From: Peter Chadwick <Peter_Chadwick@mitel.com>
To: amps <amps@contesting.com>, Steve Thompson
<rfamps@ic24.net>,
"'W8JI@contesting.com'" <W8JI@contesting.com>
Subject: RE: [AMPS] Blown TL922A... What to do?
Date sent: Fri, 8 Sep 2000 13:52:09 +0100
> Tom says:
>
> >.....the claims tank components arc from VHF parasitics on frequencies
> >far removed from the tank resonance is wrong.
>
>
> With respect, Tom, I think you're falling into the same trap Rich does -
> generalising from insufficient evidence.
With respect Peter, you seem to have not read my post.
I have measured the tanks in several PA's that Rich claims have
arcing failures from VHF parasitics.
> Consider the case where there is a resonance of the the tube output
> capacity in series with the pi tank input capacity, which series capacitor
> combination is in shunt with the inductance of the lead from the plate via
> the blocking capacitor (assumed to have a small amount of series
> inductance at VHF). The tuning C and lead inductance are series resonant
> below this frequency; you can visualise it like a pi network if you like.
> But you remember the old days of 6146s and 5763s on 2 m, where you 'series
> tuned' the plate circuit? This put the tuning C in series with the tube
> ouput C. Now if the tube output C is higher than the tuning C, then we can
> get step up to the plate tuning C at the VHF frequency. The step up will
> be in the ratio of the capacities.
These amplifiers have large air variables that nearly equal or
exceed the value of the tube output C. The contacts Rich
complains about are often on the other side of a series-inductance
that is pretty large, and the lead from the tank to the tube has
considerable shunting capacitance.
Tell me the flaw in my measurement method, exciting the anode of
the tube at the anode with a high impedance bridging source and
measuring voltage along the tank system.
That voltage steps down on any frequency I could measure from
the operating frequency to just over 1000MHz. Do you suppose the
tube oscillates at over a 1000MHz, and I missed the condition you
postulate? Perhaps we have SHF oscillations and the tank
suddenly behave in a sinister manner at 2 or 3 GHz, where the 3-
500 would be able to oscillate?
Now I didn't test every PA in the world, but the case where anode C
is more than distributed capacitance in the PA and the lead
between the two C's happens to form a step-up transformer doesn't
occur in any PA I have....even though Rich claims it can.
> Now IF (and it's admittedly a very big IF) the tube internals and the
> physical layout and the grounding of the grid (or screen - same will apply
> to grounded cathode tetrodes) is such that the feedback is in the right
> phase at the resonant frequency, then you can get an oscillation which can
> potentially (no pun intended) make the tuning capacitor arc over.
Remember we have to destroy the switch, which is further
downstream....and remember I haven't found a PA yet that meets
your speculation, although I admit could probably design one to do
that intentionally.
Please tell me, what is flawed in my method of exciting the anode
through a high impedance bridging source, and measuring voltage
along the tank with a high impedance RF probe good to a few GHz?
Wouldn't you expect to find a condition where the voltage steps
up? Let's get past that, and then talk about how the tube can
manage to develop full output at 150 MHz and not show grid
current or anode current.
73, Tom W8JI
w8ji@contesting.com
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