[AMPS] Re: Parasitics

[Tom Rauch]

> Date:          Wed, 13 May 1998 21:08:41 -0500
> From:          Jon Ogden <jono@webspun.com>
> Subject:       Re: [AMPS] Re: Parasitics
> To:            w8ji.tom@mcione.com, amps@contesting.com

Hello Jon,


I said:
> >It is possible, by unloading a tank in a conventional PA, to generate 
> >voltages of many times normal operating voltages. Underloading of a 
> >tank is the single largest cause of tank circuit arcs, and is a 
> >repeatable and measurable cause source of damage.

You said: 
> If the HF tank voltage can vary wildly and do what you say, which I 
> belive it can, what would prevent the VHF voltages in an oscillatory 
> condition from doing the same thing?
 
Very good Jon. Nothing at all, except the load presented by the 
tank. The reason the HF signal can swing so freely is the tank has 
high HF Q and is resonant. For VHF signals, the anode can swing quite 
freely but the tuning cap looks like a very low impedance.

This is easily measurable.
  
> If the tube has gain at VHF (which it does, we are all in agreement on 
> that) then what would prevent the tube from generating voltages high 
> enough to arc or discharge across the bandswitch in a runaway oscillation?

The tube, the tuning capacitor, and the series impedances of the 
leads in the PA. This is why I suggested exciting the anode (of a 
cold PA) with a sweep or single frequency swept source with a source 
impedance about average for the anode impedance.

Consider this point. How much power can you get from a 3-500Z at 180 
MHz or so if you INTENTIONALLY try to amplify or build and 
oscillator? Now imagine the likelihood that would occur by accident 
with small wire leads..

It is easy to track voltage through the tank system, and watch it 
change. The problem at VHF is often  one of transmission line 
transformer effects. 

> Why is it possible at HF and not at VHF?
 
No one said it isn't "possible", but it is "impossible" t generate 
such voltages in a tank that presents a very low impedance at VHF, or 
one that has a considerable ratio of impedance transformation in a 
downward direction.

Virtually ALL amplifiers that have acceptable VHF harmonic levels 
have very good VHF attenuation starting at the anode terminal and 
moving through the tank.

> To me, it doesn't make sense to claim that in one operating frequency 
> region an amp will do one thing, yet in another region where it can 
> easily operate, it will do something completely different.

Exactly right. Think about this conundrum. 

Some people claim VHF signals, at frequencies where tank impedance is 
VERY unfavorable for both generation and transmission of signals (and 
this can be measured and reliably confirmed to be such), can cause 
the tube to flash over, the tank capacitor to flash over, and the 
band switch to flash over.

At the same time, they claim such voltages can never be reached at 
the main operating frequency, where tank Q is highest and impedances 
are optimized!

Pretty weird science, isn't it?

> Please explain why HF energy can cause arcs and huge voltages while VHF 
> energy cannot?

Did that above. Impedances than can be measured are wrong. Now you 
do the same, and explain why very high HF voltages can not be 
generated.

> I am ignoring the presense of the VHF parasitic suppressor.  I am 
> assuming that it is either not working properly or is blown.  IMHO, a 
> properly constructed supressor will effectively limit that VHF voltage.

Not much. It's job is to load the system with series resistance. The 
optimum suppressor would have an equivalent parallel resistance 
(remember it is two components in parallel) that, when connected in 
 series with the anode lead, is greater than the impedance of that 
path.

That insures the anode is loaded by a lossy resistance, rather than a 
non-lossy reactance that mostly shifts phase.

That's why, if you look at PA's that have long thin leads in the 
tubes (look at 811A's in this example), tank to tuning cap 
connections, and so on you will see these amps need a lot of 
suppressor inductance and higher suppressor resistance.

On the opposite side of the coin you have external anode tubes with 
grid cones that terminate in a flange at the base. These tubes have a 
very short wide low impedance connection and often require nothing at 
all for suppression to obtain absolute stability.

Look carefully and you will find  PA's using tubes that provide the 
best VHF operation (tubes with the highest VHF gain and performance) 
are the most stable, while tubes with long thin leads and poor VHF 
performance are the most unstable tube and require the highest Rp 
suppressor for stability.

None of this is all that mysterious.

73, Tom W8JI
w8ji.tom@MCIONE.com

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