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[AMPS] Stability Question

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Subject: [AMPS] Stability Question
From: (
Date: Tue, 9 Feb 1999 10:54:22 -0500

On Mon, 8 Feb 99 15:55:51 -0600 Jon Ogden <> writes:
>>>Here's how I am testing:
>>>Key up amp, no RF, Ctune at min, Cload at max, open circuit on 
>>>(ie: no antenna or dummy load - no coax attached).  I figured this 
>>>be worst case.
>>Sure is and not a reliable test of the amps operational stability. 
>Try it
>>with real world tuning values and then remove the load.  A good 
>>should then be stable.
>>In a perfect world what you are trying should work but the 4-1000A 
>>that voltage is just asking to take off, why encourage it?  There 
>>even some commercial ham amps that would not pass that test.
>OK.  Well, I guess I am just of the school that wants an amplifier to 
>bullet proof.  Under "reasonable"  real world values, I have no 
>oscillation.  It's just when my turns counter on my UCS-300 gets very 
>(about 5 with a minimum of 0) that I have the problem.  It's probably 
>barely a crank from its absolute minimum.  Realistically, I'd never 
>it that low even on 10m.
>So are you saying that I don't really need to worry here?
>>>What happens is I seems to have a housing based resonance problem 
>>>one of my tubes. 
>>Just one tube or all ?
>>What bandswitch position?
>Just one of my tubes.  Perhaps it's a problem with the tube and not 
>amp.  I've a suspicion that this tube is possibly flaky.  Bandswitch 
>position is 10m. 

Does that tube produce more power than the rest?  Is it a different

>>> When the cover is off, every thing is fine.  
>>>when I put the cover on and bring Ctune close to its minimum value, 
>>>plate current starts to rise.  I must have a resonance in the tank 
>>>circuit housing.
>>>I tried adding a 10 pF vacuum cap in parallel with Ctune as some 
>>>suggested to improve stability in the past.  On paper you would 
>>>would work. 
>>Yuk !  All you are doing is increasing the chance for problems. On 
>top of
>>that the 10pf added to the approx 7pf of the Tune cap and the approx 
>>of the tube puts you well out of range for a reasonable Q on 10M and 
>>even marginal on 15M. Throw in a few more pf for strays.
>OK.  Help me out here then.  When I do tune up on 10 M, the turns 
>is well above its value from where it starts to oscillate.  No, I 
>have an idea of what value of C.  I figure it must be at least higher 
>than 20 pF.  The minimum value of the UCS-300 is 10 pF.  My guess is 
>it starts to oscillate somewhere between 10 and 20 pF.  So I thought 
>adding another 10 pF in parallel should keep the minimum capacitance 
>above where oscillation occurs (if oscillation starts at say, 15pf, 
>an addition of 10 pf will always ensure that the minimum Ctune is 20 
>which is above the oscillation value).  I understand what you are 
>about Q and that dovetails with some of our other correspondence on 
>pi-net values.  Perhaps my 10M circuitry does not have good Q and 
>why my power out is poor on 10.
>>If you have a GDO what is the oscillating freq in diode mode?
>Yes, I do.  I haven't measured that though.  I will try that this 
>evening.  How would one do this?  Just start the amp oscillating and 
>tune through the GDO to find the oscillating frequency?  Do I hold the 
>GDO just above the PA cabinet?

You probably need 3 hands. Just barely start oscillating and tune the
GDO; keep it as far away as necessary to get a half scale reading. But
keep one eye on the tube and be ready to hit the kill switch if it takes
off in melt down mode.

>>Is the tube base adequately grounded with the required clips? The 
>base is
>>a shield and MUST be grounded.
>Yes, the base has 4 clips all attached directly to the floor of the 
>And all 4 clips are in very good contact with the base.
>>>Should I add some "walls" 
>>>the housing to try to break things up?
>>Some EchoSorb on the walls or cover may help but that is a brute 
>>fix. Also try a different plate choke on the chance that it has a 
>>series resonance with the enclosure. The GDO may be of some help.
>Resonance of the plate choke in the circuit is about 22.5 MHz with 
>another resonance around 35 MHz.
>I thought about the absorber stuff.  Wish I had some!  However, I am 
>sure how effective microwave absorber will be at HF frequencies. 

There are different mixes but I have used a general type 1-10GHz to tame
a RX stage at 50MHz. Its when I am in the 13-60Ghz region that the mix
"sometimes only" can get critical. 
Rich also mentioned cavity effect in a early QST, I believe, article.

>to use this sort of trick in our 6 GHz amplifiers back at Motorola. 
>it's brute force.  I even thought of the idea of using a plexiglass 
>However, that isn't a real solution either.

Depending upon the tank coil spacing a steel lid may offer sufficient
detuning effect without absorbing too much good RF.

Bottom line...I think you should concentrate on improving the 10M
efficiency while watching for instability with other tubes.

73  Carl  KM1H

>Jon Ogden
>"A life lived in fear is a life half lived."
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