The virtues and pitfalls of using parasitic suppressor elements
around input and output of triodes and tetrodes have been discussed
in a number of articles which can be reviewed. The IRE and IEE
articles can be found in decent EE department libraries in
universities or industry:
"The Balancing and Stabilizing of High Frequency Amplifiers. with
Special Reference to Power Amplifiers for Radio Transmitters" by W.
Ure, E. Grainger and H. Cantelo, Journal IEE, Vol 72, 1933
"Parasites and Instability in Radio Transmitters" by G. Fyler, Proc.
IRE, Sept. 1935.
"Amplifier Instability in Transmitters" by Don Mix in QST, June 1948.
"VHF Parasitics in Beam Tetrodes" by George Grammer, QST, Aug. 1952.
"Improved Anode Parasitic Suppression for Modern Amplifier Tubes" by
Rich Measures, QST, Oct. 1986.
Tubes for RF Heating, Philips Application Book, by H. Dittrich
I am sure that there are others which I cannot locate right now.
However, much has been said about this, including using elements in
the grid side of tubes. My 1952 Radio Handbook also included thes
techniques as possibilities for stubborn circuits. F. Terman of
Stanford U. probably included similar work in earlier copies of Radio
Engineering.
Western Electric apparently did a lot of early work on this, putting
paralleled inductors and resistors in anode and grid leads of
triodes, but i haven't yet found where their work was published.
Probably in early Bell System Technical Journal.
Remember that the 'stopper' circuit is in the anode for the purpose
of presenting resistive damping in the parasitic resonant tube
anode/output tuning circuit while not 'appreciably' affecting the
lower operating frequencies. If the feedback mechanism for
oscillation is through the tubes Cp-g, then putting series R in the
grid may have little effect on fixing it outside of the tube. In this
case a shunt R on the grid to cathode would be better, but this has
the undesirable loading on the input RF circuit. In some cases the
only practical place is the anode. Grid loading was often used in
class B audio power amplifiers, and in some RF circuits. Bill's
statement that for a g-g triode to oscillate requires two resonant
circuits is not always the problem. One strong resonance, with
adequate feedback to the input, can make a powerful oscillator, if
the phase shift and stage gain are of the proper relationship for
oscillation.
73
John K5PRO
>
>Message: 3
>Date: Wed, 03 May 2006 20:05:44 -0700
>From: Bill Turner <dezrat@copper.net>
>Subject: [Amps] VHF Parasitics re-revisited
>To: amps@contesting.com
>Message-ID: <7.0.0.16.2.20060503200052.02255878@copper.net>
>Content-Type: text/plain; charset="us-ascii"; format=flowed
>
>For years VHF parasitic suppression has been done in the anode
>circuit of an amplifier. While it does work, I wonder if it might be
>better to apply suppression measures in the grid circuit, especially
>with a grounded-grid triode.
>
>One of the chronic problems with anode suppression is getting the VHF
>impedance high enough, but not so high that the suppression resistors
>overheat when operated on 28MHz. This is a delicate balance and one
>that not many amplifiers - even expensive commercial ones - get
>right. Many of us have found commercial amps with the suppressor
>resistors seriously overheated, having their values changed and in
>some cases, burned open, making them totally ineffective.
>
>Since it takes two resonant circuits for a g-g triode to oscillate,
>one in the plate and one in the grid-cathode, I'm wondering why there
>hasn't been more focus on the grid side? If the resonance in the grid
>side could be dampened out sufficiently, the anode side could be left
>alone with no suppressor at all. In fact, some amps such as the
>Ameritron AL-1500 already take this approach. The power level in the
>grid circuit is much less than in the anode circuit and components
>will be stressed much less.
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