I'd dispense with the grid dip and get an antenna analyzer. Terminate as I
suggest (leave the tube in socket so that the tube parasitics are in place) and
look at the output port with the analyzer. Don't forget to key the output T/R
relay (amp power off of course).
This method works fine in HF and beyond. I am doing 20 GHz work now at work
and using the same method. The terminations are 0402 resistors, not leaded
ones.
Also, not terminating the output will shift the result significantly, that is
why I prefer the terminated method with a grid dip and I'll take the broader
measurement. But I don't use my grid dip any more. Pay to get an analyzer, it
is well worth it.
73,
Steve, K0SF
----- 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: Sunday, July 15, 2007 4:27:14 PM (GMT-0600) America/Chicago
Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
The general condition where a grid dip meter dips the
deepest is with the highest possible Q, NOT when the tank is
loaded with a resistance.
If you are not getting a dip and you are using a real grid
dip meter, it is because you are looking at the tank
incorrectly or have something wired wrong.
Think of the tank as a transmission line. A normal
pi-network tank when terminated acts like a line section
with an electrical length of 130 degrees or so. In fact a
tank perfectly tuned on 7MHz for a 3000 ohm tube and 50 ohm
load resonates when open circuited about 6.9 MHz, so you
should see a dip near the operating frequency WITHOUT
termination.
Of course this is still largely a useless test, sine it does
not mean the tank is working as a matching network.
The most useful test is to terminate the tube anode with a
resistor that looks like the operating load impedance you
want (use short leads to the resistor). Then you look at the
output port and look for a low SWR.
I'm going to side with Ian on this one. The largest dip is
ALWAYS when there is no resistance, and the dip would be
very close to the working frequency. You will get the
poorest dip when you follow the advice to terminate the
tank.
You have something else wrong, or have a bad dip meter.
73 Tom
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