Tom:
I think we agree on not taking it apart and starting over and check what should
be obvious.
Without the benefit of looking at the amp, I can only throw out suggestions.
Obvious things whether the vac variable is shorted, or the tank inductor is not
switched, the tube has a plate-to-grid short (pull the tube and check) are
certainly the low bearing fruit.
I am not putting much weight on a "dip." I mean, so what ? The issue is
whether the tank is performing the impedance transform. I think we both agree
it's an obvious thing, but at this point, I wouldn't toss anything.
A plate choke that is too small or has a terrible series resonance will greatly
de-tune it. A decade coverage plate choke (as I am sure you know) is not a
trivial thing. Obviously, he can quickly disconnect the plate choke to check.
I agree that a plate block should not be it. But, a plate block that has dried
up and lost it's dielectric can change by x10 or more and be practically open ?
I've seen that before too.
Steve
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "STEVEN & NANCY FRAASCH" <sjfraasch@embarqmail.com>, "Chuck Curran"
<ccurran@wi.rr.com>
Cc: amps@contesting.com
Sent: Tuesday, July 17, 2007 8:13:51 AM (GMT-0600) America/Chicago
Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
> I just think it is something easy. For example, is your
> bandswitch shorting the section of inductor that should be
> in play ?
Any wiring error or switch indexing problem could be at
fault.
Is your plate choke much smaller than planned (that would
have the effect of a pretty strong inband zero) ?
Very unlikely. It would have to be nearly a short to stop
the dip.
Perhaps your plate blocking cap is too small a value, or
you're not setting the output T/R relay for transmit ?
Plate blocking cap cannot come into play with a dip meter,
and it has very little effect in a real amp. Consider an amp
with 3000 ohms plate load impedance. Even a blocking cap
with several hundred ohms reactance would have a minimal
effect on operation because the impedance at that point is
so high, and the capacitor adds a very high Q series
reactance.. Of course it would not affect a grid dip meter
even if it was gone, and would have only a very minor effect
on operation.
I remember when a buddy of mine and I went over to K8KAS's
house and he had a 160 homebrew amp with a single 3-500Z. We
looked at the blocking cap (it had no case or cover) and it
was 100 pF. Denny blushed and said he misread the numbers.
We were all hooting and laughing about how bad that was and
how much better the amp would work when the cap was replaced
with a proper cap. The fact is when we changed the cap the
tuning control moved several degrees in position, the load
control didn't move, and the power output and efficiency was
exactly the same within what we could read on the meters.
If you look at the effect of the blocking cap on a Smith
Chart, you'll see why it hardly matters. The real problem is
the dissipation factor or quality, not so much the value.
It has to be something like an open, a short, or a wiring or
switch problem. It isn't even necessary to have a antenna
relay closed or a blocking cap installed to test the dip
with a dip meter, although it all has to function with the
antenna analyzer and the single load resistor at the anode
of the tube.
Chuck, you should just yank the blocking cap loose and check
the dip again. Maybe the anode of a tube is touching a grid
and shorting the tank. I wouldn't start chasing nearly
impossible longshots like chokes or blocking caps without
running through a few simple fast tests first. The resonant
path is through the loading cap, the chassis, the tuning
cap, and back through the coil. The only thing in that path
that can stop a dip is an open in that path, or a short
across a component like the tuning cap or the tank coil.
Even a shorted loading cap can't screw up the dip. With only
three parts and a simple loop to stop the dip, you must be
overlooking something.
The analyzer test is more complex, and it involves the
entire system up to the load resistor you place at the
anode. But even then the choke and the blocking cap, unless
you are using 144MHz components or something very weird,
won't be an issue.
73 Tom
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