[Amps] instability issue

[n8de at thepoint.net]

Sounds like a resonance in the RF Choke.

Have you checked for that?

Do you have a frequency counter to detect the oscillating frequency?


73
Don
N8DE



Quoting N1BUG <paul at n1bug.com>:

> I'm trying to understand a problem with one of my amps. It is homebrew,
> 160-10 meters, using a 4CX1500B with screen grounded, grid RF grounded,
> cathode drive. It uses a conventional tapped coil pi network with
> vacuum variable capacitors.
>
> The amp was going into oscillation when using it on 12 meters
> (frequency of oscillation unknown, but not in-band). When it did this
> it was also destroying the vacuum relay on the output. The relay would
> fail with contacts stuck in the NO position (contact connected to amp
> output).
>
> While investigating I discovered the amp will always go into
> oscillation as soon as operating bias is applied IF there is no load on
> the output AND the LOAD capacitor is set to anything MORE than about
> 700 pF. Oscillation will stop if the load cap is reduced to something
> under 700 pf. This happens regardless of band switch position or plate
> tune setting.
>
> On most bands the amp does not break into oscillation during normal
> operation. That was only happening on 12 meters for reasons not
> understood by me. I managed to get it to stop doing that by adjusting
> the coil tap so that more output capacitance is needed. (That coil tap
> needed to be optimized anyway, as it was never in the ideal spot)
>
> Clearly this amp is not as stable as it should be. I am trying to
> understand what is going on here. I am having trouble grasping why the
> load cap seems to be the critical parameter, with coil tap and tune cap
> having essentially no influence. Any suggestions?
>
> Thanks,
> Paul
>
>
> -- 
> Paul Kelley, N1BUG
> RFI Committee chair,
> Piscataquis Amateur Radio Club
> http://www.k1pq.org
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