David:
The resistor composition will not make as much difference as will the overall
length of the resistor. Remember we're dealing with HF; therefore, I don't
think a 1/8 watt resistor with short leads will add much series inductance to
the resistive load. Remember that your tube when it is active will add series
inductance (and more important shunt C from the plate to grid structure)
because of its length and geometry,
A good rule of thumb for wire inductance (or any filamentary current) is 20
pH/mil. Therefore, 1" of wire will create about 20 nH of inductance, or 3.6
ohms of inductive reactance at 28.5 MHz. That is insiginificant in a high Z
tank circuit. I am not an expert on resistor composition effects, but metal
film (I believe) is probably the best choice, but the resistor length, not its
composition, will determine its parasitic effects for the most part. Again, I
use 0402 metal film SMT resistors in shunt all the way to 20 GHz with
repeatable results on printed circuit boards. An 0402 resistor at 20 GHz is
dimensionally much larger than a 1/8 W resistor with 1" leads at 28 MHz. I
have used 0201 resistors upside down (to emulate a microstrip TE line better)
on surface mount microstrip 75 GHz antenna arrays and they also worked fairly
well as terminators.
Good luck !
73, Steve K0SF
----- Original Message -----
From: "d cutter" <d.cutter@ntlworld.com>
To: "STEVEN & NANCY FRAASCH" <sjfraasch@embarqmail.com>
Sent: Monday, July 16, 2007 7:18:25 AM (GMT-0600) America/Chicago
Subject: Re: Re: [Amps] Grid Dipping the Pi network in a new amp?
Steve
What bothers me about the valve substitute resistor is its inductance. My
inductance meter cannot make up its mind what value my 1800R resistors are
(modern metal film). Must I find an old carbon composition for this ?
73
David
G3UNA
>
> From: STEVEN & NANCY FRAASCH <sjfraasch@embarqmail.com>
> Date: 2007/07/16 Mon PM 01:05:30 BST
> 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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> http://lists.contesting.com/mailman/listinfo/amps
>
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