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Re: [Amps] More instability problems

To: Vic K2VCO <k2vco.vic@gmail.com>
Subject: Re: [Amps] More instability problems
From: N1BUG <paul@n1bug.com>
Date: Tue, 16 Apr 2013 12:35:52 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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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