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

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Subject: Re: [Amps] Grid Dipping the Pi network in a new amp?
From: "Robert Bonner" <>
Date: Tue, 17 Jul 2007 10:02:11 -0500
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Yeah Steve I think I would have left the guns at home, big mistake.

You're only allowed to throw your pocket protector (full of pencils) at the

I have been deleting this thread up to now.  I just noticed you are using a
B&W 852.  Those work pretty simply and don't have anything wrong with them,

If you find that in the 80 meter position it barely works on 40 meters.....

They were the low impedance version of the 850A coil and are all 3/8"
tubing.  Ignoring B&W specs I wouldn't try to exceed 1000 ohms plate
impedance with one of those.  They work best with a pair of tubes or more
and low plate voltage and high current.

I prefer higher voltage and lower current and was NEVER able to get an 852
to do what I wanted it too.  I've owned 5 or so 850's in my life and 1 852.
That was the mistake Steve was referring to I made.  (The only mistake in
1979 mind you)

I'm more curious as to what your attempted power parameters are.  If they
aren't high enough plate load you will never be able to find enough
capacitance to resonate that thing.  I doubt you are running 3  -8877's with
2500V on the plates.  They made them to run a 3-1000 zero bias with 2500V
and who runs just 2500V?  A 3-1000 isnt happy until it has at least 3500V on
it.  Even at 3000V that thing is already long for the junk pile.

I then changed over to an 850 and removed 4 turns off the 80 meter section
and 2 turns off the 40 section and life was good.  I always redid the
removable 10 meter section (they had too much coil) and physically aimed it
at the blocking caps.

I may be way off the thread like I said, I haven't been following.  But
Steve drug me into it.


-----Original Message-----
From: [] On
Sent: Tuesday, July 17, 2007 7:23 AM
To: Chuck Curran
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 ?  Is your plate choke much
smaller than planned (that would have the effect of a pretty strong inband
zero) ?  Perhaps your plate blocking cap is too small a value, or you're not
setting the output T/R relay for transmit ?  It just seems like too much
work to take apart again.  These are all the things I have had to straighten
out with Bob, K0DD, but I assume you're way ahead of him.

Hey, believe me, we (and especially Bob) have made plenty of mistakes (I can
think of one at work last Thursday).  So, I guess, I would go check the
not-so-obvious obvious and make sure.

If you use your antenna bridge, remember, that it IS the source impedance;
therefore, you're not putting a 50 ohm resistor at the tank output, the
antenna analyzer is sourcing 50 ohms and you're placing the 1800 ohm
resistor (if that is what you think the design plate impedance should be) at
the tube plate connection to ground.

If you give me the tank component values and the config, I can quickly
calculate the input impedance.

Another tip: when I do PI-L networks I usually set the middle node to the
conjugate mean of the input and output impedance.  Therefore, if the input
impedance is 1.8K (real), and the output is 50 ohms (real), in between the
two L networks should be: (1.8K*50)^.5 = 300 ohms real.  There may be Q and
efficiency benefits depending on the components used to use a different
intermediate impedance, but generally, the middle of the PI-L should be at
conjugate mean.

Anyways, send me your data if you want some help.


Steve, K0SF

----- Original Message -----
From: "Chuck Curran" <>
Sent: Monday, July 16, 2007 7:00:04 PM (GMT-0600) America/Chicago
Subject: RE: [Amps] Grid Dipping the Pi network in a new amp?

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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