I've been building a 6m amp from scratch based on quad 4x150a's for more
than 3 years. I've never built any type of rf producing device before and
unbeknownst to me at the time, chose a project that was way over my head.
I've read and studied volumes of material written on 6m amps and tetrodes.
The power supply was also built from scratch and is sound. It has been
successfully tested on a 2m rf deck running a pair of 4cx250b's in push
pull. HV: 1680vdc, Screen: 360vdc, Bias: ~ -50vdc, when not jumping off the
bench idle current 400ma. Though not currrently designed for individual
adjustment and not perfect, the tubes have been selected and are very close
to matching. I'm at the point of irradicating runaway, oscillations and/or
parasitics. I have access to quality test equipment and am fairly
knowledgeable with its use. I'm conducting a cold test using a tracking
generator with spectrum analyzer to observe the output while injecting a
signal into the grids. This test is suggested in Care and Feeding Chap
5.5.2 (Feeding the Grid Circuit). This states: "The neutralization can then
be adjusted for minimum feed through." One question of many: 1) Is the
author suggesting that the tester knows the frequency of the parasitic and
that is the frequency being injected and neutralized? Continuing. I have
indications of runaway, in that, when the amp is keyed without excitation,
excessive grid, plate and negative screen current is drawn. Some of the
fixes I've tried include raising the cathodes off ground by as little as 10
and as much as 33 ohms; various configurations of parasitic suppressors,
grid swamping with various values of resistors (all non inductive) and
different methods of neutralization.
I have no intention of dropping this project, but would really appreciate
any advice or suggestions beyond that to rein this thing in.
Thanks so much for your time.
73, Roger
AI7RR
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