Chuck:
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.
73,
Steve, K0SF
----- Original Message -----
From: "Chuck Curran" <ccurran@wi.rr.com>
To: "STEVEN & NANCY FRAASCH" <sjfraasch@embarqmail.com>, amps@contesting.com
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
process.
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,
Chuck
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of STEVEN & NANCY FRAASCH
Sent: Monday, July 16, 2007 7:06 AM
To: amps@contesting.com
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
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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