To: <amps@contesting.com>
> Date: Mon, 25 May 1998 10:03:31 -0500
> From: Jon Ogden <jono@webspun.com>
> Subject: Re: [AMPS] Amplifier Experiments!
> To: w8ji.tom@mcione.com, amps@contesting.com
Hi Jon,
> Hmm...read again, Tom. I think my failure WAS a parasitic of some sort.
> To make a 3 watt, 1 Ohm metal film resistor fail, one would need several
> amps of current. To make a parallel combination of a one ohm 3 watt and
> a one ohm 2 watt fail, one would need far more current than is probably
> available. And it happened again yesterday. More on that in another
> note.
The saturated current of a 4-1000A is roughly 16 amperes. If the
"parasitic" drove the screen and control grids to 1000 volts positive,
and the anode was held at a few thousand volts, you'd only have about
16 amperes of filament emission.
Now think about an "oscillator" circuit. In order to have that much
voltage, feedback voltage would have to be nearly 100% of anode
voltage. If feedback was 100%, the tube would oscillate all the time.
It's a nice "theory", but knowing how oscillators, amplifiers,
and tubes works should cause one to toss the "parasitic" theory right
out the window.
Finally you had an intentional parasitic earlier, and you
never saw anything like this. I never have either, even though I've
built intentional instability into PA's to try and create the
claimed ill effects.
You changed tubes, and the thing popped. IMO, you probably have a
gassy tube.
> The Tune C is mounted on a bracket above the Load C.
How does the anode connect to that? And is that bracket VERY well and
directly grounded to the chassis?? How long is the path from the
anode to the chassis through that cap and bracket?
> The Tune C is a
> Jennings Vacuum variable. A line runs from the Tune C to the B&W
> inductor. The output of the inductor goes two places: One to the RF
> output, the other to the load C.
Bad move. The path to the output should ALWAYS be routed past, or
through, the loading cap. Otherwise, it does almost nothing for VHF
harmonic suppression..and you can get some nasty series resonances.
>The Load C is connected with a piece of
> copper strap about 1.25 inches wide. The strap is probably about 4 or 5
> inches long total (yes, it's not the best, but I have wanted to avoid
> changing the tank circuit layout). The copper strap is hooked to the
> sections on the top part of the cap.
I hope that strap is flashing and not braid!
> The sections are all connected by a
> piece of wire. The case and the other plates of the cap are grounded.
> Other than the long piece of copper strap running to it, the rest of it
> looked fine to me.
The RF output should be taken off the opposite end of the load cap as
the tank connection side.
> I guess I don't follow your logic. This is not an insult. A circuit
> node is a circuit node.
For dc yes. Not for RF, especially at VHF. The last thing you want to
do is extract the RF output before passing it through a low shunting
VHF impedance.
> So wether you connect the output of the amp and
> the input to the cap in exactly the same spot, or as you suggest above, I
> see no difference.
A spectrum analyzer or network analyzer will see the difference.
73, Tom W8JI
w8ji.tom@MCIONE.com
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