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Re: [Amps] Grid Dipping the Pi network in a new amp?

To: "STEVEN & NANCY FRAASCH" <>,"Chuck Curran" <>
Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
From: "Tom W8JI" <>
Date: Tue, 17 Jul 2007 09:13:51 -0400
List-post: <>
> 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 

  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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