Good Morning:
The tubes are Gi-7b's. (output capacitance = 4.6 pf/tube) The grids are
tied hard to the chassis. I cut holes in the chassis just big enough for
the grids to fit through, and then made clips (3 per tube) similar to those
used to mount 4CX250B tube sockets. (no grid fuses here HI). The strap from
the anodes to the blocking capacitors is brass (tinned at the connecting
points) about 1/4" wide and approximately 1.5" long. The plate capacitor is
a standard air variable. The blocking capacitors are mounted on a piece of
tinned copper about 3/4" wide which ties directly to the tank coil and C1.
The tank coil is 4 turns of 1/4" copper tubing and is mounted horizontally
about 1' above the chassis. It was designed to be approximately 0.47 uh. The
tank coil & C1 are tied together with a piece of #12 copper wire about 1/2"
long. The other end of the tank coil is soldered directly to C2. There are
holes drilled around the mounting position of the tubes out to the diameter
of the anodes to accommodate air flow. Chimneys are 0.093" PTFE.
That's it
Thanks,
Mike
http://members.cox.net/w5uc
----- Original Message -----
From: "Tom W8JI" <w8ji@w8ji.com>
To: "Mike(W5UC) & Kathy(K5MWH)" <w5uc@cox.net>; "AMPS" <amps@contesting.com>
Sent: Monday, July 24, 2006 6:18 AM
Subject: Re: [Amps] Pi network tuning
>> Thanks Tom. The next question is; Does it matter which side of the
>> blocking capacitor I attach the termination to ground. This is a 50 mHz
>> amp, and to attach to the anodes would mean long leads. If I hang the
>> resistor at the input to the pi-network, (across C1) the leads can be
>> short.
>
> Mike,
>
> First thing you should keep in mind is the optimum tube load impedance,
> loaded Q, and the Q you need are all just approximations. There isn't any
> need to get over-precise with what amount to a rule-of-thumb. You
> generally won't see a noticeable difference in performance if you are off
> by a factor of two or even significantly more so long as operating Q
> reasonably exceeds the square root of the impedance ratios between tube
> and load in a pi network.
>
> With a 50MHz amp you have to be mindful of distributed and stray
> impedances. The stray capacitance and series inductance of tubes, leads,
> components, and temporary connections can have a large effect on 6 meters
> when compared to bands like 40 meters.
>
> What type of tuning cap are you using? Is it a flapper plate, a standard
> variable, or are you relying on tube output capacitance? What were the
> tubes again? How are the grids grounded? Are the leads from anode to the
> tank short and wide?
>
> 73 Tom
>
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