[Amps] instability issue

[N1BUG]

Yes and maybe, Carl. I had been using the amp for some time on 12M 
without problems. But I recently moved my 12M antenna to a different 
location where it is unfortunately interacting with something else, 
producing a higher than normal SWR. The amp loads into it OK, but 
requires somewhat more loading capacitance than before. It turns out 
the amp is not stable on 12M with the load cap set where it needs to 
be for this antenna configuration. It is stable if I return the load 
cap to where it had been with the previous antenna configuration.

I think it likely this problem has been there all along, but was 
never noticed because I was operating outside the range of 
parameters that make it take off.

The bias is fed through a 150 ohm 2W carbon comp resistor with a 
.001 uF feedthrough (perhaps not enough for effective LF bypassing) 
on the cold end. I try never to use a choke on the grid.

73,
Paul


On 03/27/2013 11:18 AM, Carl wrote:
> Did it ever work well on 12M? If not or not tested Id also suspect the plate
> choke and hope the big Ameritron fits.
>
> As far as the LF oscillation I hope the bias is not fed thru a RFC as these
> tend to turn the tube into a TNT ocillator. I also suspect the antenna choke
> is open as that would likely be self resonant down there.
>
> Carl
> KM1H
>
>
>
> ----- Original Message -----
> From: "N1BUG" <paul at n1bug.com>
> To: <n8de at thepoint.net>
> Cc: "'' amps ''" <amps at contesting.com>
> Sent: Wednesday, March 27, 2013 8:26 AM
> Subject: Re: [Amps] instability issue
>
>
>> 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
>> _______________________________________________
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>>
>>
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>
>

-- 
Paul Kelley, N1BUG
RFI Committee chair,
Piscataquis Amateur Radio Club
http://www.k1pq.org