[Amps] More instability problems

[Jim W7RY]

I have a question...

Why are you using a regulator for the relay coils on your RF relays?  Are 
you sure the voltage is holding up on initial keyup? Are the relays 
chattering? It would be better if you dispensed with the regulator
and went with a simple resistor to lower the voltage to the coils if needed.

I'm curious as to your design in this area.

73
Jim W7RY


-----Original Message----- 
From: N1BUG
Sent: Tuesday, April 16, 2013 9:35 AM
To: Vic K2VCO
Cc: amps at contesting.com
Subject: Re: [Amps] More instability problems

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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