Topband: preamp construction

[Steve Flood]

Thanks Tom -  this amp design has a variable gain, the "zero-gain" point of
which is set by the correct selection of the biasing resistors.  If I train
myself to only switch bands when the pot is turned down to the zero-gain
point, I should be okay - since the amp will not be "running" and I can
switch/unterminate RF leads with no problem (?).

A second question for you if I may.  I'm also interested in someday building
a 2N5109 push-pull preamp.  It appears that it is critical that both amp
circuits be identical regarding input/output impedance and gain.  Is this
the case with push-pull amp circuits?  - and is the Magic T
splitter/combiner design on your webpage appropriate to use in a push-pull
2N5109 preamp?

73, Steve KK7UV



----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Steve Flood" <flood@ixi.net>
Sent: Friday, December 20, 2002 8:14 PM
Subject: Re: Topband: preamp construction


> > While in the process of putting a preamp together (KD9SV design, 160
> > and 80 versions in the same box), I realized that I have to run the RF
> > inputs and outputs very close together to utilize relays or
> > panel-mount switches.  The DC lines are no problem of course, but
> > after being careful with the board layout of the circuit to avoid
> > having the RF input and output too close together, it doesn't seem to
> > make sense to violate this contruction rule at the relays/switches.
>


Tom's response...

On 160 if the lines carry common signals, you can get away with a
> lot. Most likely the amp will only have 10-20dB gain, and the relay
> or switch would normally have much moore isolation than that when
> swicthing at a low impedance.
>
> I don't think that amp has gain at upper HF or VHF, so you should be
> OK.
>
> You can get into trouble if you unterminate a lead or intentionally
> switch high impedances, while the preamp is running. Most small DPDT
> relays are pretty good at 50-100 ohms.73, Tom W8JI
> >