[Amps] instability issue

[Vic K2VCO]

I had a similar LF oscillation issue with my 813 grid-driven amplifier. I fixed it by 
replacing a grid RF choke with a resistor. If you can arrange the biasing so that you do 
not have an RF choke in the grid circuit, that will probably help. Or use a different 
choke, perhaps with a ferrite core.

On 3/27/2013 8:18 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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>
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/