It is a 4CX1500B, cathode driven.
Screen: direct to chassis.
Grid: RF grounded, 6000 pF (3 x 2000 pF). Bias currently fed through
150 ohm resistor with bypass/feedthrough cap
The 50 ohm RF load resistor on the input was put there to help
provide a more predictable load for the exciter and to swamp some of
the excess drive.
The configuration can be seen here:
http://www.n1bug.com/tech/4CX1500B-6m/6m4CX1500Bsch-hires.jpg
That is my 6m amp but only component values differ between it and
the problematic HF amp. I never got around to drafting a schematic
for this HP amp.
A friend suggested converting the amp to passive grid driven
configuration. That should be simple enough with the exception of
getting the resistor network / drive level right without having a
large stock of parts to experiment with.
Paul
On 04/16/2013 11:47 AM, Vic K2VCO wrote:
I remember that it is a 4CX1500 or 4CX1000 but I don't recall the configuration
-- was it
cathode-driven or grid-driven?
You certainly do NOT need a tuned input if it is grid-driven class AB1. The
50-ohm load
provided by the resistor should provide a perfect match to the exciter, and the
impedance
won't change if there is no grid current (even if there is some, it will be
swamped by the
resistor). All you will need is some compensation for the input capacity of the
tube on 15
- 10 meters.
In this case, maybe the swamping resistor is intermittently open? If it is a
thick-film
type (looks like a transistor except only 2 leads) then it needs to be
adequately
heat-sinked or its power rating goes way down.
If it's cathode-driven, I am not sure why you would need a 50 ohm resistor. In
that case,
you WOULD need a tuned input.
--
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org
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