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

[Angel Vilaseca]

A very good point Bill!

In this particular amp, plenty of room is available for the tank 
circuit. I will consider replacing the "Fuchskreis" with a Pi or PI-L 
circuit.

But then there is the problem of the anode impedance, which should be 
quite low with the 5 tubes in parallel.I think I have an ARRL handbook 
with some charts about which value the caps and coil should have 
depending on the tube plate impedance.

I also took off the tubes and checked the emission of each tube last 
night: Heater on for a couple of minutes, then cathode and grids 
connected together and 50 Volts on the plate. Current was 0.05 mA for 
one of the tubes. 0.10 - 0.12 for three of them and 0.20 for the last one.
I compared with a NOS PL519 I had. Current was 0.36 mA.

I will see if the bias circuit of the amp, which is quite elaborate can 
compensate for such differences.


Vy 73 de Angel Vilaseca HB9SLV



Bill, W6WRT a écrit :
> 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
>
>