[Amps] More instability problems

[N1BUG]

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