Hi Roger,
Is the design single-ended or push-pull?
Have you checked out your RF deck by removing each tube and replacing it
with a carbon resistor of/at/near the resistive load (RL) for the tube
operating conditions (get this from the data sheet) - a 4CX250B has an
RL of around 4350 ohms at 2KV in AB1. If you substitute your tubes with
appropriate resistors connected anode to ground then with no volts
applied you should be able to "look in to" the output connector with a
vector network analyzer (VNA) or antenna analyzer and tune up your
output stage for a good match, ie. 20dB return loss or better.
Again, without knowing your design - you should be able to put the tubes
back in and apply heater volts and a few watts of RF drive and minimize
the coupling between grid and anode using a sensitive power meter
connected to the output, i.e. adjust the neutralization for minimum
power feed-thru. I found this technique remarkable on my W1SL 144MHz
pair of 4CX250Bs...
Mike G8TIC
On 25/07/2015 21:20, Roger wrote:
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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