On Tue, 11 Nov 1997 11:21:43 -0700 John Lyles <jtml@lanl.gov> writes:
>OK, it's perfectly fine to have such a diversity of thought on
>parasitic
>suppressors and on parasitic generation itself. By the way, I have
>built
>VHF solid state power amplifiers where they oscillate at LF. These are
>the
>bane of bipolar power amplfiers, as the LF beta is high, and
>inadequate
>bypassing and decoupling networks on the collector and base bias can
>cause
>it to want to take off a KHz to low MHz.
That is assuming the manufacturers even use bias John!
The lack of adequate bias circuits for linear operation of the majority
of VHF-UHF brick amps is a sore point with many users. It appears that
only Teletec is using an active circuit that holds up under all drive
conditions.
Thankfully, decent circuits have been published allowing a gradual
clean-up of signals.
I have it on good authority that a major brick manufacturer will soon
follow suit.
>It's a paradoxical that with tube amplifiers it's just the opposite!
>Get
>them to work at HF, and they want to oscillate at VHF/UHF. The
>principles
>are, of course, different due to the large structures involved with
>tube
>circuitry.
>
>In the present amplifier I am developing using 150-250 Kw tetrodes,
You must have as much fun at your job as Eric does with antennas! It must
be nice!
I get the feeling from your post that the driver circuits for those big
tetrodes are now SS ??
I think Tom nailed it right on the head when he predicted the demise of
first glass tubes and then small ceramic ones in this country. I sure
hope the Russians dont follow the sneaker crowd.....$2 to make a Nike in
Indonesia and sells for $100 here .
Enjoy your posts...Carl KM1H
>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). I will report how it comes out,
>in a
>few months, once we fire it up. The beauty of using low DF testing, is
>that
>we can reduce our bias pulse to just a few hundred microseconds, and
>the
>repetition rate to 10 Hz or so. Any breakup will happen in the short
>intervals where the average power is low. This is where building
>CW/SSB
>linear amplifiers for ham/industrial applications have a challenge -
>to
>keep the parasitics from doing major harm since the HV stays on, along
>with
>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.
>
>John
>K5PRO
>
>
>
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>
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