[Amps] instability issue

[Carl]

That is also a common method or shunt the choke with a 3K or so carbon 
resistor.
A homebrew amp could also mean it is full of WW2 style HV mica caps which 
are often leaky by now and make poor bypasses, coupling, and tune/loading 
padders.  For 160M included I use a .01 3KV disc ceramic for up to 2KV,  or 
a pair of .0047 at 6-10KV in parallel for big amps, as the plate choke 
bypass and a .02 1KV disc for all the LV and input coupling.

Carl
KM1H


----- Original Message ----- 
From: "Vic K2VCO" <k2vco.vic at gmail.com>
To: <amps at contesting.com>
Sent: Wednesday, March 27, 2013 12:11 PM
Subject: Re: [Amps] instability issue


>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
>>> _______________________________________________
>>> Amps mailing list
>>> Amps at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>>
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>>
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>
> -- 
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco/
>
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