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Re: [Amps] Grid Dipping the Pi network in a new amp? --- ( Itnow Functio

To: "'Chuck Curran'" <>, <>
Subject: Re: [Amps] Grid Dipping the Pi network in a new amp? --- ( Itnow Functions)
From: "Robert Bonner" <>
Date: Sat, 21 Jul 2007 15:50:39 -0500
List-post: <>
Good testing Chuck.  You confirmed what I had said earlier this week.  The
maximum plate load impedance for the 852 is around 1400-1500 Ohms and
responds best down in the 1000 range.

SO you have three options here.  Rewind your coil (cheap), buy an 852A
(medium) which you will have to take turns off, or lower your plate voltage
and raise the current (very expensive).  (The amp then will probably not
make 1500W)

It could be possible to just add a few turns to the coil.  Making the turns
closer together would also help, but not enough to make it fly.
Refrigeration copper tube is what the coil was made from in the first place,
then silver flashed.  I buy refer tubing at the local ACE hardware.  I use
it for tesla coil primaries too.

Now I did see some photos back in the olden days of a guy who taped a few
ferrite rods together and supported them inside the 80 meter portion of the
coil.  I always thought that was rather cheesy, but it works.

You are not going to want the padder caps on the plate cap if you can avoid
it.  But you will need to get the PLI up to get within the range of your
vacuum variable. Remember the Cout of the tubes does add in the C1 number.
The loading cap padder will have to handle more current and is very
susceptible to heating and moving around.  Tuning will be constantly
changing unless you have enough current ability there.

I happen to have two NOS 30 YO B&W switch contacts in the parts bin.  They
are good for adding 160 or a 3.5 position to switch in more loading cap to
hit the bottom of the band.

Once you get this working on 80, you get to sweat the rest of the bands.  I
finally found for some of my larger amps the manufactured B&W coils were
more trouble than they were worth and I started making all of my own.

Hope it all goes well.


-----Original Message-----
From: [] On
Behalf Of Chuck Curran
Sent: Saturday, July 21, 2007 2:57 PM
Subject: Re: [Amps] Grid Dipping the Pi network in a new amp? --- ( It now

To All:

I am now getting good results, that at least respond in a logical manner to
all adjustments made.  Story is below:

This morning I inspected and cleaned all connections.  I then made up a
plate load resistor with short leads and clipped it across the input cap.  I
used a measured 2080 ohm resistor, to simulate the anticipated plate load.

I followed the suggestion made to employ my MFJ-259B antenna analyzer,
connected directly to the output of the PI-NET circuit.  With the amp set up
for 80 meter operation, I quickly found a minimum SWR around 4.8 MHz.  I
then started cranking in more tune and load capacitance, but both were
already near maximum.  The minimum SWR, which was 1.3, occurred at 4.6MHz.

I added a 75 pf doorknob padding cap to the tune cap, so along with it's 284
pf I now had 359 pf available on the tune cap.  I added a measured 420 pf to
the load cap, which gave me a total of 1640 pf on the load side.  The 80
meter inductance checked at 7.0 uH  

Now I got my minimum SWR at about 3.740 MHz, much higher than expected.
While the use of the MFJ-259B worked very well, the results I got were
somewhat disappointing.  I expected to see a low SWR down around 3.6 or so,
without the padding caps.  I did expect that I might have to add padding, so
this was not a total surprise.  

I went back and re-checked my calculations, as a result of some of the
points made on the Amps list server this last week.  Actually, I decided to
purchase a new ARRL Handbook and simply load up the PI-NET calculators
included with the Handbook.  Ya, I am lazy, should of just made up my own
spreadsheet.  When I adjust Q (to 16.4) in the Handbook PI_NET calculator
until it gives 7.0 uH as the required inductance, it is suggesting 294 pf on
tune and 1699 on load.  Since I only had 1640 pf on load, possibly that is
why I had to jam in 359 pf on tune and still just got to 3.740 MHz

At this time I do feel I may realize an advantage if I increase the
inductance, since the impact will be a significant reduction in the required
tune and load cap values.  I could finish the amp without any padding caps
and the associated switch, if I simply go from 7.0 uH to about 9.2 uH.  I
will ponder that option tonight.

The grid dip meters never worked.  I did fail to mention I was placing the
grid dip pick-up coil just above the tank coil -- I was not coaxial with it
and the pick-up was not inside the coil at all.  I possibly had poor
coupling with this attempt.

Thanks for all your comments,

Chuck WA9POU

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