Yet more parasitics
To: <amps@contesting.com>
>> Date: Tue, 11 Nov 97 18:21:43 +0000
>
>Hi John,
>
>> In the present amplifier I am developing using 150-250 Kw tetrodes,
>the
>> bias is pulsed up from cutoff only 5% of the time, so the average
>> dissipation is 30 KW or so. It is CLASS A during the on-time. This
>thing is
>> for 2-6 MHz tunable. When I told one tube supplier that I was
>planning
>> Class A, they gulped and said I would be lucky if it doesn't excite
>UHF to
>> L band oscillation. The class A gain of big tetrodes is high, over
>20 dB
>> possible from the curves. The parasitic oscillation is mainly from
>the
>> large radial geometry of these tubes in the plate/screen to grid
>region. A
>> TE11 or TE21 circular mode can be excited. It is commonly seen when
>trying
>> to make characteristic curves in the test of big tubes. So one
>version of
>> my tube socket will have with a ring of lossy ferrite around the
>screen
>> grid (out of HV harms way of course).
>
>That is correct data, and exactly what Eimac and others tell me. This
>is the data that Rich Measures mistakenly applied to small tubes.
>
>Eimac had this problem with a 100 kW TV transmitter tube, the
>way I understand it the vacuum head above the cathode was large
>enough to sustain a cavity mode oscillation at or near microwave.
>
>Since the oscillation was contained inside the tube, and the tube
>drew so much current at the fundamental, the cavity mode oscillation
>INSIDE the tube slowly cooked the upper area of the grid. that's all
>that happened, the tube apopeared to run fine...no pops bangs arcs or
>sparks. The grid just slowly deteriorated.
>
>The problem was solved by adding some suppression INSIDE the tube.
>
>Unfortunately, Rich received a spin-off of the correct data
>you posted via an undocumented verbal conversation with an
>applications engineer. No one at Eimac knows if Mr. Foote gave Rich
>the correct data and Rich twisted it around to small power grid
>tubes, or if Foote goofed the data up.
>
>But the fact remains the problems Rich attributes to VHF parasitics
>were only found in ONE high power TV broadcast ultra-linear
>video PA tube, which had a near micro-wave cavitry mode
>oscillation INSIDE the tube.
>
>Unfortunately, QST published several articles and gave the incorrect
>message a "Foote" hold, and unfairly failed to tell the whole story
>to the amateur community.
>
>But what you say is correct, at least according to Varian and a
>Russian tube manufacturer. These oscillations have been found in very
>large (multi-kilowatt) high power linear amplifier tubes in Russia,
>and they are named after a Russian.
>
>> the RF. Lucky that the glass and smaller ceramic-metal tubes
>(without
>> handles!) have small interelectrode spacing, and these radial field
>modes
>> are above a GHz.
>
>That's right, and all the nichrome, carbon, steel, or ferrite you
>like in the outside-the tube- world won't cure a radial field
>oscillation INSIDE the tube.
>
>Unfortunately for hams, they were sold a false story based on a true
>fact that was totally misinterpreted. Unfortunately the wrong
>information will go on and on like the Energizer bunny beating a
>drum.
>
>73, Tom W8JI
>
>--
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>
>
>
>
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>
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