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

[Alex Eban]

...if anyone wants to get away from the tested and true PI or PI-l circuits
a better choice would be a link coupled tank circuit or a multiband circuit
similar with the Z-Match innards. This is exactly what it was designed for!
At the impedance levels encountered in multitube setups- a few hundred ohms-
you get very reasonable "Q"'s and you have to switch only the output side
coupling coils (relatively  low voltages). 
Alex     4Z5KS 

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Bill, W6WRT
Sent: Monday, June 29, 2009 8:41 PM
To: Angel Vilaseca
Cc: felipe at dxwatch.com; amps at contesting.com
Subject: Re: [Amps] Sweep tube amp by DL9AH, made by HB9AWI

ORIGINAL MESSAGE:

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!

REPLY:

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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