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

Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
From: "Chuck Curran" <>
Date: Mon, 16 Jul 2007 19:00:04 -0500
List-post: <>
To All:

Thanks for the many valuable posts in regards to my dipping the Pi-Net
output circuit.  I have read each and every one and learned a bit in the

Steve, this last post of yours has caught my attention, since I just
purchased a MFJ-259B about 6 weeks ago.  I have already tried most of the
other suggestions, with no success.  The MFJ-259B was picked up for another
issue, now resolved due to the ease of use of the antenna analyzer.  

I did not use any cleaners on the silver plated contact sections of the
vacuum variables, but last time I checked, silver oxide was conductive.
However, In order to resolve this issue by the weekend, I will dis-assemble
all three key components, check everything again and then test -- I expect
to discover my error during that endeavor!  I am considering completely
removing the tune and load caps and the B&W 852 tank coil and then
re-connecting all on the bench, and then try the analyzer.  I will do the
fully assembled version first, since it is already there and ready.

I am new to this list, and at least one of my posts seems to be missing,
hope this one goes through OK.

Thanks a million to all,


-----Original Message-----
From: [] On
Sent: Monday, July 16, 2007 7:06 AM
Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?

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

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.


Steve, K0SF

----- Original Message -----
From: "Tom W8JI" <>
To: "STEVEN & NANCY FRAASCH" <>, "Chuck Curran"
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 

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 

You have something else wrong, or have a bad dip meter.

73 Tom

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