[Amps] instability issue

[N1BUG]

Wow! Further to this... my frequency counter seems to believe the 
oscillation is occurring at ~400 to ~450 kHz! The frequency does 
shift substantially as I adjust the load cap, and to a much lesser 
degree as I adjust the plate tune cap. The band switch seems to have 
very little influence, though I cannot say it is zero.

73,
Paul


On 03/27/2013 07:18 AM, n8de at thepoint.net wrote:
> 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