[Amps] Sweep tube amp by DL9AH, made by HB9AWI

Bill, W6WRT dezrat1242 at yahoo.com
Mon Jun 29 10:40:32 PDT 2009


On Mon, 29 Jun 2009 11:26:01 +0200, Angel Vilaseca
<avilaseca at bluewin.ch> wrote:

>Advantages of this tank circuit are: simplicity, no bandswitch contacts 
>to burn nor associated wiring to cause stray resonances.
>Potential inconvenients I can think of : Safety issue when 
>bandswitching, Stability?
>Only trial and experimenting will tell!


This is an ancient design and was pretty much abandoned years ago for
good reason. Take a look at handbooks from the '30s and '40s and you
will find it there. There are two major drawbacks to it:

1. Harmonic suppression is poor. The antenna is connected across a
coil (or part of one). At higher frequencies (think TVI), the coil
acts as a simple high-pass filter, just what you don't want. Compare
this to a pi-network where the antenna is connected across a capacitor
and just the opposite happens.

2That alone should be a show-stopper, but there is one more.

2. Band switching requires switching two taps for each band. Again,
compare to a pi-network which only switches one tap. 

As soon as the pi-network became well understood it almost totally
replaced the tapped-coil output. TVI reduction was a major factor in
adopting the pi-net, as you will note in discussions of the circuit in
handbooks of the early TV era. Another mechanical factor is that a
variable capacitor is much simpler and more reliable than a variable
(or tapped) coil.

73, Bill W6WRT

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